cibse article for liverpool symposium april 2013

CIBSE Technical Symposium, Liverpool John Moores University, Liverpool, UK, 11-12 April 2013

of 17

DOMESTIC SOLAR EARTH CHARGING:

MODELLING THE PROCESS FOR AUGMENTATION OF HEAT PUMP

Architect David Nicholson-Cole with the support of Professor S. Riffat. Department of Architecture and Built Environment, University of Nottingham

Email: [email protected] ABSTRACT

This paper is a progress update on the solar thermal augmentation system for ground source heat pumps using a solarium-style polycarbonate wall-mounted ‘sunbox’ to recharge the ground, previously reported at CIBSE/ASHRAE 2012. The system is running on the author’s house in Nottingham, England. The house has a ground source heat pump, using two vertical boreholes. Real-time, diurnial and inter-seasonal solar charging processes restore the energy level in the earth, thus preventing progressive chilling of the borehole zone. This has improved the performance of the heat pump to a degree that if balanced with the 4kW PV array, it be said that the house is net-zero – generating more than it consumes, a solar fraction averaging 120% over 2.5 years.

Since CIBSE/ASHRAE 2012, there is now sufficient real data collected to construct a computer simulation, to see how it compares with the real patterns of weather and behaviour. The modelling process attempts to illustrate the effective relative energy levels from year to year and to identify the thermal elasticity of the borehole. Since CIBSE/ASHRAE 2012, there have been technical developments of the system. One has been the addition of two additional methods of solar capture. Vacuum tubes and a new roof-mounted sunbox with metal radiators are working to the same boreholes, with independent controls, pumping and metering. The previously reported wall-mounted ‘sunbox’ has been re-fronted with double skin stretched ETFE, and the paper will include a short report about adapting ETFE for use in a solar collector. The three solar collection methods provide a way of comparing the cost effectiveness, technical and visual impact of the technology of solar augmentation of a heat pump. Keywords COP, ground source heat pump, solar thermal, underground thermal energy charging, low carbon, photovoltaic, vacuum tubes, solarium, ETFE. 1.0 INTRODUCTION A previous paper by the author in CIBSE/ASHRAE 2012 [1] reported on an augmentation system for ground source heat pumps (GSHP) using wall-mounted solarium style panels to recharge the ground, thermally – referred to as the ‘Sunbox’ for the remainder of this paper (in 2010). In addition, in 2012, the author has added vacuum tubes to the east facing roof, and an additional roof-mounted sunbox to the south facing house extension, also connected to the ground loop of the heat pump.

This installation is domestic scale, operating as a real-world, real-time, full-scale lived-in experiment. The house construction is a regular British developer house with better than average insulation. The author has used ‘Active house’ [2] concepts to bring the house to Net-Zero – powering ‘up’ by gathering solar electrical energy, and powering ‘down’ by reducing the power consumption of the house and heating. This experiment has shown that PV and Solar augmented GSHP can be effective as a new build or partial retrofit solution.


of 17

The house is on the gas grid. At the time of house construction in 2006, the

justification for a GSHP instead of a gas boiler was that a mono-fuel based system, electricity, is preferable to active burning of fossil fuel sources for many reasons, cited in previous papers. It also gave the author a chance to experiment with solar energy technologies and try to reach Net-Zero. 1.1 Background: The House and Borehole The author’s occupied detached house in Nottingham, England is a 'developer' property built in 2006-7 with brick-block walls, 100mm cavity-fill insulation, and 400mm in the roof space. Total floor area is 120 square metres (Fig.1). It is not practical to insulate it further, other than hunting down air leakage gaps. It has a Swedish manufactured 2kW/6kW ground source heat pump with an integrated 185 litre water tank (Fig.2). The house has under-floor heating on ground and first floors. The originally estimated total heat and DHW requirement is 14,600 kWh. Prior to this project, the GSHP had an estimated electrical 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). The owners have an economical lifestyle.

The ground source heat pump was commissioned in March 2007. The thermal source is a twin vertical borehole set, 48m deep in dense clay-marl soil, encompassing approximately 3,600 cubic metres. The surrounding mass is infinite, i.e. there is no insulating envelope. The soil in the location of the test house is fairly uniform, consistent clay-marl, according to the report of the drillers in 2006.

The ground loop is a closed circuit of plastic pipes, of which 90m can be considered to be the active length (surface level pipes being seasonal), and the peak demand on the borehole is 4kW. This requires the ground to deliver an average of 44 watts/metre, which is safe for the dense clay-marl of the site, according to the Veissman Technical Guide. [4]. However, there is always room for improvement in performance, and the author’s research addresses solar augmentation of the GSHP.

Figure 1 - Peveril Solar house from the SE, Dec 2012. The solar heat source is combined from Sunbox, vacuum tubes and

a new small sunbox (Photo: author, 2011)

Figure 2 – IVT Greenline C6 GSHP integrating a 185L water tank. (Diagram: IVT website)

1.2 Cooling of the ground The author (house owner) is concerned that over successive years, there is a risk of deep ground cooling, reducing the COP of the GSHP. The theoretical diagram (Fig.3) 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. With a lower ground temperature, the heat pump operates less efficiently


of 17

than when first installed, and consumes more electricity. Some heat pumps (including this one) have frequent occasions when the heat pump fails to achieve a satisfactory rate of thermal extraction. This triggers direct 1:1 heating using its ‘Additional Heat’ function. In cold winter periods, this could cause a surge in grid demand when heat pumps (air or ground source) become more commonplace. During the first two years of installation (2007-2009), the heat pump run for 222 hours/annum in ‘additional heat’ mode, equivalent to an additional 900 kWh/annum. Since solar charging was introduced in 2010, it has used none.

Figure 3 – The dark line illustrates two theoretical patterns of declining ground temperatures over several seasons until a new low temperature equilibrium is reached. The orange line represents the temperature curve with solar charging (Diagram: author and C. Wood)

The theoretical shape of the curve is derived by adapting the pattern obtained by frequent measurements of deep ground temperature (Fig. 4). Starting from the end of Summer, the pattern is of a rapid fall-off once the Autumn heating season starts and solar radiation reduces. A lowest winter temperature level is met in January-February. As Spring brings warmer daytime weather and reduces the amount of energy withdrawn by the heat pump, the low temperature of the ground has a strong delta-T with the infinite surroundings, and external thermal energy is pulled in, elastically. The energy level of the ground recovers slowly in Spring because air temperatures are still cold and there is still a requirement for evening heating. The spring curve resembles a long slow climb-out, drawing energy in from the surroundings until it peaks in early September just before the following heating season.

Figure 4 - Ground Temperatures recorded at weekly intervals from August 2009 to end of January 2013. For three successive seasons, the summer high has been nearly 14ºC. For three successive winters, the winter low was higher than 10.0ºC, and when there was no solar charging the winter low was below 5ºC. The line and arrow indicates the date of installation of the solar augmentation system. (Diagram : author)


of 17

The temperature curve above is a record of real temperatures over the last three years, including three winters affected by solar earth charging. The first winter illustrates how the curve looked when there was no charging. It is regrettable that deep ground temperatures the previous years, 2007, 2008 and half of 2009, were not recorded.

In the first two years (2007-2008), the GSHP offered no energy or cost saving compared with a conventional heating system due to the higher price of electricity compared to an efficient gas boiler. In August 2009, the solar augmentation project started. In 2012, the author set out to use 32 months of collected data to construct a computer model illustrating the energy levels in the borehole and establish the curve-shape for the energy level in charged and uncharged boreholes. ENERGY MODELLING 2.0 The need for a model The domestic market for air-source heat pumps is now well established and growing, but the market for ground source heat pumps is less lively because it is associated with higher cost for no great advantage in performance. If the efficiency of ground source can be significantly improved for a reasonable cost, then advantage is returned to the ground source, especially in climates where winter air temperatures make the air source heat pump struggle.

Valuable work has been done in Canada, Italy and Sweden [5],[6], 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 [6], Chiasson [7] and Elizabeth Kjellson’s PhD [8] have been based on computer models with realistic weather data.

The solar augmentation project on the Peveril solar house was initially based on a logical hunch, rather than years of preparatory modelling. Based on common sense and preliminary literature search, the author had no doubts that an amount of augmentation could have some effect, but there are few precedents to quantify the amount required or the effectiveness, or precisely how to achieve it, technically in plumbing and control. Even if a theoretical model is developed on a computer, a real-world working installation would have to follow, to prove that the system can work reliably in real conditions with real plumbers, electricians and inhabitants, because a thermal model makes too many simplified assumptions.

In the real world, a commercial heat pump installer or purchaser would not be prepared to install a solar augmentation system unless a researcher first took the risk of modelling and/or installing a real-world system to prove that it could work and be reliable. The author has taken both these risks, and achieved net-zero in so doing.

The operating characteristics and the efficiency of any heat pump are largely determined by the temperature range over time of the large but low-temperature heat source, be it air, earth or water. The rule of thumb for ground source heat pumps is that the COP is boosted by approximately 3% for each degree (Celsius) that the evaporating temperature is raised, or the condensing temperature is lowered.[9] The SPF (Seasonal Performance Factor) [10] is a way of computing the efficiency over a long period (preferably one typical year), and varies depending on the balance of demand and energy level. If the COP is regularly stimulated by solar gains, then it is to be expected that the effect on the SPF over 12 months will be beneficial and quantifiable.

Earlier papers from this author about this project (2010, 2011) reported that solar augmentation has been effective, based primarily on meter readings. For the


of 17

first 2.5 years of the Peveril solar house, the GSHP had previously averaged an annual electricity consumption of 5200kWh. Following the installation of solar augmentation the GSHP has reduced its annual power consumption to an average of 3200kWh (swinging 600kWh either way depending on weather). Other conditions (house size, family size, water consumption, lifestyle) have remained constant.

The installation of a 4kW photovoltaic roof has an effect on the electricity consumption of the house. It has no effect on the consumption of the heat pump, which is governed by the heating demand of the house. PV has no effect on thermal performance, but as the heating is entirely electrically-based, the metered PV output is an ideal yardstick for comparison with GSHP consumption. The annual power generation of the PV roof has averaged 3250kWh in its first three years (swinging 200kWh either way depending on weather). This was contributing power to the house and electricity Grid. This figure balances with the consumption of the heat pump for heating and hot water. Both figures in the region of 3200 kWh represent a solar fraction of 100%, enabling the author to claim that the house is net-zero. Taken over the last 32 months, the solar fraction has averaged 120%.

The lowest deep ground temperature at the seasonal nadir of 2010-2011 was 5 degrees C warmer than in 2009-2010, so one would have expected only a 15% improvement. There is evidence of an accelerative effect with solar earth charging which boosts the performance beyond 15%. After heating cycles of the GSHP there is often an immediate response from the solar collector, restoring energy rapidly. It seems from this that the SPF (COP averaged over the time period) improvement is in the region of 40%.

Deep ground temperatures fluctuate depending on thermal capacity and conductivity of the store, and weather conditions on the day of the test. For a project concerned with thermal storage, thermal energy volume is a far more valuable entity to model than ground temperature. If the thermal store is untouched for several hours, no inputs or outputs, the temperature readings become more smoothly matching to the curve of energy levels.

A dynamic model of the energy flows is the next step.

2.1 Modelling – theoretical dynamic or real-world measured? Earth Energy Designer [11] is a well-known modeller for estimating borehole size. Degree days [12] give a measure of air temperatures. Records of a typical photovoltaic installation for previous years can give one a sample of ‘Sunniness’. However, this energy storage project requires an analysis of an intermittently and seasonally solar charged heat borehole. There are no reliable yardsticks for theoretical modelling of this particular system. There are too many unknowns and variables especially with a deep borehole that one cannot visit, that is infinite in size, uninsulated, and may have a variety of soil layers and conductivities below.

Therefore, a real-time real-world rig with data recording over several seasons is a means of testing the idea of solar charging. With normal weather variations, it should take at least 3 years of data to prove it with a real-time model. In August 2009, the funding was available to make a start on the PV and the solar augmentation. Three years has now been achieved. As it was a prototype, it was important to take the installation process slowly so that all the system design aspects of the prototype are resolved without too many diversions, errors or abortive costs. It was important to maintain a diary of the decision making, design and build process. [13] The system began running in March 2010. With data collected over nearly three years, it is now possible to analyse the readings, and develop a model that illustrates the performance with a performance curve, and estimates the comparative energy


of 17

levels in the earth. During the 3 years of data collection, the author’s family have experienced the improved performance and cost savings.

This energy level has to be ‘comparative’ because there is no way of knowing the precise energy level of an infinitely sized uninsulated store, but one can observe a pattern of change over the seasons. This does not eliminate the role of informed guesswork, but the data used is real, so an authentic pattern has emerged. The primary ‘knowns’ are the metered inputs and outputs. The greatest ‘unknown’ is the rate at which ground naturally replenishes heat energy, following a long winter and spring. The model goes some way to establishing a methodology for discovering how to quantify this unknown.

In April 2011, the author attempted a modelling exercise for the project, but was defeated by three things: 1, an insufficient length of time of record-keeping to produce worthwhile results; 2, an over complicated approach, in attempting to form a three-dimensional object representing the borehole in its figure-of-eight shape, to make it animate over time, and to calculate temperature (not energy level); 3, not sufficiently appreciating how important it is to have a thermal model alongside the real-world system, and giving up too easily when it was first tried.

In July 2012, the house was visited by Prof Ala Hasan of Finland [14], who encouraged the author to make another attempt. A wholly new start was made, with the more modest intention of achieving a 2-dimensional curve based solely on comparative energy levels. With a more vigorous approach and richer store of data, it only took a few days.

Figure 5 – The final diagram from the modelling project illustrates comparative energy levels, not temperature. Blue is the energy level without solar charging, and the orange zone represents the effect of charging. (Diagram – author)

2.2 Modelling – curve shape and energy volume The diagram above (Fig.5) sums up the entire modelling project. The curve shape is similar to the temperature curves shown earlier (Figs. 3 and 4), with the characteristic steep fall-off in early Winter and the long climb-out during Spring and Summer. The model represents energy levels in the borehole, based on data collected from meters at daily intervals over nearly three years. The blips in the curve faithfully illustrate some of the variations in system performance and the weather that Nottingham has experienced during the past three years.

During the long summer of solar thermal charging, it appears from the records that the ground does not get ‘hot’. It reaches a stable summer temperature and the volume that exists at that temperature gets larger, representing a larger energy volume, but not one that is hot enough to lose its energy quickly. Eventually, it can be assumed that surplus heat must get lost to the surroundings. [15]

In a short heating cycle, the heat pump is boosted by a higher temperature in the source, but over a period of a whole winter, the more important requirement for


of 17

the heat pump is the volume of energy available. Starting from a base of 12ºC, A cup of very hot tea at 90°C has less thermal energy than a chilly 300 litre water tank at 15°C. The 3600 cubic metre underground store at a stable 13.8ºC has a vast amount of energy compared with a summer norm of 12ºC or with a winter low of 5ºC. If the heat pump demands energy during a heating cycle, it lowers its ground loop output temperature, thus increasing the delta-T, and thereafter drawing more energy from the larger volume. The temperature immediately around the vertical pipes is temporarily lowered, but after a period of rest, the local energy volume will restore the temperature.

2.3 Modelling – choice of software – program or spreadsheet? The software chosen for the modelling was GDL (Geometric Description Language), the parametric programming language of ArchiCAD [16], one of the leading BIM architectural packages (Building Information Modelling). GDL [17] is used by ArchiCAD users for constructing objects, using a scripting language which describes forms, shapes, surfaces, text and more. It can be used for doors, windows, stairs, trees, metalwork, mechanisms and more, including 2D objects. GDL does not run independently, it has to be used when the environment of ArchiCAD is running. GDL uses a long established syntax which originated as a form of BASIC in the 1980s, but developed its own culture thereafter. With GDL, the program can read in a long data file of meter readings, store the numbers into arrays, apply algorithms, and then organise the results into curves and polygons.

The author has written two books about GDL [18], so is comfortable and confident using it. Some experts in Excel might be able to perform the same modelling, but for the author, a traditional programming approach comes most easily. The data is first prepared in a spreadsheet, and is then copied and pasted into a comma-separate data-file that the program can read. The program is capable of building a chart with plenty of options for titling, colour, pen thickness. The program permits a parametric approach, whereby one can tweak some parameters to compare with known or expected curves, or build in additional parameters such as a percentage for system losses. The key part of the algorithm is just a few lines of code. The resulting chart is instantly re-computed.

With the hindsight from the programming process, the algorithm could be applied in an advanced spreadsheet with tabbed pages and a final chart. The raw data is already in a spreadsheet, and additional columns could contain formulae to show gradually accumulating energy levels. It would be better to do this for weekly intervals to avoid over loading the spreadsheet’s charting routine.

2.4 Modelling – data collection The volume of data collected over three years is enormous. Having been collected manually from meters about 4-5 times a week, it is less raw and less voluminous than that from a data logger.

The illustrations below are small glimpses of the format of the data. Another tabbed page in the spreadsheet reads the meter reading columns and creates a new clean spreadsheet page purely of the data required for the model. The cleaned up spreadsheet includes columns of commas, so that the entire spreadsheet can be copied as a large text file with tabs and commas, and pasted into a GDL readable data file. The GDL routine reads in the data, considering only the commas to separate the numbers. The numbers for the dates and meter readings are stored in arrays. There need be no practical limit to the array size. It could store data for three years, or twenty years or more than a human lifetime. With perhaps 300 readings a year, an array size of 10000 is enough for more than 30 years.


of 17

Figure 6 – Metering records: frequent readings are taken of every significant meter, 4-5 times a week. The modelling program reads the dates and interpolates daily averages.

Figure 7 – Preliminary data conversion: the second page of the spreadsheet reads numbers and dates from the metering spreadsheet, and inserts commas between each number.

Figure 8 – GDL file: the columns from the cleaned up spreadsheet are copied and pasted into the GDL data file, as numeric text, with comma separation.

Figure 9 – GDL script: the arrays are declared, the data file is opened, and the data is streamed into the array and given a variable name, such as ‘gshpmeter[k], daynum[k]’.

2.4 Modelling – inputs and outputs Primarily, this stage of the model only has to know three things per time interval:

1. The electrical energy used by the heat pump, 2. The thermal energy injected into the borehole, 3. The day-number, so that the data can be formed into a timeline.

Even if a few days are missed out due to holidays or absence, the data includes the year, month, day, and the spreadsheet computes a ‘day-number’, so that correct interpolations are always made, and the day-unit spacing in the X-axis is consistent. Variations in month-lengths are thus taken account of. The model would work just as well if only a once-weekly reading was taken, although it would not have such fine levels of detail in the ‘blips’ of the curve to reveal variations that occurred due to weather or heating demand.


of 17

The model hypothesizes a theoretical bulb of energy in and around the borehole, and it needs to consider three things:

1. The thermal energy extracted by the heat pump from the borehole, 2. The thermal energy injected into the borehole, (already known) 3. The amount of energy that the borehole is assumed to draw in from or lose out

to the infinite surroundings. (This is the bit that moves into unknown territory.)

When the author first considered the model in 2011, the shape considered was the same as the real ‘figure of eight’ twin borehole. The calculation of the changing volume and surface area was of a 3 dimensional complexity that defeated the more purposeful task of calculating the energy flows.

Figure 10 – 3D representation of the borehole: Left as a single deep cylinder than can expand and contract outwards but not downwards. Right, as the actual figure-of-eight twin interlocking boreholes. (Diagram and model by author) Figure 11 – Dialog box with some of the parameters that can be tweaked to assist the algorithm. See section 2.5.

If the Peveril house had a single borehole, the depth recommended by Earth Energy Designer [10] would be 85-100 metres. For this model, a simple single cylinder of that depth is good enough to establish the principle. The theoretical cylinder has a constant depth but of varying radius and surface area depending on its energy level (Fig.10). The depth is constant because the borehole cannot change depth significantly as the energy volume changes. Only the radius changes because the energy that may be lost or gained relative to the infinite surrounding is next to the borehole, not below it. Therefore the changes in volume are proportional to the

square of the radius because the cross sectional The electrical energy used by the heat pump can be converted into an estimate of

the thermal energy withdrawn from the earth. The algorithm of the model has to make an assumption about the COP of the heat pump. In reality, COP varies hour to hour, based on workload and temperature of the source, but for the model, one has


of 17

to apply a constant such as an SPF (seasonal performance factor) that represents the average COP of the heat pump during the time of the test. If the assumed COP/SPF is 3.0, then a 2.0 kWh electric reading for that day is taken as 6.0 kWh in total, which is interpreted as a withdrawal of 4.0 kWh from the borehole. The figure of 4.0 goes into the array.

The next input to consider is the amount of energy that is drawn back to the ever-changing energy volume of the borehole zone. The author considers the analogy of elasticity, the eagerness or the reluctance of an object to return to its previous shape when deformed. This energy comes from the infinite surrounding which is assumed to be in the temperature range of 12º-12.5ºC. When the immediate energy bulb is comfortably large, but at a temperature of 13º-13.5º, the delta-T with the surroundings is small and the surface area to volume ratio is also small, so there is little elastic tension to pull energy in or lose energy out. At the lowest point of a heat pump’s heating season, in February, the energy bulb immediately around the borehole pipe’s may briefly be between zero and 5ºC, and the surface area to volume ratio is very favourable for pulling energy in from the vast surroundings, and energy will flow in more quickly.

2.5 Modelling – significant parameters The algorithm is seeking to recalculate the energy volume. Because the cylinder depth is fixed, it only has to calculate the radius of the energy bulb, at each time interval. For this, more assumptions are needed, which the model must be told. (Fig.11) These cannot be known precisely because the mass is un-insulated and there are unknowns. One benefit of a parametrically scripted model is that you can test alternative figures and examine the results. • The Peak Energy Volume: is the point beyond which you get energy losses if you

push more solar energy in. It is also the maximum energy level that the volume will try to swell elastically back to if the volume has shrunk to the extreme of winter. It is higher than the earth would reach if there were no solar charging. This is matter of judgement and guesswork, but a starting figure was 10,000 kWh. This guess was based on an estimate that the active borehole volume is 3,300 cubic metres of clay, having a thermal capacity of 0.5 kWh/ cubic metre/ deg K, and a 6 degree range before the energy volume is stressed elastically. The model is parametric, so if this guess is wrong, the curve shows it, and one can try a revised figure. At the peak energy level of summer, in early September, the surrounding ground has also been recharged naturally, so the delta-T between the borehole and the surrounding is small – reducing the risk of losses from the energy volume at the end of summer.

• Starting Energy Volume: The model starts in Autumn 2009 with a peak summer energy volume that assumes that no solar charging has been applied for the previous three years. This is nominated at 84% of the peak energy volume, at 8,400 kWh. The assumption works well in the model.

• COP: The assumption for COP (SPF) of the heat pump has been discussed above. The model’s algorithm provides a choice to calculate two curves, one based on ‘sunbox not existing’, and the other with the ‘sunbox working’. As the electrical figures are actual meter readings from the heat pump with the sunbox working, this introduces a slight inaccuracy, because the figures for the electrical consumption should ideally be what they were in 2008. It would be one tweak too many to try to fabricate a set of electrical consumption figures assuming that no augmentation has taken place.

• Recharge Adjust Factor (RAF): A key parameter for this model is what might also be explained as the ‘index of elasticity of thermal conductivity from the surrounding


of 17

mass’. As with elasticity, an inverse square law is considered for this. As one is squaring quantities in thousands with every day’s iteration, and then dividing, this RAF figure is in millionths, but how many? Not enough, and the volume never recovers, and shrinks to zero in the second year. Too much, and it swells far too quickly to an unrealistically high peak. The expectation is that the curve has some resemblance to the temperature curve. Running the program until the curve begins to resemble the curve of recorded ground temperature, it appears that a range of 35-50 millionths gives stable results.

• Sunbox system losses: It is unrealistic to expect every kilowatt hour recorded by the meter to find its way to the borehole, so the algorithm must reduce the solar energy delivered to the borehole. This is a parameter that the user can adjust.

• Sunbox uprate: The present upper sunbox is 4 square metres of collector, and the lower one is 2 sqm. What would happen if additional collectors are added? The solar capture can be increased by a percentage.

One way to judge these parameters is to run the model with ‘sunbox not existing’ and examine the curve. As the heat pump is 5 years old, one would expect the highs and lows to be similar each year, perhaps declining slightly. With practice, one gets the curve of the uncharged energy volume to balance. Then, by permitting the model to include and display the influence of the sunbox, one can see the effect of solar augmentation. (See Fig. 11) 2.6 Modelling – The algorithm A loop runs from the start to the end of the time line, progressively recalculating the energy volume (with and without recharging) at each time interval, usually one day. It adds the energy extracted, the energy injected, and computes from the difference between the energy volume at that moment and the peak energy volume an amount of energy that it will elastically pull in from the surrounding. Borehole depth is a fixed number, so from this, it calculates a value for the changing radius of the energy volume. These figures are stored in a two dimensional array rad_result[2][k]. From here, it is a matter of drawing out the lines and coloured polygons to demonstrate the curve-shape.

Figure 12 – The algorithm is a few lines, in a loop that repeats for each time interval. (Diagram: author)

2.7 Modelling – Two-dimensional diagram The hard work has been done, and it is comparatively easy to draw out the curve. In addition, there are texts for titling and captioning. There are horizontal and vertical lines to mark key moments, such as dates of the original sunbox installation, and a month in 2012 when the sunbox had a leak and was not working.


of 17

With any programming task, one is encouraged to avoid ‘coding spaghetti’ by defining each task as a distinct subroutine. Each point’s location is PUT into memory, and then a polygon POLY2 is drawn, getting the numbers from memory with a GET() statement, resulting in the coloured polygons.

These diagrams are not the entire algorithm, they are screen captures of the programming interface, capturing key parts of the code. Both diagrams: author

Figure 13 – a screen capture of the executive script for the 2D drawing showing how it is organised tidily into a set of subroutines.

Figure 14 – a screen capture of part of the routine that draws out the polygons, using data stored in an array.

Figure 15 – [repeating Fig 5] The final two curves, illustrating energy levels over time, with (red) and without (blue) solar charging. (Diagram: author 2012 of period 2009-2013)

2.8 Modelling – Weather interpretation The final line (red polygon) shows the augmentation effect of solar earth charging. Both lines act as a visual diary of seasonal weather in the UK. 2010 had a very cold spring, a reasonably good summer and a very cold autumn-winter, with the coldest December on record. With ground source heat pumps, there seems to be an inertia of about 2 months, whereby the lowest energy level is during February when the heating is still on, but sunlight has not yet been strong enough to restore energy levels. 2011 had a very mild spring, a good summer and a very mild autumn-winter, so the energy level rose as a combination of reduced depletion and increased solar charging. In 2012, the rainy and cold spring lasting well into July is revealed by a set of wobbles in the graph until air temperatures stabilised and the heat pump no longer required energy for house heating. Has the temperature of the ground risen significantly in summers? It appears that it does not exceed 14ºC. It is much higher than this during the day; often the daytime ‘return’ temperature from the deep borehole is 16-24ºC. The ground temperature is always tested at midnight, with no input or outputs of energy after several hours of ‘rest’. What appears to be the case, circumstantially, is that the


of 17

temperature gradients flatten out, and the energy level in the ground increases by widening, blurring ‘rings of warmth’ and filling an enlarged radius at just below 14ºC. It may also be that with successive years, the reserve of energy deep down is built up, making the borehole more resilient if one particular winter is colder than usual. This appears to be happening now in Spring 2013. The rainy summer and autumn of 2012 and the sunless spring of 2013 are causing the energy level to reduce, but it remains secure enough for the GSHP consumption to be comfortably stable. This observation can only be converted into certainty with another two years of monitoring, and another real-world installation, with buried thermocouples. 3.0 Solar sources – Vacuum tubes The existing sunbox can be characterised as ‘Low-Temperature, High-Volume’. It will operate happily for 10-16 hours a day taking advantage of a 3-4 degree delta-T, and store 8-16 kWh/day even in bright conditions without direct sunshine.

The author is frequently questioned by visitors and critics as to the possibility of using industry-standard products to achieve the same end. Can a ‘High-Temperature, Low-Volume’ technology work to provide energy to a borehole? Since April 2012, the author installed two square metres of evacuated tubes [17], to add to the existing solar circuit. These are high performance vacuum tubes which produce high temperatures providing there is some sunshine.

Tubes are normally installed in a pressurised closed circuit, providing high temperature energy to a water tank. In this installation, they are heating a metal heat exchanger, with daily temperatures of 22-30ºC. The solar controller is set for indirect heating to a swimming pool via the heat exchanger.

There is enough of a human and technical story in the problem-solving process of the installation of the vacuum tubes to justify another paper, but this paper is only to cover the thermal modelling in detail.

In brief, the low-temperature high-volume low-complexity solution, the sunbox, is approximately 4 times as productive during summer, and continues to work in winter, whereas the tubes have done nothing during the winter. The roof surface faces nearly east, so there has been some performance during summer, but in winter, the angle of incidence is too acute to stimulate the tube array into action. Condensation on the interior of some tubes suggests that for some of them, the vacuum is no longer airtight.

Figure 16 - Vacuum tubes, 15 in number closely spaced, totalling 2 square metres. The advantage of the Varisol is that they have a mini-manifold head detail so that by connecting them in series in a modular way, you can have any number in an array, at 70mm intervals. They would be better if vertically oriented. (Photo – author)

Figure 17 – The only effective way to use the tubes is with a heat exchanger. If the tubes are used directly with the ground loop, the tube nodules get chilled by cold glycol, and the controller turns off the pump before the warmed liquid has had time to reach the borehole. The controller can handle two pumps. (Photo – author)


of 17

3.1 Solar sources – the addition of an additional solar collector facing south During December 2012, the author has fitted 2.0 sqm of black painted metal radiators in a polycarbonate sunbox on the south roof of a house extension, constructed during August-December 2012. This has its own metering, controller and pump, and will be monitored and compared with the original sunbox and the vacuum tubes. As this is similar in some ways to the existing sunbox, the author is looking forward to observing the comparisons of performance of plastic and metal collectors. The geometric properties will be noted: vertical wall mounted panels (hand-built) are expected to do better in winter, whereas a unitized roof unit sloping at a pitch of 15º should do better in summer.

(The name Surya simply means Sun-god in Hindi and is the name used by the author to name them.)

Figure 18 – Schematic Circuit diagram of Peveril Solar house solar system. The original sunbox is top left, vacuum tubes are top right, and additional roof mounted sunbox at 15º pitch, bottom left. (Diagram – author)

Figure 19 – View of both sunboxes, the upper one with 4 sqm of plastic panels fronted with ETFE. The roof mounted one is fronted with 6mm optically clear polycarbonate over 2sqm of metal radiators. (Photo – author)

3.2 Change of enclosure materials The front panels of the original wall-mounted sunbox were changed to ETFE (ethylene tetra fluoro ethylene) in place of the previous triple skin polycarbonate. These are sloping at a pitch of 70º and face almost perfectly south. ETFE can be stretched tightly over an metal frame and heat-sealed into a double-glazed unit, or it can be used in the inflated cushion technique used on the Eden Centre (Cornwall) or the Beijing Olympic Aquadrome.

The lightness and transparency of ETFE to solar thermal energy is better than any other material that could be used externally [20]. Its durability, strength, flame resistance and self cleaning properties are exceptionally good. The author was persuaded to make this change despite the cost of scaffolding and re-framing because the sunbox is seen as an ever advancing research project. The original sunbox had already proven itself as a means of reducing GSHP consumption.

The ETFE arrived ready-made in a heat-sealed, double glazing configuration around a welded aluminium frame. It was essential to avoid any sort of drilling, so the inner frames were clamped (not riveted or screwed) into the weathering outer frame.

The front panels were installed at the end of October 2012. More seasons are required to reach a firm conclusion about the ETFE. It appears favourable because in


of 17

successive periods 1 November to 10 February, the sunbox solar capture was 2010-2011 452 kWh; 2011-2012 437 kWh; and the current winter period 2012-2013 480 kWh.

CONCLUSION 4.0 The accelerative effect of solar earth charging The metering of the systems proves that there has been a great reduction in electrical consumption of the ground source heat pump, annually from over 5200 kWh to an average of 3200 kWh. The improvement was expected to be approximately 15%, based on the observation that the deep ground temperature in the lowest part of the winter is 5 degrees warmer than the time before the solar charging was installed (using the previously mentioned rule of thumb that COP is improved by 3-4% for each extra degree C in the energy source). The improvement has been closer to 40%, and ground temperature has been consistently higher than the previous uncharged levels. [21]

Figure 20 – Plan view of twin boreholes illustrates how the region immediately around the pipes may be warmer or cooler than the surroundings. It also show that with twin boreholes, the regions of over lapping circles could result in a cold zone if solar charging does not occur, but conversely can result in a comfortable ‘nursing’ effect, retaining energy between the pipes. (Diagram: author)

Figure 21 – Schematic section: Theoretical thermal contours. The region immediately around the pipe will have warming and cooling fluctuations immediately around the borehole pipes, even during a single day, as the GSHP comes on and then rests, and solar energy is pumped down intermittently, depending on weather. Short term local warmth or local chilling will affect performance. (Diagram: author)

A perceived improvement in annual consumption of 40% needs some explaining. Watching the system as it performs, there is evidence of a short term effect that is greater than that revealed by daily or weekly readings. After a heating cycle by the heat pump, the glycol is briefly cool enough for there to be a good delta-T between the borehole and the temperature in the sunbox. What follows is that for a period after a heating cycle, the sunbox circuit is very busy. In effect, it is performing a rapid restoration of energy level, and it continues until the borehole is closer to the temperature it was before the heating cycle. • During Winter, in daytime, the liquid temperature may be 10-16ºC in the sunbox, enough to provide a rapid recharge for the borehole which may temporarily have been lowered to zero-5ºC. With the good insulation of the house, the heating is only needed 8am-8pm, so it does not have to draw energy from the borehole all night. The borehole temperature has 12 hours of rest allowing it to even out. • In the Equinox, the effect is palpable, with warm daytime temperatures charging the borehole up to more than 16ºC before the cooler evening. • In Summer, there is a continuous dumping of energy to the borehole during the day, with borehole daytime temperatures rising to 16-20º on good days. When the heat pump comes on for hot water purposes, it finds an immediate resource of higher-


of 17

than-expected warmth, it completes the hot water heating cycle very quickly, after which the borehole temperature may still be higher than 12ºC. There is rapid restoration to the daytime summer temperature.

The computer model assumes daily energy quantities injected or withdrawn, over a period of years as a linear process, but it is clear that the accelerative effect brings in a factor of Time that further reduces GSHP energy consumption.

4.1 Question - Alternative circuit, delivering solar energy direct to GSHP? Warmed liquid from the solar panels always goes through the ground loop first before being fed to the GSHP. The author is often asked why the solar panel liquid cannot just be fed direct to the GSHP, to get the most instant delivery of thermal energy. The plumbing would be easy, but operationally it would be too troublesome and the electrics would be more complex. The prime reason it should not is that in summer, the GSHP is on for only about an hour a day for hot water, so the solar panel circuit can get up into the 40s. The sudden introduction of a liquid in the 20s or higher would be interpreted as a serious malfunction by the GSHP, which would promptly result in a shut-down. Secondly, while this could be overcome by using solenoid valves and a mixer valve to avoid the aforementioned risk, that would make the system too dependent on everything working correctly, and difficult to repair when it did not.

The author has experienced failure of a solenoid valve which caused a failure in the pump, which then caused a leakage failure. With the possibility of higher circulating temperatures there is a higher risk of expansion, and leakage through joints. The use of the ground loop as a ‘damper’ is very safe by keeping temperatures low. Solenoid valves can be omitted, with much simpler check-valves employed to make sure that the liquids take the correct route around the circuit. 4.2 Question - Where next with modelling? The model described in the paper is using data from more than three years of real-world real-time observation and recording, producing a chart of what has happened in the past. A future development would be to adapt the modelling algorithm for an un-built installation, and predict the future. A key idea has been established using a real-world experiment, that of quantifying the Recharge Adjust Factor, or the ‘index of elasticity of thermal conductivity from the surrounding mass’. This factor must be peculiar to the soil above which the building is situated. A theoretical source data set could be constructed by using Degree Days and PV records. The model could make assumptions about the likely heating demand of the GSHP considering the thermal properties of the building and the degree days for previous typical years in that location. It could do the same for the likely solar capture from the solar panel or sunbox by combining degree days and PV capture for the proposed orientation. Although it could not provide guaranteed figures, it could indicate a trend, and provide criteria to help the designer to establish appropriate sizes for thermal store and solar thermal panels. This is a direction for future research. 4.3 Concluding statement Thermal modelling has provided a stronger theoretical base for what seemed to the author to be ‘a good idea’ for research in 2009. Solar panels and circuit connection are relatively cheap compared to the cost of a GHSP, Loop and house although there is still a cost. Subject to reasonable ground quality and house design, the author (as an architect) could never in conscience specify a GSHP without including solar augmentation as an essential add-on to achieve a performance benefit that could enable the building to achieve the magic target of ‘Net-Zero’.


of 17

Acknowledgements The author would like to acknowledge the support of Dr Ala Hasan of the Aalto University, Finland, and of Professor Saffa Riffat of the University of Nottingham, UK.

References Full details and reporting of the project are visible on the author’s website at http://chargingtheearth.blogspot.com

[1] ‘Domestic Solar Earth Charging: Carbon Zero hybrid retrofit achieved by balancing PV with solar earth charging for augmentation of heat pump’. D Nicholson-Cole, CIBSE/ASHRAE Technical Symposium, London April 2012. [2] 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 [3] Veissman Technical Guide 08/2006 p19 [4] Active House concept explained http://activehouse.info/vision [5] Anneberg residential area evaluation: Anneberg, Stockhom, Sweden http://www.ateik.info/northsun2005/pdf/plenar/Magdalena_presentation_North_Sun.pdf [6] Drake Landing Solar community, Okotoks, Alberta, Canada. http://www.dlsc.ca/how.htm [7] Trillat-Berdal, V., B. Souyri, et al. (2007). "Coupling of geothermal heat pumps with thermal solar collectors." Applied Thermal Engineering 27(10): 1750-1755. [8] 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 [9] Yumus A. Cengel & Michael A. Boles, 1998. Thermodynamics: An Engineering Approach, 3rd Ed. WCB/McGraw-Hill, Boston, USA. [10] http://www.engineeringtoolbox.com/heat-pump-efficiency-ratings-d_1117.html [11] Earth energy Designer (http://buildingphysics.com/manuals/eed.pdf) [12 Degree Days on line calculator] http://www.degreedays.net/ [13] Diary of the process http://chargingtheearth.blogspot.com [14] Prof Ala Hasan, Aalto University, Helsinki, Finland (president of the Nordic affiliate of IBPSA, the International Building Performance Simulation Association) [15] Author’s blog: http://chargingtheearth.blogspot.co.uk/p/key-performance-indicators.html [16] http://www.graphisoft.com/products/archicad/ [17] http://www.graphisoft.com/products/archicad/parametric_objects/ [18] GDL Cookbook (2001) D. Nicholson-Cole, Marmalade Graphics, Object Making with ArchiCAD (2001, 2004) D. Nicholson-Cole, Graphisoft, Budapest. [19] Varisol tubes http://www.kingspansolar.com/products/varisol.aspx [20] ETFE technical properties: http://www.architen.com/technical/articles/etfe-foil-a-guide-to-design [21] http://chargingtheearth.blogspot.co.uk/2012/07/accelerative-effect-of-solar-earth.html

http://chargingtheearth.blogspot.com/

http://www.decc.gov.uk/en/content/cms/what_we_do/lc_uk/2050/2050.aspx

http://www.engineeringtoolbox.com/heat-pump-efficiency-ratings-d_1117.html

http://buildingphysics.com/manuals/eed.pdf

http://www.degreedays.net/

http://chargingtheearth.blogspot.com/

http://chargingtheearth.blogspot.co.uk/p/key-performance-indicators.html

http://www.graphisoft.com/products/archicad/

http://www.graphisoft.com/products/archicad/parametric_objects/

http://www.kingspansolar.com/products/varisol.aspx

http://www.architen.com/technical/articles/etfe-foil-a-guide-to-design

http://chargingtheearth.blogspot.co.uk/2012/07/accelerative-effect-of-solar-earth.html

cibse article for liverpool symposium april 2013

Documents

solar collector

solar fraction

solar electrical energy

house construction

authors house

house extension

active house

ground source heat pumps