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Domestic Solar Earth Charging Carbon Zero hybrid retrofit achieved by balancing PV with solar earth charging for augmentation of heat pump Architect David Nicholson-Cole BArch Department of Architecture and Built Environment, University of Nottingham david.nicholson-cole@nottingham.ac.uk Abstract This paper reports on an experimental augmentation system for ground source heat pumps using solarium style panels to recharge the ground, thermally. The system is running on a house in Nottingham, England. The house has a 4kW photovoltaic array and a ground source heat pump for heating and hot water, using twin 48 metre boreholes. Real-time, Diurnial and Inter-seasonal solar charging restores the energy level in the earth, which prevents progressive frosting of the borehole zone. Measured over a year, it can improve the GSHP performance by a significant percentage – it has been sufficient to reduce the annual electrical consumption for heating and hot water to less than the annual power generated by the PV array on the roof, thus achieving zero carbon emission. Keywords COP, ground source heat pump, solar thermal, underground thermal charging, low carbon, photovoltaic
1.0 Introduction This paper reports on an augmentation system for ground source heat pumps (GSHP) using solarium style panels to recharge the ground, thermally. This is domestic scale, fitting a single house as a real-world, real-time, full-scale lived-in experiment. At the time of house construction in 2006, the justification for a GSHP was that a monofuel based system, electricity, is preferable to active burning of fossil fuel sources for four reasons: 1. If we burn fossil fuel or biomass, we cannot avoid emission of CO2, even if we find other ways to offset or compensate. 2. Governments fully intend to increase the proportion of renewable electricity by 2020-30, to meet European targets. This will include large scale wind power, but also includes thousands of home generators using photovoltaic panels. [1] 3. The technology of heating with electricity is becoming more efficient with well installed and designed heat pumps which can use renewably sourced electricity to extract heat from a renewable source. 4. Individual buildings and groups of buildings can home-generate enough electricity to meet their electrical requirements for space heating and with good design, for hot water too. As a bonus, electricity is capable of being metered precisely, yielding the researcher with definitive proof of the carbon balance. It is a fundamental tenet of renewable energy that solar heat is free, even though most of it arrives at times when we do not need it, in summer. We need considerable ingenuity to use this clean energy source successfully. We can all agree that future domestic buildings in cold climates can and should be insulated to excellent or even Passivhaus levels to reduce their heating Page 1 of 14
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CIBSE ASHRAE Technical Symposium, Imperial College, London UK – 18 and 19 April 2012
requirements. We can also agree that we need to find ways to make millions of existing houses more efficient by insulating. Good insulation must always come first. However, there is still a need for effective and affordable technology to get close to carbon zero. This experiment shows that PV and Solar augmented GSHP can be effective as a partial retrofit solution. Ground source heat pump (GSHP) is a well established technology with hundreds of thousands of units (80% domestic) installed worldwide. There are about 4,000 GSHP installed annually in the UK [2] and 17,000 Air source heat pumps installed in the same time. Compared with Scandinavia, few systems have been installed in the UK, in which metered gas is one quarter the price of electricity. In the UK, the domestic heating industry is primarily geared to providing compact gas-fired condensing boilers, replacing older ones. The UK’s DECC report on heating [3] shows that more than 90% of centrally heated homes are using Gas as a heating fuel. The UK’s Carbon Plan [4] repeatedly states that the future of domestic heating is heat pumps (air and ground). The installation cost deters householders, despite small subsidies such as the Renewable Heat Incentive, unless they are off the gas grid, in which case their primary practical choices are oil or heat pump, supplemented by the recent surge of interest in woodburning stoves. The majority of marketing of heat pumps in the UK is for Air source, and ASHP installations in the UK 2010 were 13,000, more than 3 times that of GSHP [2]. The UK Energy Savings Trust Getting Warmer report in 2010 [5] reported a general satisfaction with ground source heat pumps, but because there was also some dissatisfaction in the survey, it highlighted the need to improve the sales process, installation and performance, control systems and training. The operating characteristics and the efficiency of a heat pump are largely determined by the heat source. This supplies the low-temperature heat for ‘pumping up’ to usable temperatures. The theoretical and practical coefficient of performance (COP) is dependent on the temperature difference between heat in and heat out. The development of measures to improve the utilisation of heat sources is vital to widening their deployment. A ‘rule of thumb’ is that the COP improves by approximately 3% for each degree (Celcius) the evaporating temperature is raised, or the condensing temperature is lowered [6]. The experiment reported in this paper shows that the efficiency can be further improved if there is direct access to solar heat. 2.0 The House and Borehole The author’s occupied detached house in Nottingham, England is a 'developer' property built in 2006-7 with above average insulation, and a floor area of 120 square metres. It has a Swedish manufactured 2kW/6kW ground source heat pump with an integrated 185 litre water tank. The house has underfloor heating on ground and first floors. The insulation of the house is 100mm cavity fill. It is not up to ‘Passivhaus’ standard, and it is not practical to increase it further, other than hunting down air leakage gaps The estimated total heat and DHW requirement is 14,600 kWh. Prior to this project, the GSHP had an estimated consumption of 5,200 kWh, giving an average COP of 2.5-2.9. Variations are due to weather statistics (degree days) and patterns of occupancy (e.g. visitors taking more baths). The ground source heat pump was commissioned in March 2007. The thermal mass is a twin borehole set, 48 metres deep into dense clay-marl soil, encompassing approximately 3,600 cubic metres. The ground loop is a closed circuit of 40mm Page 2 of 14
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CIBSE ASHRAE Technical Symposium, Imperial College, London UK – 18 and 19 April 2012
plastic pipes. 90 metres can be considered to be the active length (ignoring the top 3 metres), and the peak demand on the borehole is 4000 watts. This requires the ground to deliver an average of 44 watts/metre, which is safe for the dense damp clay-marl of the site, according to the Veissman Technical Guide. [7]. However, there is always room for improvement in performance.
Figure 1 - Peveril Solar house from the South East 2010. (Photo: author)
Ground source heat pumps are mostly supplied with heat from a closed loop, operating vertically (in borehole/s), horizontally (in a long trench) or to a compact collector (a pipe-grid that combines depth and width). The author counts himself fortunate that there was insufficient garden space for anything but a borehole. It is common sense that underground thermal energy storage is safest if it is deeper, being out of reach of seasonal variations, providing there are no water bearing gravel layers to remove the stored energy. The heat retrieved from the ground is Solar heat deposited over a vast time. The amount of heat retrievable from Magma heat is negligible [7]. It can take many decades for naturally acquired solar heat to find its own way down to a depth of 48 metres. With the investment already made in a borehole, the system has a pair of pipes reaching that depth, with a pump attached. It seems axiomatic to the author that solar heat can be pumped down today, tomorrow and every other sunny day, instead of waiting for decades. During the first two years of use, the house consumed an annual average of 8,500 kWh of electricity, and it is assumed that 4,800-5,200 kWh of this was caused by the heat pump, the rest being cooking, lighting and power appliances.
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Figure 2 – 3D schematic view of house and boreholes. (Diagram: author) Figure 3 – IVT Greenline C6 GSHP including 185L water tank. (Diagram: IVT website)
After the initial joyful assumption that the house was ‘heated from free heat in the soil’, the electrical consumption from 2007 to 2009 looked increasingly expensive and inefficient, until for the author, it became unsatisfactory. The author believes that by accepting the concept of the ‘Active house’, using efficient energy-generating and energy- distributing technology, it is possible to achieve zero carbon emission with a less-than-Passivhaus. This research is aiming to achieve that, and this retrofit project was started in August 2009. 3.0 Progressive ground chilling and solar charging The house owner is concerned that over successive years, there is a risk of deep ground cooling, reducing the COP of the GSHP. The diagram (Fig.4) by NicholsonCole and Wood illustrates a gradual decline, with a progressive failure of the ground to recover fully from the heat loss of the previous season, until it reaches a new equilibrium with winter and summer temperatures being lower than in year one – and the heat pump operating more ineffectively than when first installed. There are increasingly frequent occasions when the heat pump fails to achieve a satisfactory thermal balance. This triggers direct 1:1 heating using its ‘Additional Heat’ function. In cold winter nights, this could cause a surge in grid demand when heat pumps (air or ground source) become more commonplace.
Figure 4 – The brown line illustrates declining ground temperatures over several seasons until a low temperature equilibrium is reached. The orange line represents the temperature curve with solar charging (Diagram: author and C. Wood [8, 9]) Figure 5 – Theoretical thermal contours. The region immediately around the pipe will have greater fluctuations, even during a single day, as the GSHP comes on and then rests. Short term local warmth or local chilling will affect performance. (Diagram: author)
Rybach [10], and Trillat-Berdal, Souyri et al. [11] state that the use of a geothermal heat pump with vertical borehole heat exchanger to heat buildings can cause an annual imbalance in the ground loads; then the coefficient of performance of the heat pump decreases and consequently the installation gradually becomes less efficient. Valuable work has been done in Canada, Italy and Sweden [12],[13], to demonstrate that solar thermal charging of boreholes can improve the performance of ground source heat pumps. These are all for institutions or district heating schemes, and to date, few examples of single houses thus augmented are known. Studies for single dwellings by Trillat-Berdal [11], Chiasson [14] and Kjellson [15] have been based on computer models with realistic weather data. Kjellson [15] investigated, using computer simulation, a combined solar collector and ground source heat pump in a dwelling in Sweden. The results show that there are advantages with recharging the borehole; firstly this may increase seasonal performance of the heat pump. In Page 4 of 14
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CIBSE ASHRAE Technical Symposium, Imperial College, London UK – 18 and 19 April 2012
addition it may give a possibility to use shorter boreholes and achieve higher heat extraction from the borehole. Kjellson’s results also show that it is particularly useful to recharge the ground if the boreholes are close to each other, providing there is a manifold in the piping to ensure that the neighbouring boreholes all receive equal thermal storage. Experimental work by D. Maritan 2008 [16] in Northern Italy shows a significant benefit to the ground temperatures from solar charging an array of compact underground collectors using evacuated tubes. Closely spaced boreholes are normally considered to be disadvantageous because they have reduced surface area to the surroundings, making it less easy to recharge after the winter. But if the boreholes are charged with externally supplied heat, the opposite is the case, as they ‘nurse’ their charge, reducing losses to the surroundings. The work of Dr Chris Wood [9] with thermal charging of foundation piles in 2011 supports the idea that a cluster of thermal piles of 8-15m deep are more effective at nursing heat than just one or two of 60m or more. The probability of the boreholes hitting ‘bad’ layers is reduced if the boreholes are frequent and not deep. The combined length of boreholes must still meet the requirements of the Veissman Technical guide [7], and it would be foolish false economy to shorten them. 4.0 Defining a PV-based target In an effort to make the house carbon-emission free, the owner-author began this project in October 2009 by proposing solar thermal charging of the boreholes. The aim of this project widened to include the notion of limiting the heating energy requirement to what can be produced by a 4kW PV array in the UK climate. The first practical action was the installation of 22 photovoltaic panels. In the UK, domestic PV installations are limited to 4 kW by the Feed in Tariff system. Larger ones are permitted, but there are strong financial incentives not to exceed 4 kW. The PV roof of the Peveril Solar house at 53ºN in Nottingham faces east with a 40º pitch and generates 3,300-3,400 kWh annually. Another house 100m away with a south facing PV array of the same size has generated approx 3,750kWh. This numeric range of 3,300-3750 provides the target to meet with the GSHP consumption. The author’s next research target was to find ways to reduce the GSHP power consumption. If the annual power consumption of the GSHP is also in the range of 3,300-3750 kWh, then the activity of space heating and DHW is not emitting Carbon, measured annually. For the house, the annual electrical consumption of the GSHP was estimated to be in the range of 4,800-5,200 kWh prior to the project [17]. Since early 2010, a combination of solar augmentation and more disciplined thermal management has reduced the total GSHP consumption to a range of 2,600-3,300 kWh, thus meeting and exceeding the target. The proportion of the GSHP workload devoted to househeating only is approx 1,800-2,400 kWh (depending on weather), so with quiet satisfaction, it can be considered that the house meets carbon zero targets, based on a better than zero balance of metered kilowatt hours. [18] PV power arrives at a time when it is least needed, in the summer daytime. As the number of people with PV is a tiny fraction compared with those who do not, we can apply an ‘Electricity Storage’ concept, using the National Electricity Grid. During the Page 5 of 14
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summer, the home-generators are delivering clean electricity to be used by their neighbours; in the winter, they are drawing electricity from the Grid. As the UK adapts to more renewable and intermittent power sources, more clever storage concepts will be needed. A summer saturation of PV microgeneration is unlikely to happen because the majority of today’s houses have unsuitable roofs, with hipped geometry and chimneys, and the UK landmass is mostly north of 50 degrees N. The National Grid will always be larger than the amount produced by small home-generators. Countries which have a surplus of renewable energy have already found ways to store the surplus, notable examples being Norway and Austria. [19]. 5.0 Solar charging method The primary means of improving the COP of the GSHP has been to augment the ground loop with a solar system that captures and stores free heat to the thermal mass below the house. Since early 2010, solar-cooker style ‘sunboxes’ have been installed on the south wall, connected directly to the ground loop. These are controlled by a thermostat which activates a solenoid valve. They are self designed and built by the author. These have now been running for two years and have revealed good performance data.
Figure 6 – Schematic circuit diagram of solar ground charging system (by the author). A normally operating conventional ground loop is as shown (right). The entire ground loop is diverted through the Sunbox when conditions are good for solar thermal charging (left). The illustration is dated 2011, because there are adaptations planned for 2012 (Diagram: author)
The system works in three ways: • In Winter this provides Real-time heat to the GSHP if delta-T is good. Winter sun occurs frequently, and provides an immediate boost to the GSHP. This varies from 10 minutes a day on cloudy days or up to 5 hours on sunny days. • In Equinox, this provides Diurnal augmentation. Daytime heat is stored, the region immediately around the pipes is warmer than the surrounding soil, and heat is retrieved for use in cold evenings. • In Summer, the system works Interseasonally – it quietly continues for months to store heat into the borehole for use during the winter. Air heat and direct solar heat both contribute to the collection. Continuous metering of temperatures shows that [18] the earth does not get ‘hot’ but the energy level rises - the low grade heat moves further out into the volume around the pipes.
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6.0 Construction of the system There have been two versions of the sunbox system during the experiment. The original idea in August 2009 was to hang a bare grid of 40mm black pipes on the south wall. Then it evolved to using matt black swimming pool panels, exposed to the air. As the thinking progressed, the design included a glazed microclimatic enclosure to exploit the greenhouse-effect, enabling the system to work in equinox and winter In March 2010, the first design was constructed as two small glazed solariums (which have been called ‘Sunboxes’) surrounded with aluminium alloy reflective panels. These reflectors enhanced solar capture compared with their performance without the reflectors. The sunboxes were monocoque construction made of a single skin of 6mm structural polycarbonate. The first sunboxes resembled vertically mounted solar cookers, using the greenhouse effect. The insulated brick wall to which they are mounted makes an incidental contribution in summer, by heating to >40º on hot days and releasing an estimated additional 5.5 kWh into the air space late into the evening, long after sunset. Angling the front face to 70 or 75 degrees would have been more effective, but solar energy installations with projections greater than 200mm from the wall require planning permission in the UK.
Figure 7 – Cutaway image: link from sunboxes to heat pump. (Diagram: author) Figure 8 – First design (2010) of south facing solar sunboxes, with reflectors (first version). (Photo: author)
Figure 9 – Construction process of the improved design for Sunbox system, combining two small ones into a single large volume, Aug 2011 (photo: author). Figure 10 – Completion of improved Sunbox design Aug 2011 with weather station above.
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(photo: author). The 700mm projection provides shading to the first floor south facing windows in the hottest part of summer.
The owner has planning permission to exceed 200mm. The local authority have been very supportive of such an innovative project taking place in their district, and have participated in ‘Technology Open Day’ events at the house. During the summer of 2011, the two sunboxes were completely de-constructed and rebuilt as one single large sunbox with sloping front faces of 70 degrees. The intention is to increase solar capture in all seasons, and to increase the air volume in the box relative to the liquid volume in the black chillers. The volume was increased from 1.1 cu metres to 2.8 cu metres. The new envelope is a monocoque structure of triple wall polycarbonate, with no-cold-bridge design at the junctions. The side edges of the boxes are caulked with sealant to improve air-tightness. The front panels hinge up for maintenance access. The whole assembly is fastened to the wall with 8 x 14mm anchor bolts. The chillers within are 4 square metres of black polypropylene swimming pool heater panels. They are directly plumbed into the ground loop of the GSHP. The circuit is thermostatically activated when conditions are good – either the sky is bright with infrared, or the Sun is shining, or there is a good delta-T between the sunbox airspace and the ground loop. A 3-port valve opens, and a circulating pump diverts the entire ground loop through the solar sunbox. If all available heat has been downloaded, or if conditions are not good or the chillers have over-cooled the space, the valve closes and the ground loop is restored to its original circuit. If conditions are improving, the sunbox warms up again, and the thermostat restarts circulation. This process has worked for an average of 2,400 hours/year. An energy meter monitors the delivery of energy in kilowatt hours and in cubic metres. What makes this different from a normal solar panel arrangement is the ‘direct’ plumbing arrangement - the entire ground loop is serially pumped through the panels, because they have large diameter piping (28mm generally, and 40mm at the panel entry-point, and 22mm where the pipes meet the pump and energy meter) (Fig. 6). When the heat pump is sleeping, the sunbox flow rate is slow, at 5 litres/minute. When the GHSP is driving the circuit, the flow rate is boosted to a rapid 18 litres/minute. A conventional solar panel or array of evacuated tubes could be used, but these would require a ‘parallel’ plumbing connection with a non-return valve, and they would work by ‘diluting’ their heat into the existing ground loop at the slow flow rate. When the original sunboxes were taken off and replaced with the new model in August 2011, the black chillers, plumbing circuit, and electrical controls were left unchanged, as they have worked perfectly. The author plans in Summer 2012 to fit a bank of 16 evacuated tubes on a parallel plumbing circuit, just to compare performance. Polycarbonate is less reflective than glass, and would not be any more of a nuisance to neighbours than a large window or wall mounted solar panel. The sunbox is unlikely to be occupied by bats or birds as the temperature regularly exceeds the mid forties Celsius on sunny days. Normally, there is no gap large enough for creatures, but the maintenance access louvres could be left open by mistake. Page 8 of 14
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7.0 Performance monitoring of ground and system Daily monitoring and datalogger records have shown improved GSHP efficiency, compared with the previous year. The team have had problems with extracting data from the dataloggers, so all results reported here are based on daily meter readings with weekly, monthly and annual summaries. The annual figures are re-computed every week, by comparing meter readings with the reading of precisely the previous year. All results are posted on the web [18]. Meters are either the official government approved meters supplied with the house or the PV system, or are comparable products of reputable manufacturers. It is estimated by the heat pump manufacturer that the GSHP must draw 9,00010,000 kWh from the earth. [17] The Sunbox system, nicknamed ‘Surya’ by the author, captured an annual average of >3,000kWh of thermal energy in the first year. Therefore, there remains a large contribution of approximately >6,000 kWh that must come from ‘Mother Nature’. It also helps to have a well insulated building with high thermal mass, as the GSHP can be turned off for 11 hours/night by a timeclock without any problem, even at the coldest peak of winter. This reduces consumption compared with a lightweight building. Prior to the Sunbox system, the estimated average COP was 2.6-2.9, comparing heat produced with electricity consumed, over 2 winters. Since the Sunbox system was commissioned, the annual consumption for GSHP has declined to less than 3,000 kWh/year for all heating, hot water and circulating floor pump. At the time of writing, the precise figure is hovering about 2,700 kWh. This is hundreds of kWh below the PV based target range. This figure could be further reduced by 200 kWh if the power demand of the circulating floor pump is deducted.
Figure 11 – Comparison of Degree Days in Nottingham (Red, indicating the level of space heating required, monthly) and the workload of the GSHP for heating (Blue curve, in kWh/month), from Oct 2009 to Feb 2012 (diagram: author) Figure 12 – Deep ground temperatures measured since Aug 2009 to Feb 2012. The Sunbox system was installed in March 2010. The trend is very visible, with the ground temperature falling to below 5º in winter 2009-2010 and only falling to 10.0º in winter 2010-11 and 201112. The overall curve is smoothed, with flatter troughs, although the ground has not got significantly hotter at the peaks. (diagram: author)
So, let us weigh up the figures: for a total heating requirement of 12,500-14,600 kWh, the GSHP is consuming 2,700-3,300 kWh. This simple arithmetic suggests that the COP over one year is now averaging a COP of about 4.4-4.8, far higher than its practical expected performance – from observations, it appears that the GSHP is beneficially boosted by the active contribution of direct solar heat. Page 9 of 14
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The annual power consumption purely for the space heating requirement is approx 2,000-2,200 kWh deducting the effect of the Hot water. The roof mounted PV power generation has touched 3,400 kWh. If a Passivhaus permits a consumption of plus 15 kWh/M2/year [21], this ‘Active House’ [22] has a balance of minus 10 kWh/m2/year. The long term result of solar ground charging has not been to heat the ground significantly but to maintain stability by preventing deep chilling, speeding up ground recovery after winter, and thus improving the COP in both halves of the winter season. During the last two winters of 2010-11 and 2011-12, the deep ground temperature did not fall below 10.0ºC (Fig.12). The GSHP performed with reduced electrical consumption relative to the level of heating demand, as indicated by the GSHP-Degree Days chart (Fig.11). When tested after some hours of rest, the overall borehole temperature has never exceeded 14.0ºC. However, the short term effect in Diurnal operation is to provide a ‘warmer-than-the-surroundings’ zone of 16-17 ºC immediately around the borehole pipes on sunny days, assisting the GSHP more quickly in the evenings than if the entire borehole zone was warmed. After a heating cycle has completed, the Sunbox system circulation performs a rapid restoration of temperature in the immediate local zone around the pipes.
Figure 13 – Combined graphs of Sunbox, PV, GSHP and Degree days reveal interesting peaks of the sunbox at Equinox times, and consistently higher kilowatt hour capture than the PV during the winter months. (Diagram: author) Figure 14 – Sunny autumn day performance. The thermostat is able to judge two channels (actual temperature or delta-T). If conditions are right, it opens the solenoid valve, which relays power to the pump. Even in Oct. 2011 (as above when the external air temperature was 14ºC), heat is sent to the borehole. (Photo: author)
The storage of heat underground appears to be efficient when used this way, providing the thermal energy does not get lost to water courses. Under the Peveril Solar house, the soil is solid Marl (clay-rock mixture) all the way down. If the rate of storage is >3,000 kWh/year and the rate of withdrawal is 9,800 kWh/year, then that heat has no time to escape. The delta-T is not sufficient for it to escape because the borehole temperature only needs to rise to a maximum of 14ºC or 15ºC for the GSHP to be substantially augmented. By October, when long term summer heat is beginning to move away, the GSHP is demanding the heat back. The GSHP has never had to resort to ‘additional heat’ since the Sunbox system was installed, saving perhaps 1,000 kWh/year. The power consuming activity of the 30W pump in the Sunbox system is largely met by the large PV roof because the majority of its working time takes place in daylight. Page 10 of 14
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The thermal performance of the Sunbox system varies seasonally (Fig. 13). During Summer, it averages 1.1kW, running for long hours in warm conditions at the slow pump speed. During Winter it captures less in total, but when it is running, driven by the faster pump in the GSHP and controlled by 6 degs of delta-T, it averages 1.61.8kW. In Equinox, the total monthly capture peaks briefly because there is a combination of warm daytime temperatures and more frequent activity by the GSHP, averaging at about 1.4kW for the hours in which it runs. 8.0 Comparison with solar thermal heating for water The annual performance of using solar thermal energy to heat the ground is higher than heating a water tank. This compensates for the potential risk of heat-loss of heating a large un-insulated mass. The ground is too large and cool to suffer from ‘stagnation’. The heat is free, so it is possible to pump all summer. For many more hours there is a beneficial delta-T, even during sunny winter days. Regular solar panels are governed by a thermostat that compares the store temperature and the panel temperature. For water heating, the tank is often up to its optimum, nobody is taking a bath, there is no delta-T and the pump does not run. This is stagnant hot water and the panel may have to drain back to avoid damage. The British SAP calculator [20] assesses the contribution of 4 sq metres of solar thermal panel or evacuated tubes to be in the region of 1,100 kWh/yr. The Sunbox system in the author’s project has run for 2,400 hours in a year and buried >3,000 kWh in their first year. 9.0 Conclusions: PV limits and comparable systems The underground thermal energy storage project on the Peveril Solar house in Nottingham is the only example in the UK known to the author of real-world implementation on a single house. The active ‘virtuous pentangle’ of PV, GSHP and solar charging has succeeded in achieving ‘Carbon Zero’ for heating and hot water. In the UK, the 4kw limit on PV would make it impossible to achieve absolute carbon zero for everything (heating, DHW, lighting, power, and cooking) unless significantly backed up by other factors. A newly designed house can enjoy Passivhaus levels of insulation and/ or biomass heating to soften the effect of cold snaps in winter. Retrofit to existing houses requires the designer to take an ‘Active house’ approach, making the best of what is there, and deciding what can be fitted.
Figure 15 – The ‘virtuous pentangle’ of House in the centre, with Grid, PV power, GSHP, Borehole and Sunbox system working together. White=Electricity, Red=Thermal Energy.
(Diagram: author) Page 11 of 14
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The average British family household is deemed to use about 3,500 kWh for annual lighting and power and 21,000 kWh of gas energy for heating and DHW. As 4 kW of PV can only produce about 3,300-3,750 kWh. It seems clear that even with a heat pump, 4kW of PV would not meet the entire energy demand – but the experiment shows that it can fully meet ‘regulated’ demand, such as heating, and produce enough in addition to provide hot water. The Passivhaus standard permits a house of the same size to consume 15 kWh/sqm x120sqm = 1800 kWh/year for heating. [21] With the ‘Active House’ concept, it is possible to reduce this figure to below Zero kWh/sqm/year. [22] A 6kW photovoltaic installation could produce 5,000kWh at the latitude of Nottingham [23], and with this quantity, the household could balance its entire power equation of all consumption and generation, even for lighting, power and cooking – providing it has a roof large enough for 25-30 PV panels. The Peveril Solar house is occupied and functioning efficiently, so it will be necessary to find other houses and GSHPs to experiment with if the technology is to be tested further. It is a difficult task to find another GSHP user with a borehole willing to conduct another experiment. In the UK market it is rare for householders to use GSHPs, and if they do, they are more frequently persuaded by installers to use horizontal loops, or to use an ASHP (Air source HP). There is no users’ club or association through which to make contact with other owners. 10.0 Conclusions: Future application of the idea This paper proposes that there is a case for solar augmentation on single houses, in the long term future. In the economic environment of British housing, district heating scale systems are unusual. Apartments exist only in main cities and towns, and for these, shared heating systems are acceptable. The scale of construction of a apartment block makes it easier to consider a communal GSHP, a cluster of boreholes and a rooftop or facade of communal solar thermal panels. The majority of UK dwellings are houses in suburban locations where owner occupation is predominant, and heating facilities are not shared. UK residents go their own way, like motorists, whereas in Scandinavia, people are content to be like public transport users, living with district heating, even in clusters of detached dwellings. Successful underground thermal storage schemes have worked for groups of houses in Sweden [24] and Canada [25]. Combined photovoltaic and thermal panels are an option for the future; liquid cooled PV makes more electricity, and the surface area of a PV installation that is also thermal will store more kWh of heat underground, annually than the kWh of electricity. There is no risk of ‘stagnation’, due to the infinite size of the ground and the thermal area of a PVT array is likely to be 20-28sqm instead of the 4 sqm on the Peveril Solar house. For the same surface area, thermal capture could be 3-4 times the electrical capture because PV panels work best if they discharge heat efficiently. [26] The author is of the opinion that individual houses opting for a GSHP can economically justify the drilling of a borehole if they have a small front garden or a front drive. Houses in urban areas do not have enough space for horizontal ground loops. Boreholes cause least risk to foundations, compared with deep trenches. Page 12 of 14
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Using a borehole, the investment in a GSHP would be further justified if it is allied to a low-cost solar augmentation system as described in this paper. The idea could be applied to clusters of foundation piles under a building, but this is still in the testing stage.[9] There is an analogy to the requirement for petrol powered cars to have catalytic converters, to reduce carbon emissions. In the 1950s the catalytic converter was a new invention, a unique, expensive idea. Forty years after its invention in the 1990s, it was accepted, and universally fitted as a mandatory requirement for all cars. Using financial incentives such as the Green Deal, the government could encourage future GSHPs to be solar-augmented, if ground conditions permit. At 2012 prices, the all in cost of a GSHP could be as much as £18,000 including the GSHP, installation, boreholes, underfloor heating and fan assisted radiators. In essence the addition of solar charging is merely ‘teeing’ into the ground loop, and does not require an expensive water tank. An additional cost of £1,000-1,200 for a solar thermal installation could enhance the performance by 30% year-on-year for only 6-10% once-off addition to the capital cost. Theoretically, the boreholes could be downsized by a cost of £1,200, but it would be a foolish economy, in case a future occupant turned off the solar charging pump. The author is an architect, and could not, in conscience, specify a GSHP in future without solar augmentation. This does not have to use custom-built sunboxes as on the house in Nottingham. It can also be done with commercially available solar flat plate panels or arrays of evacuated tubes. The most important things are: • Firstly, to design the building well with excellent insulation and orientation, and planned with space to enable panels and pipes to be installed. • Secondly, use technology to get and store free solar heat by any means. Acknowledgements The author would like to acknowledge the support of David Atkins of Ice Energy Ltd, Oxford, UK. The author acknowledges the written and advisory contributions of Dr. Chris Wood, Professor Saffa Riffat and engineer Blaise Mempouo of the University of Nottingham, UK.
References Full details and reporting of the project are visible at http://chargingtheearth.blogspot.com [1] DECC (Dept of Energy and Climate change) pages and publications on Energy policy to 2050. http://www.decc.gov.uk/en/content/cms/what_we_do/lc_uk/2050/2050.aspx [2] Johannes Fritsch, BRSIA Heat Pumps Study, Multi-Client study, Jan 2011 http://www.gshp.org.uk/documents/HHIC%20BSRIA%20Summary%20UK%20Heat%20Pum ps.pdf [2] General Information Report 72: “Heat pumps in the UK- a monitoring report” Energy Efficiency Best Practice Programme Document, Building-related project (www.bre.co.uk/brecsu/) , UK [3] DECC Article http://www.decc.gov.uk/assets/decc/statistics/publications/trends/articles_issue/562trendssep10-heat-use-article.pdf p. 40 [4] DECC, Article about the Carbon Plan 2010 http://www.decc.gov.uk/assets/decc/What%20we%20do/A%20low%20carbon%20UK/1358the-carbon-plan.pdf [5] Energy Saving Trust Report: “Getting Warmer – a field trial of heat pumps” http://www.energysavingtrust.org.uk/Publications2/Generate-your-own-energy/Gettingwarmer-a-field-trial-of-heat-pumps
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CIBSE ASHRAE Technical Symposium, Imperial College, London UK – 18 and 19 April 2012
[6] Yumus A. Cengel & Michael A. Boles, 1998. Thermodynamics: An Engineering Approach, 3rd Ed. WCB/McGraw-Hill, Boston, USA. [7] Veissman Technical Guide 08/2006 p19 [8] Nicholson-Cole D., and Wood J.C. (2009) “Charging the Earth – The Solar Way!” available online at : http://chargingtheearth.blogspot.com/2009/11/meetingchris-wood-diurnial.html [9] C. Wood, ‘Firm Foundations’, Geodrilling International, July/Aug 2011 (possibly password needed) [10] Rybach, W. J. E. a. L. (2000). Sustainable Production From Borehole Heat Exchanger Systems. Proceedings World Geothermal Congress 2000 S. I. o. Geophysics. Kyushu - Tohoku, Japan. [11] Trillat-Berdal, V., B. Souyri, et al. (2007). "Coupling of geothermal heat pumps with thermal solar collectors." Applied Thermal Engineering 27(10): 1750-1755. [12] Nick Wincott, lecture Ecobuild (London) March 2011 http://www.gshp.org.uk/documents/9.NicWincott.pdf and http://www.ecobuild.co.uk/uploads/nic-wincott-2.pdf (possibly password needed) [13] Goran Hellstrom, lectures National Energy Foundation 2005 http://www.gshp.org.uk/documents/GSHPsinScandanavia-Hellstrom.pdf [14] A. Chiasson, C. Y. (2003). "Assessment of the viability of hybrid geothermal heat pump systems with solar thermal collectors." ASHRAE Transactions 109(2): 487-500. [15] E. Kjellson, 2004, Solar Heating in Dwellings With Analysis of Combined Solar Collectors and Ground Source Heat Pump, Report TVBH 3047, Dept. of Buildings Physics, Lund University, Sweden, 2004, 173pp [16] D. Maritan, ‘The Case of a Geothermal Heat Pump System with a compact ground heat exchanger in dry soil and solar panels recharging’ 9th International IEA Heat Pump Conference, 20 – 22 May 2008, Zürich, Switzerland, http://www.geotherm.it/Ricerca_scientifica_geotermia_files/P.4.45_maritan.pdf [17] Assessment method used by the manufacturer IVT, and supplier Ice Energy Ltd. http://www.vpw2100.com [18] Metering records over 20 months are at http://tinyurl.com/peveril-metering [19] World-Nuclear.org reporting that Norway obtains 99% of renewable from Hydro http://world-nuclear.org/info/inf10.html [20] SAP calculation in UK takes into account water consumption and stasis risks http://www.solardesign.co.uk/sap/sap2009.htm [21] Passivhaus Institute standards http://www.passivhaus.org.uk/standard.jsp?id=18 [22] Active House concept explained http://activehouse.info/vision [23] JRC European Commission, PV Geographic Information System http://re.jrc.ec.europa.eu/pvgis/ [24] Anneberg residential area evaluation: Anneberg, Stockhom, Sweden http://www.ateik.info/northsun2005/pdf/plenar/Magdalena_presentation_North_Sun.pdf [25] Drake Landing Solar community, Okotoks, Alberta, Canada. http://www.dlsc.ca/how.htm [26] Walthamstow Fire Station: London, England. http://www.newformenergy.com/walthamstow
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