Domestic Solar Earth Charging Carbon Zero hybrid retrofit achieved by balancing PV with Solar Earth Charging for augmentation of GSHP using solarium style collector
Author: David Nicholson-Cole Dept. of Architecture and Built Environment University of Nottingham Nottingham, United Kingdom firstname.lastname@example.org
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. Direct and Interseasonal Solar charging prevents progressive frosting of the deep ground. 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 equal 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. Introduction This paper reports on an experimental 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. 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.
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. 
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.
Individual buildings and groups of buildings can home-generate enough electricity to meet their
Co-Authors: Prof Saffa Riffat and Dr Blaise Mempuou University of Nottingham Nottingham, United Kingdom email@example.com
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 Passivhaus levels to reduce their heating 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, but 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 retrofit solution. The Ground source heat pump is a well established technology with over 400,000 units (80% are domestic) installed worldwide and about 45,000 installed annually . However, in the same period, very few systems have been installed in the UK, in which gas is ¼ the price of electricity, and whose heating industry is primarily geared to providing gas-fired condensing boilers. The UK Energy Savings Trust report no 72 in 2010  reported dissatisfaction with ground source heat pumps – highlighting the need to improve installation and performance. 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 heat. 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 . This experiment shows that the efficiency can be further improved if there is direct access to solar heat.
2. Research experiment A.
lighting and power appliances. The estimated total heat and DHW requirement is 14,600 kWh, giving a COP of 2.5-2.9.
House and Borehole
The author’s occupied detached house in Nottingham, England is a 'developer' property built in 2006-7 with above average insulation. It has an IVT Greenline 2kW/6kW ground source heat pump (GSHP) with an integrated 160 litre water tank. The house has underfloor heating on ground and first floors. The insulation of the house is not up to ‘Passivhaus’ standard, and it is not practical to bring it to this standard. However, by accepting the concept of the ‘Active house’, using efficient technology, it is still possible to achieve zero carbon emission.
Figure 3. 3D view indicating relative scale and position of sunboxes, PV roof, heat pump and twin boreholes (diagram by author)
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, and this retrofit project was started. Figure 1. Peveril Solar house from the South East. (photo: author)
The ground source heat pump was commissioned in March 2007. The thermal mass is a twin borehole set, both 48 metres deep into dense clay-marl soil, encompassing approximately 3,600 cubic metres. The ground loop is of 40mm plastic pipes. The author is very fortunate that there was insufficient garden space for a horizontal ground loop, or this experiment with solar charging would not have been possible 3 years later.
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 (1) by Nicholson-Cole 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 were increasingly frequent occasions when the heat pump failed to achieve a satisfactory thermal balance, and switched to direct 1:1 heating.
Figure 2. IVT Greenline C6 GSHP incorporating 185Lwater tank. (diagram from IVT website)
During the first two years of use, the house consumed 8,500 kWh of electricity annually, and it is assumed that 4,800-5,200 of this was the heat pump, the rest being cooking,
Figure 4. Declining ground temperatures over several seasons (diagram by author and C. Wood [12, 17]). Regular solar charging can maintain the thermal mass at equal to or better than the quality of the first year of installation. 
are permitted, but there are strong financial incentives not to exceed 4 kW. The practical home generation for this number of PV panels at 53º latitude in one year is in a range of 3,2003,500 kWh, depending on azimuth and pitch. The PV roof of the Peveril Solar house in Nottingham faces east with a 40º pitch and generates 3,300 kWh annually.
Figure 5. Theoretical thermal contours. The ground temperature is obtained once a week by churning the ground loop around (cold) for 20-25 mins until an average figure for the whole loop is then recorded. The GSHP is switched off 4 hours earlier to allow ground temperature contours to flatten out. (diagram by author)
Rybach , and Trillat-Berdal, Souyri et al.  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 ,, 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 TrillatBerdal , Chiasson  and Kjellson  have been based on computer models with realistic weather data. Kjellson  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 addition it may give a possibility to use shorter boreholes and higher heat extraction from the borehole.
This number provides the target to meet with the GSHP consumption. The author’s research project was to reduce the heat pump power consumption until the figures equate. If the annual power consumption of the GSHP is also in the range of 3,250-3450 kWh, then the workload of space heating and DHW is not emitting Carbon, measured annually. For the Nottingham house, the annual electrical consumption of the GSHP was in the range of 4,800-5,200 kWh prior to the project. Since early 2010, a combination of solar augmentation and more disciplined thermal management has reduced the total GSHP consumption to a range of 3,2503,450 kWh, thus meeting the target. The proportion of the GSHP workload devoted to house-heating is approx 2,400 kWh, so with quiet satisfaction, it can be considered that the house meets carbon zero targets, based on a zero balance of metered kilowatt hours. D.
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, controlled by a thermostat and solenoid valve. They are self designed and built by the author. These have now been running for more than a year and have revealed good performance data.
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 neighboring boreholes all receive equal thermal storage. 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  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 a smaller number of 60m or more. C.
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 installing 22 photovoltaic panels. In the UK, domestic PV installations are limited to 4 kW by the Feed in Tariff system. Larger ones
Figure 6. Schematic circuit diagram of solar ground charging system (by the author). A normally operating conventional ground loop is as shown (left). The entire ground loop is diverted through the Sunbox when conditions are good for solar thermal charging (right). If liquid from the Sunbox is too hot, it does not harm the GSHP because it is passed through the ground loop first.
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. If days are grey and cold, the GSHP uses heat stored in previous days or weeks.
• In Equinox, this provides diurnal augmentation. Daytime heat is stored, the region immediately around the pipes is warmer than the surrounding soil, and heat retrieved for use in cold evenings. • In Summer, the system quietly continues for months to store heat into the borehole for use during the winter. Air heat or direct solar heat are all welcome. The earth does not get ‘hot’, the low grade heat moves further out into the volume around the pipes.
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 greenhouseeffect, enabling the system to work in equinox and winter.
Figure 8. The first design (2010) of South facing solar sunboxes, with reflectors (first version). (photo: author)
The owner obtained planning permission to exceed 200mm, and during the summer of 2011, the boxes were completely de-constructed and rebuilt as one single large box with sloping front faces of 70 degrees, to increase solar capture in all seasons, and to increase the air volume in the boxes relative to the liquid volume in the black chillers. The volume was increased from 1.1 cu metres to 2.8 cu metres. The envelope is of triple wall polycarbonate, with no-coldbridge design at the corners. The projection is 700mm.
Figure 7. Cutaway image showing link from sunboxes to heat pump (diagram by author)
In March 2010, the first design was constructed as two small glazed solariums (which we call ‘Sunboxes’) surrounded with aluminium alloy reflective panels. These reflectors enhanced solar capture compared with their performance without the mirrors. The sunboxes were made of a single skin of 6mm structural polycarbonate with aluminium framing. 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, after sunset. After 6 months of testing, the aluminium reflectors were added in October 2010, boosting their performance slightly. Angling the front face to 70 or 75 degrees would have been more effective, but British planning laws limit the projection to 200mm.
Figure 9. Construction process of the improved design for Sunbox system, combining two small ones into a single large volume, Aug 2011 (photo: author).
The chillers within are 4 square metres of black polypropylene swimming pool heater panels, donated by Ice Energy of Oxford, England. They are directly plumbed into the ground loop of the GSHP. The system is thermostatically activated when conditions are good – 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 links the ground loop to the solar sunboxes.
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 form. If conditions are improving, the Sunbox warms up, and the thermostat restarts circulation. This process has works for an average of 2,400 hours/year. 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). When the heat pump is sleeping, the flow rate is slow, at 5 litres/minute. When the GHSP is driving the circuit, the flow rate is boosted to a rapid 15 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.
It is estimated by the manufacturer that the GSHP must draw 9,000-10,000 kWh from the earth.  The Sunbox system has stored >3,000kWh of heat below in the first year. Therefore, there remains a large contribution of approximately 6,000 kWh that must come from ‘Mother Nature’. Prior to the Sunbox system, the estimated average COP was 2.6-2.9. Since the Sunbox system was commissioned, the annual consumption for GSHP has declined to less than 3,350 kWh/year for all heating, hot water and circulating floor pump. At the time of writing, the precise figure is 3,238 kWh, below the target range. This figure could be reduced by 300 kWh if the power demand of the circulating floor pump is deducted. The simple arithmetic from yearly meter readings suggests that the COP over one year is now averaging at about 4.4, far higher than its practical expected performance – from observations, it appears to be artificially boosted by the active contribution of direct solar heat. See the Metering records 
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.
Figure 10. Completion of improved Sunbox design Aug 2011 with weather station above. (photo: author). The 700mm projection provides shading to the first floor south facing windows in Summer.
3. Annual Performance A Monitoring of system and ground 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. Meters are either the official government approved meters supplied with the house or the PV system, or are comparable products of reputable manufacturers such as SuperCal, Landis & Gyr, or Elster.
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) The Sunbox system was installed in March 2010 (black line). Thereafter, the GSHP worked with less electrical consumption even though the 2010-2011 winter was more severe than the previous year. (diagram by Author)
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 improving the COP. During December 2010, the coldest UK December in 150 years, the deep ground temperature did not fall below 10.0ºC, and 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 14.0 ºC. However, the short term effect in Diurnal and Real-time operation is to provide a ‘warmer-than-the-surroundings’ zone of 16-17 ºC immediately around the borehole pipes, 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 which follows performs a rapid restoration of temperature in the immediate local zone around the pipes.
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 delta-T, it averages 1.85kW. 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.
Figure 12: Deep ground temperatures measured since August 2009. The Sunbox system was installed in March 2010 (black line). 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 2010-2011. (diagram by Author). The overall curve is smoothed, with flatter troughs, although the ground has not got significantly hotter at the peaks.
The storage of heat underground appears to be efficient when used this way, providing the heat does not get lost to water courses. Under the Peveril Solar house, the soil is solid clay-rock mixture all the way down. If the rate of storage is >3,000 kWh/year and the rate of withdrawal is 9,80 0 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.
Figure 14. The control for the sunboxes is an AKO thermostat, able to judge two channels (actual temperature or delta-T). If conditions are right, it directs electrical power to the solenoid valve, which relays power to the pump. Even in February (as in the photos above when the external air temperature was 10ºC), heat is sent to the borehole. Above: temperature 30ºC in Sunbox. Below: temperature 15ºC of liquid flowing into borehole. (Photo: Author)
B Comparison with solar thermal for water heating The annual performance of using solar thermal panels to heat the ground is higher than heating water tanks. This compensates for the potential risk of heat-loss of heating an un-insulated mass. The ground is too large to suffer from ‘stagnation’. 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 drawing off hot water, there is no delta-T and the pump does not run. This is stagnant hot water.
Figure 13. Combined graphs of Sunbox, PV and Degree days reveal interesting peaks of the sunboxes at Equinox times, and achievement of higher kilowatt hour capture than the PV during the winter months. (diagram by author)
The activity of the 30W pump in the system is largely met by the large PV roof because the majority of its 2,400 hours of working time is in daylight. The thermal performance of the Sunbox system varies seasonally (Fig. 13). During Summer, it averages 1.1kW,
Charging the ground is effective because the ground is cold and vast, and for many more hours there is a beneficial delta-T, even during sunny winter days. The British SAP calculator  assesses the contribution of 4 sq metres of solar thermal panel or evacuated tubes to be in the region of 1,000 kWh/yr. The Sunbox system in the author’s project has run for 2,400 hours in a year and buried more than 3,000 kWh in their first year. The author hopes that with the new Sunbox design, performance will improve. Greater air tightness, a skin of insulated polycarbonate and angling of the front face, have improved thermal capture by 7.5% compared with the previous design. A doubling of the panel area to 8 sq metres would be advisable in future.
The author has installed a similar sunbox at a house nearby, using metal chillers. The occupant has a GSHP with twin 60m deep boreholes, and large amounts of PV installation. This has not been rewarding because the occupant has been lazy about sending in data records, and the occupant did his own plumbing and made some mistakes, causing loss of pressure in the ground loop.
B 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 solar thermal panels. Figure 15. The ‘virtual pentangle’ of House in the centre, with Grid, PV power, GSHP, Borehole and Sunbox system working together.
A PV limits and Comparable systems The solar project on the Peveril Solar house in Nottingham is the only example known to the author of implementation on a single house. The ‘virtuous pentangle’ of PV, GSHP and solar augmentation 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 make the best of what is there, and to decide what can be fitted. The average British family household uses about 3,500 kWh for annual lighting, cooking and power. As 4 kW of PV can produce about 3,300 kWh, this would require the house to use less than zero for heating and hot water, which is not possible. Even the Passivhaus standard permits a house of this size to consume 15 kWh/sqm x120sqm = 1800 kWh/year for heating.  A 6 kW photovoltaic installation could produce 5,000 kWh at the latitude of Nottingham , and with this quantity, the household could balance its entire power equation of consumption and generation – 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. There is no users’ club or association through which to make contact with other owners.
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 detached dwellings. Successful underground thermal storage schemes have worked for groups of houses in Sweden  and Canada . 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 heat underground, annually. There is no risk of ‘stagnation’, due to the infinite size of the ground and the area of a PVT array is likely to be 20-28sqm instead of the 4 sqm on the Peveril Solar house.  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 garden or a front drive. There are few houses in urban areas with enough space for horizontal ground loops. Boreholes cause least risk to foundations, compared with deep trenches. 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 can be applied to horizontal ground loops if they are insulated above, or to clusters of foundation piles under a building. 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. By the late 1990s, it was law, and universally fitted. It took 40 years from invention to becoming a mandatory requirement for all cars. Using financial incentives, the government could require GSHPs to be augmented, if ground conditions permit. 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. With suitable adaptations to the plumbing, it can be done with solar flat plate panels, arrays of tubes or PVThermal panels. The most important things are to design the building well first, then get free solar heat by any means.
Acknowledgements The first named 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 Mempuou of the University of Nottingham, UK.
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