Heriot Watt University: ICIT, Orkney Campus
MSc Renewable Energy Development
Renewable Energy Technology
The studies key objective was to advise a UK based property developer looking to develop on the Scottish mainland a 500 home residential project on how they could best optimize its energy self-sufficiency potential. As a Renewable Energy consultant hired for the task; I have considered commercially available technology only since it’s assumed the project planning is underway. By researching and identifying an actual Scottish listed brownfield site, the assignment takes on real world applicability akin to a renewable development feasibility study. A brownfield site re-development selection has a high potential for local authority and community support. The following renewable energy generation technologies were considered;
- Community Energy From Waste (EfW)/Biomass Combined Heat and Power Plant
- Small to Medium scale wind turbines.
- Solar Photovoltaic (PV) modules
- Solar domestic heated water (SDHW)
Geothermal, i.e. ground sourced heat pumps or a possible District Heating scheme were not part of the Universities project remit but may potentially have significantly helped the developments heat supply needs.
Using the latest Scottish register of Vacant and Derelict Land (2014), the study was able to focus upon a suitable brownfield development site lying on the western fringe of Forfar in Angus County, Scotland; Orchard Bank Business Park. A residential development so close to the A90 link road could form a ‘green energy’ commuter hub for workers looking at alternatives to Aberdeen and Dundee housing. The renewable systems could also attract technology suppliers to the region creating employment opportunities. All report assumptions are specific to the current renewable’s support from the Scottish Government and local social and physical climate.
A renewable, self-sufficient energy generation strategy was evaluated on this site for a new 500 home development. Wind, solar and biomass energy conversion technologies were explained and summarised to the reader before their site applicability assessed. Commercially available technologies were identified but it should be noted that economic prospectivity was only inferred and not explicitly determined.
The final energy generation scenarios took into account the regional electrical and thermal household energy demands using typical UK averages over a typical year. Given Scotland’s high latitudes and long winters, the thermal demands (overwhelmingly space heating rather than hot water demands) potentially could be six times that of the developments electrical demands in winter months. Additionally, a higher energy demand could be expected prior to working day hours (around 4am) and immediately after normal working hours during weekdays (around 8pm).
Biomass conversion systems are forecast to have a high chance of successful operation at the site and able to deliver a continuous baseload electrical and thermal energy supply throughout energy demand fluctuations periods. A reliable locally sourced resource exists and includes; energy from Forfar waste supply (EfW), biogas from nearby sewage works and wood chips/pellets from government recognised nearby suppliers.
Medium size (<500kW), south west facing wind turbines placed on the sites periphery could supply all of the developments electrical requirements for six months of the year over the winter period, with electrical surplus being sold on as profit to the local grid.
With regards to solar; although nearby solar PV farms have been approved and built, their performance at this site is questionable in reality given the low percentage of clear sunny days – even though in this study the average parameters do indicate rooftop mounted solar PV cells could deliver the developments electrical demand for seven months of the year. Rooftop mounted solar water heating panels however would be more efficient at collecting solar energy at this latitude and provide hot water to homes directly for at least six months of the year over summer months to meet steady demand usage.
Two final integrated technology solutions were created; scenario one which identifies the best technical solution, and scenario two which follows a least planning problems and more socially acceptable solution. Both scenarios would require the housing developer to minimize space heating requirements in new homes via passive heating design and efficient home insulation, include electrical heating system options and insulate delivery hot water pipework to help overcome the high winter space heating demands being met by biomass combined heat and power units alone. Electrical and hot water heating demand could be met entirely from renewable technologies, with efficient hot water storage systems in built to meet morning demands. The assignment brief was not configured to take into account geothermal energy utilization via ground source heat pumps for example which should also be considered to meet space heating demands in any final solution.The use of natural gas or any fossil fuel resource was not considered in this forward thinking ‘green’ energy project.
Flexible technology integration would use grid connected electrical systems, district heating connection to business park users and be designed with possible expansion in mind.
The next step beyond this feasibility study would be to identify if the political and social demand exists for such a residential development to redevelop this unused brownfield site. If so, to then to approach property developers to assess their investment interest before commissioning a more detailed and collaborative study between a renewable technology consultant and the developer. A thorough economic analysis would be a key output of any feasibility study.
What this report does achieve is a positive renewable technology feasibility baseline to promote further discussion.
Renewables generation in Scotland
In March 2015, Scotland had 7.4 GW of installed renewable electricity generation, with an additional 8.9GW under construction or consented to. In 2014 around 49.8% of Scotland’s electricity generation came from renewables, close to the 50% 2015 target. The Scottish Government has set itself a target of 100% of gross annual electricity consumption to be from sourced from renewables by 2020. Wind power makes up almost 72% of the existing installed renewables capacity, with hydroelectric schemes making up the bulk of the remainder (22%). Solar PV and the utilisation of Biomass conversion exist in small quantities with wave and tidal energy yet to be commercially operational (Figure 1).
Scotland’s Renewable Heat Action Pan (2009) set out an 11% target (based upon forecast usage) for the renewable generation of non-electrical heat demand by 2020. In 2013 the calculation method changed to reference current, rather than forecast heat demand; however in 2012 using the old method – Scotland generated 4.1% of forecast 2020 non electrical head demand from renewables. Biomass generation accounted for 79% of this figure in 2012, and 83% in 2013 (Figure 2).
The UK Governments Department of Climate Change (DECC) has a Renewable energy planning database siting the technology and status of renewable energy projects in Scotland (Table 1). It can clearly be seen that each of the commercial technologies in this exercise have either fully penetrated the local energy market, or are rapidly growing in popularity – thus confirming each ones applicability to this housing project as a source of self-sufficient renewable energy generation.
Figure 1 – Renewable Electricity Capacity by technology, Scotland (Scottish Government 2014)
Figure 2 – Renewable Heat Capacity by technology, Scotland (Scottish Government 2013)
Table 1 – Pipeline Renewable Projects in Scotland by Technology and Status (Sept 2013)
Site and Resource estimation report structure
The various stages of developing a sites renewable energy potential are shown in Figure 3 below. This study focuses mainly upon the outer ring steps one and two, with passing reference to some of the additional later stage issues. The Project Assumptions listing is located in Appendix A and creates the boundary conditions for this report.
Figure 3 – Stages for developing evidence base for renewable energy potential (SQW Energy)
The report is structured as follows;
- Estimation of the developments likely peak energy requirements over a typical annual period, as well as considering seasonal and diurnal demand variations.
- Identification and description of a specific site on the Scottish mainland registered as a brownfield site for focus. A brownfield site would have considerable local and regional support for a ‘Green Energy’ re-development.
- Potential commercial energy generation technologies comparison, with each ones basic functioning explained. Important parameters such as efficiencies, capacity factors, reliability, technical lifetime, comparable technology capital/maintenance costs and key technical aspects of each is discussed.
- Resource assessment of the selected sites available energy on an annual basis.
- Recommended suitable commercial technology scenarios appropriate to the site with strategic reasoning.
- Technologies scale, deployment and integration within the development, including demand-supply mitigation strategies.
- Project conclusion and way forward.
- Project Assumptions and References in an Appendix
Estimation of the housing developments annual energy consumption demands
UK and Scottish Government statistics were used based upon the stated assumptions to determine an average annual electricity and heat usage (using gas as a proxy) by household and hence for the development as a whole. The average instantaneous per household power demand calculation used both UK DECC and Scottish Government data;
- Data sense check – DECC UK average domestic per household consumption (2014);
- Electricity = 4000 kWh pa
- Heat (temperature seasonality corrected) = 14250 kWh pa
- Total = 18250 kWh pa
- Study utilised data;
- Scottish Government mean household annual power consumption (2011-2013);
- Electricity = 4600 kWh pa
- Heat (no stipulation of seasonal correction) = 15100 kWh pa
- Total = 19700 kWh pa
- Scottish Government mean household annual power consumption (2011-2013);
- Instantaneous power consumption average per household per hour (365 days)
- Electricity = 4600kWh/8760hours=525We
- Heat (no stipulation of seasonal correction) = 15100kWh/8760hours=1723Wth
Both DECC and Scottish Government consumption results broadly agreed. Slightly higher Scottish household energy consumption can been attributed to increased heating and lighting needs for higher Northern latitudes and statistical averaging over different populations, i.e. the 2014 Scottish population was 5.4 million, versus the UK as a whole of 64.1 million people. For a new housing development of 500 semi-detached/detached homes, the annual energy usage was calculated to be;
- Total annual power demands needing to be met by self-sufficient renewables;
- Electricity; 500 homes x 4600 kWh pa per home = 2300 MWh(e) per annum
- Heating; 500 homes x 15100 kWh pa per home = 7550 MWh(th) per annum
- Average instantaneous power systems rating;
- Electricity; 500 homes x 525W per home = 262.5kWe
- Heating; 500 homes x 1723W per home = 861.5kWth
Seasonal variations in Energy consumption
Seasonality in Scotland has a large impact upon how this annual energy demand is spread across the year. Energy demand also varies widely during the day and week. Fully self-sufficient renewable energy generation would need to be sized to anticipate changing demand patterns to avoid peak usage power unavailability to the community.
Patterns of electricity demand/consumption in Scotland were studied by the University of Edinburgh in 2006 as part of a Scottish resource assessment project looking at historical data. Although out of date, the study still serves to demonstrate both the longer and shorter term variations in consumption patterns.
Scottish Power (SP) and Scottish Southern Electric (SSE) were two of the big electricity suppliers to their regions at the time. Since electricity could not (and still cannot) be stored in large volumes, the electrical power demand curves observed in Figures 4 and 5 below display short term consumption demands over a longer term three year period, and over a typical day within those three years respectively.
System maximum demands occur once per year, in 2003-2004 SP recorded theirs on the 20th December, whilst SSE recorded theirs on 28th January. Both utilities exhibit summer drops in demand as temperatures rise and the hours of daylight increase. Winter power demand peaks in January, with high demand three months either side. Figure 4 also shows this summer/winter demand spread, but also shows the demand variation for electricity and heat during a 24hr typical day; maximums occurring on weekdays, and minima occurring at weekends. Daily power demand jumps at awakening times (6-8 am), with another jump when people arrive home after work (4pm) for the evening; then dropping down again as people go to sleep (8pm onwards.)
From these graphs, a spread of between 35%-40% occurs in peak electrical energy demand/consumption over both longer and short term periods in Scotland;
- 40% spread between weekly maximum and minimum electrical demand.
- 35% spread between peak weekly winter versus summer electrical demand.
- 35% spread between daily summer weekend minima and winter weekday maxima.
Figure 4 – Annual electricity load profiles for SP/SSE utilities (weekly minima/maxima shown)
Figure 5 – Daily load profiles (SP–top)/ (SSE – lower) N.B.(Demand Curves; Max (weekday)/average winter /average summer /Min demand (weekend))
Just under half of the total final energy consumption in the UK is used for heating purposes according to the UK Energy Research Center (2014). From this national heat consumption average; 63% is used for space heating and 14% for water heating – mostly for domestic (57%), and to a lesser extent industrial (24%) and service (19%) usage. Gas burning provides the energy source for 80% of domestic heat use. It’s this fossil fuel burning that locally sourced renewable heat generation in this project development is looking to replace.
Hot water heating consumption variations were analysed by The Energy Saving trust in 2008. Typical cold water inlet to various hot water cylinder heating systems had distributed monitoring sensors placed around 120 UK homes. Home users then utilized the hot water for domestic bathroom, washing and kitchen purposes. Following a year’s data collection and data quality post processing, the following conclusions were determined within a 95% statistical confidence level;
- Mean consumption of 122 litres day.
- Occupancy numbers, rather than geographical region and boiler type were the key demand drivers.
- Mean hot water delivery temperatures were between 49-53oC (not 60oC as assumed).
- Water heating time average estimated to be 2.6 hours per day.
- Household heated water required generally between 8-10am and 6-11pm.
- Hot water consumption remained relatively even throughout the year (Figure 6).
Figure 6 – Annual pattern of UK volumetric hot water consumption (EnergyTrust)
Space heating demand however in the UK is subject to far larger seasonal variations, with a peak winter heat load being several times that of average heat load (Figure 7).
Figure 7 – Heat/Electricity demand variability, domestic/commercial (Imperial College 2010)
What this means in practice is that for the housing development to be designed to be self-sufficient throughout demand fluctuation periods – the power rating of the renewable energy generation system would need be over-sized. The development could export or sell excess electrical energy to the grid and heat energy to nearby industry and commerce if required. Figure 8 below illustrates this concept for a fictional single onshore wind farm (capacity factoring/availability included).
Figure 8 – Pictorial representation of a fictitious 40MW onshore wind farm power (TheGreenAge)
Increasing the developments household average electrical demand by +20% was deemed to better be able to meet the developments peak instantaneous supply throughout the year, i.e.;
- Electricity; 500 homes x 525We per home = 262.5kWe +20% = 315kWe
For heating demands a 20% technology over-sizing only forms part of the final solution to meeting space heating demands given the very wide seasonal variation, with water heating being less problematic. Pipe transfer heating losses between household and remote heat generation can exacerbate space heating issues. So an emphasis on localized renewable heat generation technologies and redundancy in the final strategy solution was considered. Some integration section suggestions on electrical space heating and passive heating building design form additional heat supply mitigation methods. Hence the developments finalized heat target supply was;
- Heating; 500 homes x 1723Wth per home = 861.5kWth +20% = 1033kWth
Identification of suitable Scottish brownfield housing development site
Utilizing the latest Scottish Vacant & Derelict Land (SVDL) register; a filtering of suitable brownfield sites that could viably support a 500 home development with self-sufficient energy generation on-site was carried out. The following criteria for site selection were used;
- Previous site usage
- Agricultural, commerce, educational to minimize site clean-up costs.
- Type of site
- Urban vacant (no buildings) preferred over Derelict land due to reclamation costs.
- Development Potential
- Developable ideally with shorter term planning consents.
- Amount of time vacant
- Land disused for longer periods was likely to require greater clean-up. Hence, only post 2000 brownfield sites used.
- Site Size
- Assuming a low, medium and high housing development density as per a typical local authority guideline, i.e. 15,30 and 45 units per ha (West Lothian Local Planning, 2009).
- Land Area
- A 500 home development would need between; 11.1-16.7-33.3 (ha) in space.
- 3 hectares additional space would be required for renewable technologies;
- Scottish Planning Policy dictates a minimum 2km large scale wind turbine site separation from the edge of any residential sites. On site smaller medium scale turbines (<500kW) are more practically suited to this development but would need locating away from each turbines wake influence and housing, hence an additional 2ha space was included.
- Solar PV farms would need a large off-site land area, unless rooftop mounted systems were used. Along with roof mounted Solar Domestic Water Heating; no additional on-site space was required since roof top systems were the optimum Solar solution.
- Commercially available medium (>200kW) biomass CHP systems would require less than 1 hectare additional space (incl. fuel storage/heat storage facilities).
- Hence, the Total brownfield site area estimation required was between 14-21-36 hectares depending upon the chosen housing density.
- Nearby Settlements
- A nearby settlement of greater than 2000 people would provide access to existing infrastructure, excess energy demand and a source of waste for biomass.
- Site ownership
- Ideally a single ‘known’ ownership site to simplify purchasing/leasing.
Many other more detailed issues of site selection would require further investigation by the Developer, however these go beyond the scope of this study.
The result of this filtering produced two possible sites; Denny High School, Glasgow (12.9ha) and Orchard Bank Business Park, Forfar (21.4ha) – the latter being the selected given its greater area (flexibility to adjust housing density and renewable configurations). Additional Forfar site favourability is that renewable energy projects had already gained planning consent in Angus County(2014), i.e. nine solar PV farms (8x<5MW) and two approved <5MW onshore wind farms at Govals Farm (six turbines) near Forfar and Frawney (seven others had initial planning withdrawn or refused on first application, with public opposition being the main challenge).
Orchard Bank Business Park (Figure 9) is situated in Forfar – the administrative capital of Angus and located 21km north of Dundee with a population of 14000 people (2014). There are no operational rail links in the town, the nearest operational airport is located in Dundee. Drumshade recreational airfield is located 6.5km to the West of the site. There is no active airfield or MOD facility nearby. Since June 2014, Angus Council has in place a comprehensive recycling service. Roughly 2/3 of non-recyclables are sent either to landfill at Lochhead or for incineration Dundee. Surrounding Forfar are rolling hills used for fertile agriculture, with 87% of Angus County land used for agriculture and forestry. National Forest Estates exist 14km to the East of Forfar (Dubton), and 24km NW around the Backwater Reservoir. The nearest Scottish Nature Heritage site is 5km to the NNW at Loch Kinnordy.
The OBBP site itself (56.6ON, 2.90 W latitude and longitude) lies opposite the Forfar Loch Country Park on the western edge of the town in open/flat area away from existing housing on a green, vacant plot with no buildings on it. Its Adjacent to main A90 road running between Dundee and Aberdeen. Nearby the OBBP site there is sewage treatment works, a timber product yard, council offices and a community health center amongst other users.
Figure 9 –Orchard Bank Business Park vacant Brownfield site location, Forfar, Scotland
Potential commercial renewable energy generation technologies
The four applicable technologies chosen and explained further in this section were;
- Medium Scale Biomass CHP
- Wind Turbines
- Solar Photo Voltaics
- Solar domestic water heating
Medium scale (>500kW) Community Biomass Combined Heat and Power Plant (CHP)
There are many site specific configurations of CHP power plants, however they all have at the basic level a similar mode of operation (Table 2). The plant primarily generates electricity via a prime mover (gas, steam turbine or ICE engine driving an alternator) using a fuel combustion process. High (150-2000C) to Low (<1000C) quality heat is recovered from boilers by heat exchangers from the process as a by-product and exported via pipelines making the facility highly efficient.
Most appropriate Development technology focus;
Historically, the majority of Medium scale Biomass Community CHP plants are steam turbine systems for generating high heat to electrical power ratios and low grade heat/hot water for household usage. However, small to medium scale gasifiers with gas compression engine CHP system technologies were also considered as a recently commercial emergent technology. It’s uncommon to find cost-effective CHP applications for gas turbines below 5MW, with gas micro-turbines not yet commercially demonstrated. The flexibility of fuel supply, fuel storage and usage influences the fuel costs and is a key technology consideration.
Biomass Fuels are preferred, either in solid or gaseous form – replacing fossil fuels as a carbon neutral fuel energy source. Liquid biofuels produced from energy crops are too expensive a feedstock for CHP. The ‘UK Biomass Strategy’ provides a legally binding definition for the wide range of biomass fuels;
- Plant growth (woody or grassy), from forestry, farming and energy crops.
- Animals (manure/slurry), from agriculture and sewage.
- Human Activity, by-product gas from landfill/sewage sites, and solid manufacturing, food processing and municipal wastes.
The availability of sufficient biomass resource depends upon local population sizes and local agricultural activities. They will however be highly predictable, and if diversified highly reliable as a fuel resource.
Energy Conversion techniques;
- Direct combustion of solid biomass – a burning process with fuel and air/oxygen.
- Anaerobic Digestion of wet wastes to produce biogas (Accelerated decomposition of biomass feedstock in the absence of oxygen using AD digesters).
- Advanced thermochemical processes, e.g. Gasification to produce syngas. Solid biomass fuels can be converted into syngas, with composition dependent upon the feedstock. Gasification is the thermal degradation of carbonaceous feedstock by partial oxidation.
Energy Conversion Science;
Steam turbines utilize the ‘Organic Rankine Cycle’ (thermodynamic cycle of a heat engine). Direct combustion processes release energy by burning a wide range of solid fuels with air to produce high temperature gases in a boiler, using either moving Grates or Fluidized Beds configurations (varying methods of fuel/air mixing and fuel movement in the chamber). Heat generated is then applied externally to a closed loop working fluid; usually water (or an organic fluid like Toluene) to produce steam for a steam turbine to generate electricity. Discharged steam then provides a secondary source of recovered heat energy (Figure 10).
Gasification with gas spark/compression ignition engine technologies work in the same way as pure liquid fuel in normal ICE engines. Gasification creates intermediate products like methane, hydrogen and carbon monoxide – collectively called syngas. Gasifier designs vary depending upon method in which oxygen carrier (air, steam, pure oxygen) is introduced, i.e. updraft, downdraft or cross-draft. Burning syngas in a gas ICE rather than biomass directly in a steam boiler/turbine achieves greater electrical efficiencies (Figure 10).
Table 2 – Tabulated range of Biomass combustible CHP Plant working characteristics (UK Gov.uk)
Figure 10 – L(Steam boiler/generator), M (Spark ICE), R(Compression ICE) Systems (UK Gov.uk)
Medium Scale (>100kW-500kW) Wind turbines
Atmospheric pressure differentials arising from the imbalance of global heating drives pressure balancing winds that can be harnessed by wind turbine blades to drive a generator to produce electricity.
Most appropriate Development technology focus;
Given the current popular opposition to large scale (>500kW) wind turbines; the technology focus for this development will be smaller, commercially established medium size horizontal axis turbines between 250kW-500kW rating size to provide on-site electricity. Figure 11 displays a pictorial representation of turbine physical sizes. Given a medium housing density, peripheral land on the OBBP site could optimally locate a number of medium scale turbines, inclusive of near road and housing topple safety and noise buffer distances. The recommendation of numerous, individual household small scale household turbines (<100kW) was avoided given the collective, rather than isolated nature of the housing development with noise and aesthetics in mind.
Figure 11 – Small scale relative wind turbine sizes (RenewableUK.com)
Wind speeds increase over wide, empty and featureless land areas and with height from the lands surface due to reduced wind-land friction losses. Wind concentration densities at a site over different temporal periods determine electrical generation characteristics and can be highly unpredictable in nature. Prevailing wind systems, topographic highs and coastal areas present optimal, unidirectional wind sites for turbines.
Energy Conversion techniques;
Power generated by the turbine is directly proportional to the swept area of the blades (swept area being proportional to square of blade radius), air density (colder the air, greater the density and power generation) and most importantly to the cube of the wind speed. In addition, the turbine design will possess a power extraction coefficient, and a gearbox and generation efficiency factor controlling final power delivery.
The most common commercial designs are horizontally axis wind turbines (HAWT). Typically consisting of three blades shaped like aircraft wings – turbine rotation is created by the interaction with oncoming wind (upwind design) and the angle of the blades leading edge to it. The blades can be either fixed or variable pitch in design, the latter helping to improve the turbines conversion efficiency but being more expensive. Wind vanes and anemometers provide data for the turbines yaw system to keep the turbine pointing into oncoming winds. Within a certain wind speed range, lift is induced and the blade rotors spin. This rotational movement is translated via a rotor draft shaft and a gearbox (unless a direct drive system) to a fixed magnet electrical generator all sited within a protective housing called a nacelle (Figure 12). Generated power can then be transferred via a ground based electrical inverter to the electrical grid system, or domestic electrical circuits directly.
Figure 12 – Components of a typical commercial wind turbine (Heriot Watt University)
Energy Conversion Science;
Key to the kinetic wind energy conversion is the aerodynamics of the aerofoil blades, which act in the same manner as an aircraft’s wings (Figure 13). Lifting forces acting perpendicular to the oncoming wind are created by the upper convex aerofoils surface being under-pressured relative to the concave shaped aerofoils underside. Drag forces counteract the lifting forces parallel to the oncoming wind direction. The resultant force from these two components acts to rotate the blade relative to its pivotal hub, creating rotational mechanical kinetic energy via the rotors drive shaft.
The secondary conversion from rotating mechanical to electrical energy takes place in the electrical generator working on the electromagnetic induction principle. Stationary magnets (stator) create a powerful magnetic field that is cut through by a rotating magnet (rotor) attached to the driveshaft. This magnetic flux generates an electrical DC current which can be synchronised and inverted to a grid friendly AC current.
Figure 13–Forces acting on aerofoil blade cross section of a wind turbine (Heriot Watt University)
Solar Photovoltaic Panels
Photovoltaic cells harness the energy properties of photons contained within the suns solar insolation using the photoelectric/voltaic effect. Multiple cells are configured as individual panels and can be free standing or roof mounted at an optimum interception tilt angle to generate a flowing electrical charge. Thin Film PV technology was ignored considering its current ‘niche’ development status.
Most appropriate Development technology focus;
Given the large and separate land area that would be required for electricity generation at this developments power demand scale at this latitudes insolation levels; more convenient, individual household rooftop solar systems were considered more appropriate. As with wind technology, photovoltaic devices area proven technology, increasingly affordable and commercially available.
Incoming solar radiation reaching the Earth’s surface consists of packets of photons containing light energy. The Earth atmosphere reflects (ca. 34%) and further filters the remaining incoming insolation according to wavelength, with the direct (clear atmospheric days) and diffuse (water vapour, gas and cloud scattered) light available for solar photovoltaics comprising largely of visible, infra-red and UV light wavelengths. Around 44% (of 66% not reflected) being usable insolation for photo-electricity. The annual solar intensity at a site defines renewable technologies power generation levels and is greater at lower latitudes and during summer months due to the longer length of day light hours. Other factors influencing the daily solar intensity arriving at a PV device are the atmospheric state, incoming radiation angle and sun-PV cell azimuth orientations. Proportions of direct to diffuse arriving light, air ambient temperatures and a devices energy transmission and absorption characteristic determine energy conversion efficiencies.
Energy Conversion techniques;
Using overlaid and doped (chemically treated) thin semi-conductor layers – usually silicon – within each cell a p-n junction is created. Sunlight’s photons falling on the p-n junction transfer energy by promoting some electrons to a higher energy level which are then free to conduct electricity (Figure 14). Metallic conductor elements then conduct the DC current load away from the panel into a buildings integrated circuitry (Figure 15). The last decade has witnessed rapidly improving manufacturing methods with both energy conversion efficiency and cells cost effectiveness benefits.
Figure 14 – p-n junction photoelectric effect & Solar PV cell construction (Heriot Watt University)
Figure 15 – Building integrated solar PV system (Heriot Watt University)
Energy Conversion Science;
The photovoltaic effect (PE) dislodges outer shell electrons by light photons (energy quantized packets) whose frequency (inverse of wavelength) exceeds a semi-conductors specific threshold. Photon energy threshold is important because it means even at low solar intensity levels the PE effect can occur. Semi-conductor metals (mono-crystalline/poly-crystalline silicone) react best to the PE effect, requiring only small energies transfers from UV and Visible light mainly to imbalance charges that are naturally balanced via by current flow. Future stacked p-n junction semi-conductor cells under research will be able to harness a wider range of insolation wavelengths across the IR-Visible-UV band.
Rooftop Domestic Solar Water Heating
Active solar heating involves the conversion of direct and diffuse solar radiation to heat by the thermal interaction of an absorber via photo-thermal conversion. Flat Plate or Evacuated Tube devices are common and commercially available in the UK to provide low grade domestic heat supply of around 50-60oC via integrated water storage heating circuits.
Most appropriate Development technology focus;
Whilst Flat Plate systems are more affordable and would provide low grade heat for large water tanks – they are prone to conduction losses and efficiency reductions as the difference between collector working fluid and ambient air temperatures increase. Evacuated Tube systems however can reach higher temperatures with smaller efficiency losses over a wider range of temperature differentials because the working fluid circulates in a vacuum suiting smaller domestic water tanks demands. In the UK climate; even in the coldest winter months Evacuated Tube systems can generate heat effectively from both direct and diffuse light (Table 3 & Figure 16), albeit at a more expensive cost (ca. 25% more than flat plate systems).
Table 3 – Domestic solar water heating system characteristics (TheGreenAge website)
Figure 16 –Evacuated Tube solar domestic water heater/efficiency plot (Heriot Watt University)
See earlier Solar PV section.
Energy Conversion techniques;
Evacuated tube heat collectors consist of a ‘tube-in-tube’ system of an absorber element situated in a pressure proof glass tube vacuum. A number of these heat absorber tubes are connected to a support structure/manifold to transfer the energy to point of use using a transfer fluid. The vacuum insulates the absorber from convection and conduction losses improving capture efficiencies. The heat absorbing working fluid vaporises in the evacuated tube, rises up and exchanges its heat to the manifold transfer fluid before condensing and dropping back down the tube in a cyclic manner. The tubes are collectively gathered in a panel and roof mounted in a south facing optimum tilt angle to maximise solar irradiation interception. These rooftops systems are integrated into a buildings hot water storage tank. A controller module regulates using system temperatures the circuits pump which controls fluid circulation. (Figure 17).
Energy Conversion Science;
Incoming radiation heats up to vaporisation temperatures a low boiling point working fluid like non-toxic glycol which ascends as a gas up the tube. The gases high temperature heat energy is transferred to a home’s hot water heating tank via two heat exchange processes that take advantage of the latent heat of vaporisation. Once the energy has been exchanged – cooling and phase change to a condensed liquid returns the working fluid to its closed system starting point whilst obeying the laws thermodynamics (Figure 17).
Figure 17–Hot water system integrated SDWH and phase change diagram (TheGreenAge website)
A comparison of each technologies key parameters been summarized in Table 4 below.
Table 4 – Considered renewable generation technologies comparable key parameters (Laurie 2016)
Assessment of the residential developments available energy resource and annual variations
When considering renewable energy and the developments energy self-sustainability, an additional set of site selection criteria required consideration;
- EfW/Biomass local fuel types, supply (fuel transport distances being particular important to project economics) and storage.
- Local Wind regimes; i.e. mean wind speeds, wind speed distribution, temporal variations etc.…
- Local Solar insolation; i.e. solar intensity and seasonal variations, latitude etc.……
- Electrical integration infrastructure; i.e. ease/cost of grid connection.
- Noise and project aesthetics; Community quality of life and social popularity.
- Environmental impacts;
- Natura2000/Ramsar/SSSI sites
- SPA/SAC/SCI/Nature Reserves areas
- Architectural/Historic sites
- Airfields/Airports interference.
- Site Access for construction and maintenance work.
The resource assessment focuses on the first three in the list above to estimate the available resource and hence each technologies site suitability. Estimations were performed in four steps;
- Reaffirm the developments peak and annual power demands (Inclusive of seasonal adjustment).
- Taking a conservative approach; determine each technologies approximate power sizing needed to achieve the OBBP developments peak energy demand on a stand-alone basis.
- Identify a suitable commercial technology and key energy metric for each resource.
- Assess the sites ability to meet the key energy metric to meet peak energy demands. Identification of energy seasonality’s and any site constraints effecting deployment.
Individual technology power ratings are for resource evaluation guidance only. The challenge of balancing between over-sizing and under sizing individual generating technologies to meeting peak seasonal demands will be answered later in this report by combining technologies into a final integrated generation strategy, with mitigation measures in respect to any surplus-deficit power balances also mentioned. Assessing power delivery on a technology stand-alone basis initially should ensure a more reliable and complete final generation system because of redundancy and cost optimization flexibility – for example; in reality should one installed technology perform poorer than expected or be more expensive to install than forecast, then the other technologies can be scaled to make up power demand shortfalls.
All technologies availability and reliability have assumed to be 100% for this resource assessment, with the impact of Capacity Factors implicit in their individual resource assessments. The final strategy over-sizing and technology redundancy will mitigate against these important factors. The technology sizing targets used were;
1 – Target instantaneous power demand;
- 500 homes Peak Electricity = 315kWe (inclusive of 20% seasonal peak)
- 500 homes Peak Heating = 1033kWth (inclusive of 20% seasonal mitigation)
Associated achievable annual power demand;
- Electricity for 500 homes x 4600 kWh pa = 2300 MWh(e) per annum +20%
- Heating for 500 homes x 15100 kWh pa = 7550 MWh(th) per annum +20%
- (Heating and Electricity power surpluses will be sold to the grid/community)
2 – System scale preliminary selection criteria;
- (A/B)Biomass CHP Peak electrical or heating demand (whichever is lowest).
- (C)Wind turbines Peak electrical demand only.
- (D)Solar PV Peak electrical demand only.
- (E)Solar Heating Peak heating demand only.
(A) Steam turbine Biomass CHP Plant;
- Target Peak electricity power supply =>0.35MWe
- (Associated heating power supply =>1.05MWth (1:3 power to heat ratio used)
(B) Gasifier ICE gas engine Biomass CHP Plant;
- Target Peak electricity power supply =>0.75MWe
- (Associated heating power supply =>1.05MWth (1:1.3 power to heat ratio used)
(C) Wind Turbine system;
- Target Peak electricity power supply =>0.35MWe
- (Associated 2300MWhe annually)
(D) Solar PV system;
- Target Peak electricity power supply =>0.35MWe
(E) Solar Water Heating system
- Target Peak heating power supply =>1.05MWth
3 – Identified commercial technologies and Key energy metrics;
(A/B) Biomass CHP systems – Identified Technology;
Steam turbines and Gasification ICE Gas engines can utilise a range of biomass fuels. An assessment of each fuel source’s volume and suitability to the Development is heavily influenced by its relative cost and vicinity to the Forfar brownsite location. Energy intensive manufactured biomass fuels like liquid bioethanol/biodiesel were considered too expensive to utilise. Dry fuels (plant biomass/treated biogas) were also preferred to wet fuels (i.e. slurry/manure) since they have lower moisture content and hence higher energy transfer potential. Each Biomass/Waste CHP plant is site and fuel source specific in design – a wide range of examples are already heavily utilised within the UK. Example system commercially available;
- Waste to energy plant;
- Baldovie Waste to Energy CHP plant (DERL)
- Biomass CHP plan;
- RiversideDene ,750kW biomass boiler at Cruddas Park House (Vitai Energi)
- AD ‘biogas’ CHP plant;
- Cumbernauld, Glasgow (Energen Biogas)
- Biomass gasifier plus ICE gas engine;
- ArborElectroGen 200/400/800 modular units24.(Arbor Heat & Powers Inc.)
- MSW/Biomass fuel weights available locally (dry tonnes)
- Biogas Volumes available locally (Mm3)
- It should be noted that a wide value variation exists in literature for each fuels energy content. Different calculation methods and a range of appropriate Calorific Values were used to determine a sensible range/average on a single fuel use only basis to meet entire developments demand.
- Generation efficiencies and plant availabilities (%)
(C)Wind turbine system – Identified Technology;
Medium scale commercial wind turbines are predicted to be able meet the OBBP Developments peak electrical demand. The turbines performance characteristics versus the key metrics can be integrated with the sites assessed resource to confirm the technologies viability. Example system commercially available;
The Endurance Wind Power inc.X35 was selected based upon its rating, best in class record and commercially availability in UK;
- Rating: 225kW.
- Rotor diameter: 35m and Swept Area: 962m2
- Average design wind speed: 7.5m/s, Cut in speed: >5m/s.
- Fixed pitch-stall control blade design, Cut out speed: 20m/s.
- Rated power is not achieved until >12m/s wind speed.
- Free standing monopole tower: hub height 30.5m or 40.2m.
- Fixed position: no yaw mechanism.
- Asynchronous induction 3-phase generator: 50Hz AC grid connected.
The turbines published performance and power generation used in the resource assessment calculations are shown in Figure 18. A standard air density and no turbulence were assumed. Energy conversion efficiencies are implicit to the wind speed vs. power curves provided by the vendor, as is power per turbine swept area calculation.
Figure 18 – Endurance X3; 225kW performance characteristic’s (Endurance Wind Power Inc.)
- Wind speed (average/annual wind speed probability distribution in m/s).
- Wind shear with height (at three heights in m/s).
- Wind direction (average annual, 16 point compass).
- Time spent annually within wind turbines operational envelope.
(D/E)Solar systems – Identified Technology;
Solar Photovoltaic Systems (SVS) and Solar Domestic Water Heating (SDWH) systems are commercially available and employed in the UK by a wide range of suppliers. Example rooftop systems;
SVS system – Suntech’s Hypro STP290S-2024 SVS was chosen as an example technology due to its high efficiencies at low solar intensities;
- Monocrystalline silicon cells, with 60 cells per panel
- Max panel power rating @25oC: 290W (@NOCT (45oC: 215kW)
- Panel physical area: 1.62m2
- 25 year performance decrease: 18.8%
- Module overall efficiency: 17.8% @25oC (of total irradiance)
- Normal operating cell temperature (NOCT): 45oC
- Temperature coefficient of Pmax: -0.40%/oC
SDWH system – Navitron SFB47 range of Evacuated Tube systems were chosen due to their high conversion efficiencies across a wide range of ambient temperatures. They are also the preferred solar water heating technology of the Scottish Government;
- Evacuated tube solar water heater, 30 tubes per panel (SFB4730)
- Panel physical area: 3.54m2
- Tube aperture area: 1.88m2
- Max power rating: 1126W (@1000W/m2 & optimum incidence angle)
- Module overall efficiency: 60% (of usable irradiance)
- Transmissivity: 0.91 and Absorptivity: 0.92
- Heat transfer coefficient: 2.101(W/m2K)
- Domestic water heating typical temperature target: 50oC
Energy conversion efficiencies are implicit to the solar irradiance vs. power curves provided by the vendor, as is the power per panel area calculation.
- Roof top area for siting panels (m2)
- Aperture area per panel (m2)
- Optimally inclined surface solar energy (W/m2/d)
- Solar intensities annual/daily variations
- Length of daylight (hours)
- Ambient Temperatures (oC)
4 – Forfar site Resource Assessments with seasonal variations;
Biomass Resource Assessment;
Table 5 below itemizes the various biomass fuel masses and volumes necessary to achieve peak power targets utilising two different methods. Note these are the masses/volumes required to deliver continuous electricity and heat to the development for the entire year as a baseload generator.
Table 5 – Fuel mass/volume required for different types of fuel to achieve the power targets (Laurie 2016)
The following Forfar sites could each supply the OBBP Developments biomass to power requirements on a stand-alone basis. The presents a nearby and diversified resource feed to the CHP;
- Angus County Council recycling programme/Forfar waste recycling centre (EfW)
Waste prevention and re-use according to ‘Waste Framework Directive’ is prioritised. Energy recovery being a secondary MSW option. Forfar waste recycling centre lies within 1km of the OBBP site;
- Currently 2/3 of the annual domestic waste in the Angus area (36000t18) is sent to Baldovie ‘Waste to Energy’ plant (34MW) in Dundee (DERL).
- The required annual MSW resource range exists in the Angus area for both CHP technologies/calculation methods, i.e. 2130-7500dtonnes.
- No data is available publicly on a per day basis.
Since Angus council is required to pay a per tonne ‘gate-fee’ to DERL this could come as a free and continuous resource.
- Forfar sewage works
Forfar sewage works is well positioned nearby to pipe anaerobically digested biogas directly to the development. At present, no such biogas recycling facility exists. The nearest biogas generators are >20km away at Peacehill and Keithick Farm. Lochhead landfill site is too far away (70km) to utilise its biogas. With a population of 14000 people (+2000 from the Development) nearby. An estimation of biogas yield from sewage for a year from Forfar alone;
- Bachmann (IEA, 2015)16 estimates 18-26L of sewage per day, per person.
- Using low end and water density;
- 18L = 0.018m3/d x 16000 people = 288m3/d of sewage supplied daily.
- 288m3/d = mid-range for Danish centralized biogas plants (Table 6).
- This amount of sewage yields: 7100–14800m3/d of biogas (Table 6); with a mid-range=10950m3/d of biogas.
- Annually, continuous biogas generation from Forfar sewage works;
- Biogas yields = 10950m3/d x 365 days= 4Mm3 per year
This number is greater than the calculated biogas fuel requirement for both steam and gas engine CHP plants annually (1.2-2.05Mm3 pa), even when using a range of methane content energy densities. This resource presents an untapped, nearby free and continuous energy source.
Table 6 -Parameters of centralized biogas plants in Denmark (Dohanyos et al., 2000).
- Bill Watson Angus Biofuels (Forfar)
Biofuel supplier to the Scottish ‘Biomass Energy Supply Framework’, located just outside Forfar town. Sells sustainable wood fuel sources – logs, chips and pellets, natural dried chips with low moisture content <25% (lower fuel cost than pellets) and less bulky pellets for smaller boilers and dry chips for larger installations.
The facilities daily capacity is 100 dry tonnes woodchip per day maximum output15. Depending upon technology utilised, the per day requirement for the development is between 6-9 dry tonnes per day (365 day operation). The business is established and possesses modified delivery vehicles and services, thus minimising onsite storage facilities. Unlike MSW and Biogas, the economic implications of purchasing this processed fuel would need careful consideration at the next stage of the development planning.
Biomass Seasonal resource variation;
All of the Biomass generation sources are assumed to deliver continuously throughout the year. Three local sources have been identified and their Resource capacity assessed.
Biomass Site Constraints;
Aesthetic and noise impact of CHP operations on the developments desirability. Good road access required for fuel transportation. Some fuel storage space considerations. Cooling stack prevailing wind considerations.
Wind Resource Assessment;
A number of databases were utilised for the wind resource assessment to gather a range of appropriate calculation input values;
- NOABL UK (cross checked with DECC wind speed database)
- NOABL uses 1km2 grid squares and assimilates METAR weather stations located predominantly at airfields (reported hourly).
- NASA wind maps
- NASA averages data over ten years on 1o resolution grid.
- Dundee airport weather station
- Dundee airport collected METAR station averages over ten years.
Database findings require calibration to the sites own local and prolonged wind measurements at a later project stage. Together the databases give a good cross section indication of the sites key wind characteristics and seasonal variations (Figures 19-22).
Figure 19 – Average Wind speed 10m above ground (NOABL renSMART) : DD8 1AX
Figure 20 – NASA Power Wind Map at 10m (UK Postcode: DD8 1AX)
Figure 21 – Wind direction distribution at ground level (2000-2010 – Dundee/Riverside airport)
Figure 22 –Annual average Wind speeds at ground level (2000-2010 – Dundee/Riverside Airport)
Site Wind resource characteristics;
- Average annual wind speed variation; 5-8.6m/s at 10m height.
- NOABL data at finer resolution more trustworthy then NASA data, hence received more emphasis in this assessment.
- NOABL Wind shear effects – wind speed with height (Average annual wind speed);
- At 25m hub height; 6m/s (i.e. 17% higher than 10m)
- At 45m hub height; 6.7m/s (i.e. 24% higher than 10m)
- Dundee Airport Wind direction highly unidirectional; SW-WSW-W (70%)
- Dundee Airport database approximate time the wind speed is within turbines operational range;
- 55% at ground level.
- 68% at 45m hub height.
Wind Technology characteristics integration (Endurance X35 Turbine);
Table 7 and 8 below summarises the OBBP sites forecast power generation. There is sufficient wind resource at the site to support the successful deployment of five 225kW wind turbines to meet the developments entire peak and annual electricity demands for six months out of the year. The number actually deployed will depend upon site restrictions.
Table 7–X35 turbine Annual Power Generation using Vendor curves (Laurie 2016 (Standard air density/no turbulence/Dundee airport annual wind distribution @45m hub height)
Table 8 – X35 turbine OBBP site Power calculations (Peak and Annual power generation, Laurie 2016)
Wind Seasonal resource variation;
Both databases illustrate lower wind speeds averages are experienced during the summer, with highs in the winter. The potential monthly power generation profile using 5 x 225kW turbines is shown in Figure 23;
- May-August @45m hub height – average monthly wind speed does not exceed the X35 turbines ‘cut-in’ speed (30% of year). This correlates well with the Dundee Airport database giving the NOABL data greater validity.
- Average 6.7m/s wind speed @45m may not be met/exceeded between April-September, i.e. 43% of the year.
The Vendor’s turbine power curve charts were conducted at a standard air density (1.225kg/m3 at 15oC). The METOffice data in Table 9 (later Solar Resource section) has a colder local OBBP site average temperature range between 3.5-13oC across the year, this will lead to an improved turbine performance due to denser air.
Figure 23 – 5 x turbines monthly peak electrical power (@45m/Av WS/Direction adjusted)
Wind Site constraints;
Topple safety distances and noise impact considerations – namely the nearby A90 road, new homes, council offices, health centre and businesses. Turbines would need to be staggered and offset from each other’s turbulence wakes. Loch Forfar to the North and the OBBP boundary constrain optimum siting.
Solar Resource Assessment (Rooftop);
Solar Photovoltaics (SPV) and Solar Heating Systems (SHS) utilised the following databases. As per the housing development assumptions; dwellings are assumed to be semi-detached and detached. The ‘Energy Saving Trust’ assumes these standard dwelling type and size for its electricity and heating calculations, i.e. roof top areas and water heating cylinder tank sizes. Site Resource sources and assumptions;
- PVGIS solar map (Figure 24)
- PVGIS uses 1km2 grid squares and assimilated METAR weather stations located predominantly at airfields.
- NASA solar maps (Figure 25)
- NASA average data over ten years on a 1o resolution grid.
- MET office historical weather data for Scotland (Table 9)
- Data averages over entire Scottish region.
Database findings require calibration to the sites own local solar intensity measurements at a later project stage.
Figure 24 – Average Solar Energy Intensity (PVGIS renSMART); UK Postcode DD8 1AX
Figure 25 – Average Solar Energy Intensity (NASA); UK Postcode DD8 1AX
Table 9 – PVGIS Solar database and MET Office data for Scotland and OBBP site (Laurie 2016)
Site solar resource characteristics;
- Solar Intensity;
- Average annual = 2.6-2.8kWh/m2/day
- Annual range = 0.27-5.17kWh/m2/day
- Average daylight hours = 12 hours
- Average Annual irradiance during daylight hours = 197W/m2/d (200W/m2/d)(Which occurs around March and September)
- Historical Scottish weather average conditions
- % direct sunshine hours (pm) daylight hours = 25.5%
- Average mean monthly temperatures = 7.8oC
Solar Technology characteristics integration SPV (Suntech STP290S);
Utilising the vendors per panel power curves (Figure 26) and local average conditions, Table 10 summarises the OBBP sites power generation potential from Solar PV.
Figure 26 – Suntech’s Solar PV STP290S Power performance curve vs. solar intensities
Table 10 – Suntech’s Solar PV STP290S OBBP site power calculation table (Laurie 2016)
The solar resource investigation found sufficient roof space for a maximum 12-20 solar PV panels per semi-detached-detached home respectively. Using the vendors Solar PV 200W/m2 performance power curve was utilised as closest to Scottish conditions for calculations and configuring at least 12 panels per home, Solar PV could deliver each homes instantaneous peak electrical power demand during daylight hours using the average solar intensity (@200W/m2).
With local ambient temperatures averaging closer to 8oC, the colder operating temperatures will ensure PV panel efficiencies are maintained. The 18.8% performance efficiency reduction over 25 years could be mitigated by placing an additional panel on each roof (the 12th panel).A major concern with the calculations however is the sites low average amount (25.5%) of unobstructed direct sunny sky during daylight hours. Solar PV cells work most efficiently with direct sunlight, with cells 40% as effective in heavy cloud as direct sunlight (TheEcoExperts.co.uk).
Solar Technology characteristics integration SDWH (Navitron SFB4730);
Utilising the vendors per panel power curves and local average conditions, Table 11 summarises the OBBP sites power generation potential from SDWH. The solar resource investigation found that by utilising all available roof space, SDWH panels can supply 26% (semi-detached/6 panels) and 44% (detached/10 panels) of the instantaneous peak thermal power to homes (200W/m2/d site average solar irradiance). Over an average 12 hours of daylight; these number of panels are capable of warming heater storage tanks to 50oC for domestic space heating and hot water for both house types (8oC average ambient air/200W/m2/d irradiance).
Table 11 – Navitron SFB4730 ET OBBP site power calculation table (Laurie 2016)
Solar Seasonal variation;
Peak heating and electrical demand over winter correlates with the lowest solar irradiance intensities and shortest daylight lengths. Both solar generation systems suffer from a peak generation drop during winter months (Figure 27 & 28). October through to March power generation falls below required targets.
Figure 27 – Possible monthly peak electrical power generation profile from Solar PV panels (Laurie 2016)
Figure 28 – Possible monthly peak thermal power generation profile from SDWH Panels (Laurie 2016)
Solar Site constraints;
None apart from homes design needing to have a favourable south facing rooftop orientation and inclination at the OBBP sites.
Recommended technology scenarios
First and second choice scenario combinations were selected to provide a feasibility stage project flexibility – an optimum generation and cost strategy first, and then a ‘least planning and social acceptance problem’ choice second. Both solutions would meet the developments forecast electricity and heating demands throughout the year with system redundancy for both electrical and thermal power generation. All technology sizings were based upon the OBBP sites annual average solar irradiance, sunshine daylight hours, wind regimes, temperatures and biomass supply possibilities.
Figure 29 shows a predicted 2020 technology cost from a quantitative analysis produced for DECC in 2009 – these costs forecasts help to economically implicitly guide the final combination choices from a secondary cost effectiveness perspective.
Solar Photovoltaic electricity generation was not utilised in either scenario because of the OBBP sites low and unpredictable direct sunshine hours per day risk required for effective Solar PV electrical conversion. In addition, the need to have on-site heat generating redundancy prioritized roof space for SDWH panels.
Figure 29–Projected (2020) technology costs of sub 5MW electricity supply (DECC, 2009) (Technology cost based upon total fixed cost component, a scalable kW marginal component and a maintenance component with a learning rate element, fixed exchange/interest rates before VAT).
- Scenario 1 (Optimum cost AND technology strategy);
- Baseload: (A) EfW/Biomass Steam CHP
- Opportunistic load: (C) Wind turbines + (E) Rooftop SDWH
- Scenario 2 (Least planning AND social acceptance problems strategy);
- Baseload: (B) modular Biomass Gas Engine CHP
- Opportunistic load: (E) Rooftop SDWH
(A) Direct combustion EfW/Biomass CHP boiler/steam turbine;
- System Sizing; Single unit – 0.35MWe/1.05MWth (1:3 ratio)
- Energy Demand/Supply; developments entire electricity and heat ‘dispatchable’ supply could be met annually with little seasonality issues. OBBP site perfectly located near to established biomass suppliers and waste generation resources. Minimal need for storage facilities and good infrastructure access.
- Resource supply; combination of wood chips/pellets and MSW primarily. However AD biogas/digestate from the nearby sewage works should be given further consideration. Given the diversity of fuel sources – supplies are considered constant/reliable throughout the year.
- Economics; lowest resource cost of all the technologies if utilising energy from waste. If using biomass, not as cheap as installed medium scale wind generation.
- Key Issues; social acceptance due to large CHP plant aesthetics and noise presenting planning permissions and residential desirability problems.
(C) Medium scale (<500kW) Wind Turbines (Endurance X35 WT);
- System Sizing; five WSW facing turbines (each 225kW) to provide electrical power during wind generation opportunities and provide electrical generation redundancy to CHP plant.
- Energy Demand/Supply; developments entire peak electricity supply possible from October to March. Daily resource unpredictability precludes wind from being a sole electrical supply source.
- Resource supply; good unidirectional regional wind data suggest adequate free resource to meet peak turbine generation ratings, but with seasonal dips.
- Economics; cheapest per kW technology costing if EfW supply not taken advantage of. Should be utilised instead of CHP power when available.
- Key Issues; social acceptance issues and supply loss during summer months. More unpredictable generation than CHP plant. Low power despatchability.
(E) Roof mounted Solar Domestic Water Heating (Navitron SFB4730);
- System Sizing; between 534kWth (semi-detached)–918kWth(detached) peak thermal generation from 6 to 10 SDWH panels respectively on each South facing roof. Provides WATER heating redundancy to the CHP plant.
- Energy Demand/Supply; housing development water heating supply to compliment CHP ‘dispatchable’ heat supply. Good matching with daily evening and March to October monthly demands only, CHP plant required at all other times.
- Resource supply; generally poor matching heat generation supply to demand, especially during fewer daylight hours winter months and in the mornings. However on average over the entire year can provide each homes hot water heating needs to 50oC given average daylight hours (12hrs), even with large ambient to working fluid temperature differentials.
- Economics; similar to domestic PV, a more expensive installation option initially than biomass CHP but more cost effective over the projects life time with a free resource.
- Key challenges; poor match between seasonal demand and supply. Low power despatchability. Initial installation costs. Heat piping and home insulation to reduce losses.
(B) Modular gasifier plus ICE gas engine (arborElectroGen 400);
- System Sizing; 2 x Modular units 0.8MWe + 1.05MWth (1:1.3 ratio)
- Energy Demand/Supply; developments entire annual and peak diurnal ‘dispatchable’ electricity and heat supply. Assumed little supply seasonality.
- Resource supply; more limited fuel range than a boiler/steam turbine system. Uses coarse biomass such as wood chips or briquetted digestate from the Sewage plant. Good local nearby fuel sources.
- Economics; low resource cost. Using biomass however, fuel not as cheap as installed EfW or onshore wind generation.
- Key issues; additional heat supply redundancy required to cut fuel costs and single source risk. Two modular ICE units allow electrical redundancy and continuous generation. Biomass fuel sourcing requires additional local suppliers. Modular/smaller design makes the CHP more aesthetic pleasing and quiet. High power despatchability.
E) Roof mounted Solar Domestic Water Heating (Navitron SFB4730);
See scenario one description above.
Finalised Technologies deployment and integration within the development
Both technology combinations require careful consideration of their siting at a later project planning stage to maximise their efficiencies and minimise their impact on the residential development. The technology combinations in each choice aim to create generation redundancy, meet peak electrical and thermal power loads and be cost effective over the technology lifetimes with excess electricity sold to the local grid, and excess heat delivered to nearby offices and businesses via a district heating scheme. Given the new build house design, it’s envisaged that each home will have a high energy efficiency rating via passive heating design, installed insulation in walls and piping and take a ‘Whole-House Systems Approach’ as endorsed by the UK governments ‘zero carbon’ approach for new homes from 2016 onwards. The ultimate aim being to reduce each households power demands, especially for space heating.
The technology integrated power generation profiles for each scenario represent a typical supply profile given the sites resource evaluation at this preliminary stage, and given the technology commercially available at the time of writing. Ideally home design flexibility and technology expansion space would be included into the critical elements of the power generation scheme, e.g. the chosen Biomass CHP system to allow upsizing if required.
Figures 30-33 graphically captures examples of how the different Scenario’s monthly electrical and thermal power generation under average OBBP site conditions could look. In case of any single system breakdown; grid electricity can be imported and housing design mitigation for alternate heating sources investigated, i.e. electrical heating systems.
Figure 30 – Scenario#1; Monthly Electrical power generation to meet Development targets
Figure 31 – Scenario#1; Monthly Thermal power generation to meet Development targets
Figure 32 – Scenario#2; Monthly Electrical power generation to meet Development targets
Figure 33 – Scenario#2; Monthly Thermal power generation to meet Development targets
To ensure sustainable, renewable and continuous power generation, the following flexible deployment and system integration design should be considered;
- The CHP plant will operate in ‘Parallel Mode’, i.e. the plant/development sites electrical system operates in conjunction with the local area electrical supply system via switchgear connections. Provides ‘Top-Up’ power, ‘Back-Up’ failsafe power and ‘Export’ power. Special metering facilities are required. The CHP plant requires generators equipped with synchronising to match alternator phase power, protection equipment to provide automatic and instantaneous disconnection in the event of local system instabilities and must not be capable of introducing excessively high peak currents (via a major fault) to the local system.
- Asynchronous selected wind turbine technology is designed to be grid connected from the onset and provide flexibility in electrical supply between the housing development and grid. Biomass CHP electricity generation can be subordinate to wind power electricity during winter months by informative forward weather forecasting and potentially battery storage systems smoothing.
- Power generation priority will be given to free wind and solar sources over biomass generation to reduce fuel purchase, transportation and storage costs.
- Essential technology maintenance in both scenarios will be carried out for each technology at the point where maximum system redundancy occurs;
- Scenario #1 – Wind turbines summer, CHP in spring/autumn, SDWH in winter
- Scenario #2 – ICE GI Mod 1 or 2 in summer, SDWH in winter
- With MSW/Biomass steam turbine systems, a slow heat demand response time can be smoothed by incorporating liquid heat accumulator energy storage systems. Biomass gas engines have a faster demand response time but may also benefit from utilising accumulators.
- Solar domestic generation water heating and Biomass CHP heating will operate in balance to support the developments water heating demands. Throughout the year, biomass CHP systems will be required to meeting morning hot water demands given the daylight hours needed for SDWH systems to warm cylinder tank waters unless some form of thermal storage system is incorporated.
- Key to the renewable energy generation in meeting space heating demands wide seasonal variations is the reduction in waste heat through better building design standards, smart metering, effective heating controls systems and educating energy usage behaviour to consumers in line with the UK Governments ‘Future Heating Strategic Framework30.
Project conclusion and way forward
A Forfar OBBP housing development could be an attractive residential project with a high potential to be fully self-sufficient in renewable energy generation, and even profitable to both residents and local businesses economically and environmentally given current Government subsidies. Siting the development on a business park and marketing it as a ‘Green Development’ could attract and encourage renewables support services to the location providing job opportunities to the 1.9% of unemployed people in the Angus County region (Angus Council, 2016). The way forward from this report is to approach Residential Developers and Angus County Council to gauge their interest and enthusiasm for such a scheme, with a high likelihood of support given the Scottish Governments ambitious renewable energy goals.
Report Assumptions used to define project boundary conditions
Project Timing; considers technology usage now.
A number of optimal site criteria only were used to identify a suitable brownfield sites that may qualify for a 500 home development and could in reality support on site renewable energy generation. Its assumed that private land owners, public and local authority support would be agreeable for solar PV or onshore wind farms if appropriate. Also assumed the Developer agrees the site is developable.
Based upon Scottish Government statistics, a 500 homes development within a single local authority (LA) by a private enterprise represents a large scale individual development (Average 2009-2014 LA annual development = 339 homes). In keeping with the size of the developments ambition, I am assuming the main housing type is either medium sized semi-detached or larger detached housing with rooftops designed to cope with solar panel installations.
Energy Usage; Scottish Government statistics for annual household electricity and gas consumption were utilised for the most recent years available;
- Scottish Regional variations for the developments energy consumption statistics were ignored as an over complication at this feasibility stage.
- Energy consumption calculation statistics assume the selected ‘home’ types are new builds (higher efficiencies than older housing), with between 3-5 people occupying each home. No distinction was made for property tenure type or household income.
- This would create a project development population of between 1500-2500 people.
- All homes assumed to have the same energy usage levels and patterns.
- Given Scotland’s access to North Sea gas and its dependence as a space heating fuel source, I have assumed a new development would replace this gas usage for heating with renewable generated heat.
- The communities total heat demand has attempted to be met using renewable generation. In-house electrical heating systems are becoming more popular but this is not yet reflected in the energy consumption figures. Electrical heating could be considered as a mitigation strategy in future should the development energy under achieve expectation.
- Typical UK technology ‘capacity factors’ identified.
- Biomass supply is assumed to be clean of contaminants. No account has been made for various sources of water, H2S, Ammonia or volatile solids
- European Policy context and Energy Framework policy; considered supportive.
- Brownfield siting; by locating the project at vacant/derelict brownfield site, Scottish Government project to its regeneration would generally be supportive.
- Local authority and Scottish Government scheme support; considered favourable as published in the National Planning Framework 3 and Scottish Planning Policy document(1). The political allegiance of individual Authorities is assumed to not be a planning consent hindrance. Planning and guidance consent is devolved to local authorities who are instructed to support, guide, manage impacts, grant permissions and consider decommissioning from the onset.
- Electrical Market Reform (EMR); The latest renewables support mechanism; ‘Contract for Difference’ which replaces the Renewables Obligation mechanism will be deemed favourable to this project, i.e. providing a long term price certainty for low carbon generation.
- Renewables Supports Incentives; It’s assumed that the Renewable Heat Incentive (RHI), Feed in Tariff (Fit) for self-generation projects below 5MW and Home Energy Scotland renewables loan schemes will be available to homeowners buying into a property development possessing small scale renewables generation.
UK commercial/installed technologies; only have been considered.
- This will simplify and strengthen local authority planning integration consents, whilst providing existing tried and tested technology supply chains and expertise. Preference will be given to utilising the Scottish regions suppliers where possible.
- Technology performance has assumed to be as per initial installation and operate indefinitely. Technology replacement, repair/upgrade costs have been noted only.
The study is a technical feasibility report only and does not include a detailed economic assessment. However it does refer to one particularly relevant report23 that compares sub 5MW UK technology generation costs to guide this study’s conclusion.
No direct scheme funding strategy is presented. However the Renewable Energy Investment Fund (REIF) set up in October 2012 is operated by the Scottish Enterprises Scottish Investment Bank and offers loans and equity investments for renewable energy projects, with an initial focus on marine, community energy (via Community and Renewable Energy Scheme) and district heating.
Assumed to be favourable to each of the available renewable for energy supply to the new housing development. Reflective public consultation in line with the Aarhus Convention will be adopted for nearby communities/residents.
Grid Access and Reinforcement;
By identifying brownfield sites near to existing larger scale settlements, its assumed the local capacity will exist to export excess electrical power as well as access local substation spare capacity for renewable project power tie in and distribution. It’s also assumed spare local demand capacity is present to accept this surplus energy.
Assumed local commerce and industry will accept piped surplus heat energy.
The selection of currently installed commercial renewable energy technologies in Scotland should secure access to the equipment and expertise required for a successful installation and operation.
Region specific historical data was assumed for the available forecasting energy resource. Differences in regional consumption patterns based on local climate variations were not considered, as was any forecast change in prevailing climatic conditions.
Systems Performance, Operation and Monitoring;
Post installation operation, monitoring and optimisation of renewable generation systems will assumed to be outsourced to an industry recognised company, and to not be considered a technology selection issues within this report.
- Scottish Government; Scottish Planning Policy, (February 2010).
- Scottish Vacant and Derelict Land Survey (2014).
- 2020 Route map for Renewable Energy, Scotland –update (October 2013) & (September 2015).
- West Lothian Council residential development guide (2008)
- Scottish Government; Energy Statistics Summary – March 2016.xls
- Scottish Government; Private Sector new housing starts and completions – 2015.xls
- DECC; Consumption in UK (2015).
- DECC; Renewables Energy database (2016)
- DECC; DUKES capacity factors from renewable electricity generation (2010-2014).xls
- University of Edinburgh; Matching Renewable Electricity Generation with Demand (2006)
- SQW Energy report; Renewable and Low Carbon Energy Capture Methodology; Methodology for English Regions (Jan 2010).
- DECC; CHP technology – a detailed guide for CHP developers (2008).
- Technology data for Energy Plants (Denmark ENERGI Styrelsen – 2012).
- Carbon Trust; Small Scale Wind Report, Policy insight and practical guidance (2008).
- Angus Biofuels: Forestry Engineering Group (2011).ppt
- IEA N Bachmann; Sustainable biogas production in MSW treatment plants (2015).
- Viesmann Group: Producing and Using Biogas
- Angus CC; Environmental Strategy Action Plant (2002)
- RenewablesUK; Generate your own Wind Power (2014)
- RenewablesUK, Small and Medium scale wind strategy: current and future potential of sub-500kW wind industry in UK (November 2014)
- Endurance Wind Power Inc; Endurance X35 225kW turbine specification brochure.
- R.Thomson/G.Harrison, Edinburgh University; Lifecycle costs/carbon emissions of wind (2015)
- Element Energy; Design of feed in tariffs for sub 5MW electricity in Britain (July 2009)
- Arbor Heat and Power Inc.; Arbor ElectroGen 200 400 800 brochure
- Suntech Hypro STP290 solar PV brochure.
- Navitron Solar Hot Water SFB47 brochure.
- The Scottish Government; Low Carbon equipment and building regulations – a guide to safe and sustainable construction. Solar Water Heating Systems (March 2010).
- Energy Saving Trust; Measurement of Domestic Hot Water Consumption in Dwellings (2008).
- UKERC; Future Role of thermal energy storage in UK: Research Report (Nov 2014).
- DECC; The Future of Heating: Meeting the Challenge (Mar 2013)