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Solar earth charging: Peveril Solar, Nottingham • SET Vancouver 2012 : Paper 225 • David A Nicholson-Cole (with Prof S Riffat). • • Architectural staff in the Dept of Architecture & Built Environment, University of Nottingham

• Peveril Solar house, Nottingham • Solar heated all year round • Net-zero, due to credit balance of PV generation

and annual GSHP consumption • Project from Aug 2009 to present day. • Multiple solar systems tested - Sunbox, Tubes, PVT. • Three years of monitoring meters.

Sunbox is an array of swimming pool panels contained in an insulated polycarbonate solarium connected to the ground loop. The author is unable to travel to Vancouver due to the illness of his wife. I’m grateful to the kind person who is reading this. The Peveril Solar house in Nottingham is entirely solar heated, all year round, with better than Net Zero performance for heating and hot water. This is a hybrid retrofit, achieved on a well insulated British developer house. The solar project has been going for three years now, principally to apply interseasonal solar charging to a borehole underneath a single house. It uses that heat to augment the ground source heat pump. The Sunbox is a 4 square metre array of swimming pool panels contained in an insulated polycarbonate solarium. The entire system has been hand built by the author, an architect who often wishes he was an engineer.

Solar earth charging: Virtuous Pentangle • Net Zero balance, thanks to

energy storage The Grid • PV generates 3,300 kWh Sunbox • GSHP uses 3,000 kWh +Tubes • Solar stores 3,000 kWh 6 m2 • Net-zero made possible by Solar House Solar thermal augmentation of 2 120 m borehole used by GSHP •Two solar thermal The Borehole systems: Sunbox & Tubes

• Real-time, Diurnial

The Earth

and Inter-seasonal charging • GSHP previously used 5,200 kWh annual, now it is 3,000 kWh. • Accelerative effect of solar charging - 40% improvement, not 15% expected

Clay+Limestone 3600 m3

PV roof 4kW

GSHP 2kW normal 6kW panic

•Accelerative Effect - TIME plays a

part. The energy restoration after each GSHP heating cycle is immediate, leaving borehole better prepared for next heating cycle.

The Virtuous Pentangle diagram illustrates the interactions of the system. The Photovoltaic roof is an installation of 4 kW facing East, but it is enough to bring in 3,300 kWh annually. Before 2009, the heat pump used an estimated 5,200 kWh/year. With solar charging, it is now maintaining an annual consumption of less then 3,000 kWh/year, thus the house is Net-Zero. The deep ground temperature at the lowest point of the year is 5 degrees warmer than in previous winters. One might expect a 15% improvement to the seasonal performance factor. The improvement is measured at 40%. This may be due to the accelerative effect of solar charging. After each heating cycle, the delta-T is larger. Frequently the system will perform immediate restoration of energy levels in the borehole, making it more ready for the next heating cycle.

Solar earth charging: Ground chilling • Borehole chilling effect

worsens over several years.... • ... depends on conductivity of earth, borehole depth, shading of surroundings of building, other heat pump users, rate of extraction etc • Temperatures of deep earth can be measured • Note: Curve shape: •steep fall-off •long slow climb-out. • Energy Volume is more significant than Temperature •This model is searching for Energy Volume

Above: Regular chilling of ground until a new low equilibrium achieved - the GSHP becomes inefficient and uses ‘additional heat’. Left: deep ground temperature curve over three years. Characteristic shape is steep fall-off in early Winter, and long slow climbout during Spring, through to late summer. Below: The curve of Energy Volume in the Model

Deep boreholes for ground source heat pumps are at risk of progressive chilling. They are dependent on decades worth of slow natural solar charging. A house with high demand for heating or hot water will cool the ground enough to lower the temperature to a point where the heat pump is struggling. It may seek ‘additional heat’ by one-to-one electrical heating. Ideally, the ground recovers during the summer. If the house is surrounded with other houses covering the ground or using heat pumps, or is situated on east-west streets with tree shading, then deep ground temperature will not recover fully. It will level off at a new lower equilibrium, making seasonal performance of the heat pump much worse. The curve shape of ground temperature reveals a steep falling off in early winter, with a long and slow climb out during spring and summer. Temperature is our way of guessing the energy level in the ground. For a thermal model, the Energy Volume is the only thing that matters, as temperatures vary in rings around the borehole pipes.

Solar earth charging: the need for a Model • Project started in 2009 based on logical

hunch - knowing there is a benefit, but not knowing the numbers. • Real world - Peveril installation is testing Sunbox,Vacuum tubes (and PVT) all in one building. Each is monitored. Too complex for existing spreadsheets to forecast or model. • Theoretical model is needed, to understand energy flows and to predict future system. • For this to progress to manufacture, it needs credible track record of both modelled and installed examples - human & architectural factors needs to be researched. • 2012, the author set out to write the model using Geometric Description Language (GDL), a programming system within ArchiCAD BIM software.

Above: Circuit diagram for three solar systems together. Below: Website for the project, with every thought and work stage recorded. http://

In 2009, the author proceeded directly with a real-world, real-scale, real-time inhabited installation. Life is too short to spend three years monitoring the “Before” condition, thus losing all the benefits of the PV feed in tariff and the saving on the heat pump operation. He was convinced that it would work, without knowing the numbers. Funding was available, so the project began. It has been running for three years and there is enough data from meters to analyse the performance. We now know that it works. A theoretical model is now required for understanding the energy flows. This model enables future installations to be planned, with a sound methodology for calculating solar panel areas, borehole depth, heat pump capacity etc. In 2012, the author set out to write the script for a model, using Geometric Description Language, a programming language within ArchiCAD software.

Solar earth charging: Re-defining Borehole • Real borehole is twinset - 48m deep, 5m apart,

overlapping - ideal for solar charging. • For Modelling purposes, regard the Energy Level as a Virtual Volume, as a single deep cylinder, approx 85-100m deep. • As volume increases, can only expand outwards, not downwards. • Therefore: Energy Volume proportional to square of Radius. • Imagine energy volume expanding and contracting as inputs and outputs occur. • When energy level is low, energy sucked in from infinite surrounding mass. • Surface area to volume ratio changes with expansion/ contraction, proportional to square.

• It’s necessary to assume a consistent thermal conductivity of the earth.

It was necessary to picture the Energy Level in the borehole as a Virtual Volume. The real borehole is a twinset of overlapping cylinders. An attempt to write the model in 2011 failed because the twinset is too complex for a first attempt. Defining the Energy Volume as a single cylinder of fixed depth means that one can calculate the inputs and outputs and add these to the volume. As the volume changes, only the Radius is changing. This changes the surface area to volume ratio beneficially. When the volume is small, the delta-T is high, and it has a stronger elastic pull on the energy around. When it is very large, there is only a small loss to the infinite surroundings. In the real world, earth has layers, but for a model one has to assume consistent thermal conductivity.

Solar earth charging: Data collection, conversion 6. Daily monitoring of all electric and thermal energy meters in a giant multi-tabbed spreadsheet (See it at http:// ) 7. Another spreadsheet reads relevant columns, places in new columns with separating commas 8. Data is copied and pasted into text field of GDL data-file 9. GDL script reads file and puts all data into live ‘Array’ in memory Electric and solar thermal energy meters are read almost every day. These figures are stored in a giant multi-page spreadsheet. If a few days are missed out, the days between are interpolated. Another page in the spreadsheet reads relevant columns from this and places the meter readings into new columns with separating commas. This data is copied and pasted into a text field of a GDL data-file. The GDL script of the energy modelling calculation reads this file, links the data to the time-line and puts all data into Arrays in memory as dates, day numbers and energy flows, inward and outward.

Solar earth charging: defining Parameters GDL allows one to build a parameter table, to display to the user some of the constants required.

•Peak summer energy volume: the

maximum charged level beyond which energy will leak away. •Starting energy volume - at the start of the timeline, in this case Aug 2009. •COP/SPF of heat pump: reading in the electric consumption, the algorithm determines how much energy is pulled from the earth. •System loss: all systems have losses in pipework, and in top part of borehole, and liquid left in pipe after pump stops.

•System upgrade: What happens

if the solar panel area is increased? •Borehole depth: a constant that allows the Radius to be a variable.

The GDL front end allows one to build a parameter table, to display to the user some of the constants required. Peak summer energy volume is the maximum charged level beyond which energy will leak away, and towards which the winter shrunken energy volume wishes to expand. Starting energy volume is whatever you estimate at the start of the timeline. In this case it was August 2009. The Seasonal Performance Factor of the heat pump is the COP averaged over the year. By reading in the electric consumption, the algorithm can determine how much energy is pulled from the earth each day. System losses must be allowed for because they really occur, whatever the meter tells you. There are losses in the pipework, and in the top part of borehole, and warm liquid is left in the pipe after the pump stops. System upgrade allows one to consider what happens if the solar panel area is increased. The Borehole depth is a constant that allows the Radius to be calculated at each time interval.

Solar earth charging: Parameter RAF, Algorithm Recharge Adjust Factor • RAF: This is the most useful discovery of the model - a technique to quantify natural recharge which occurs all the time, not only in summer. • Think of as: “index of elasticity of thermal conductivity as energy is brought in from the surrounding mass”. • Large numbers are involved, so the RAF for this model is in the region of 35x10-6. • View the graph without charging, get the peaks and troughs to level nicely: the RAF is then correct.

• The main algorithm is this short loop. • Running through the timeline (daily

intervals over 3 yrs), incrementally adjusts Energy Volume based on inputs and outputs, and then calculates the elastic Recharge from the surroundings. • Fills two arrays with values for RADIUS, one that allows solar charging and one that assumes zero charging.

For the author, the most useful finding has been discovering what he calls ‘RAF’ : a means of quantifying the natural recharge process - an elastic process of pulling external energy in as the energy volume shrinks with each time interval. The recharge adjust factor works best with a value of 35 millionths for this particular installation and soil condition. The algorithm calculates two energy levels at each time interval, one with solar charging and one without. From this, two values for Radius of the Energy Volume are stored in an array.

Solar earth charging: Model results • The array contains daily

figures for the Radius of the Energy Volume. • These can be converted into 2D polygons • Graph can show the energy level without charging, with charging, or in this case with both options displayed. • Vertical black lines show significant moments on the timeline. • Dates are printed along the baseline. • Tweak parameters if necessary.

Above: Final chart, Blue is Uncharged, and Red is the cumulative effect of charging relative to Weather. (2010 was cold, 2011 was a very warm.)

Curve Shape is the end result of the process. Below: Subroutines to draw out diagram. Right: Most of the drawing routine.

The array has been filled with two values for the RADIUS of the Energy Volume. These can be converted into 2D polygons. With two values, the graph can display the result without solar charging, with solar charging, or in this case with both options displayed. Vertical black lines show the significant moments on the timeline. Dates are printed along the baseline. If it looks crazy, the constant parameters such as RAF and Peak may need adjustment. At the end, one is trying to establish a Curve Shape, not precise energy levels. Curve Shape demonstrates the pattern of behaviour over three seasons relative to Weather.

Solar earth charging: Conclusions Vacuum Tubes (and PVT) • Low-temp, large-volume collector proved more effective, versatile, simple than high-temp collector (needs heat exchanger). Subject for another paper! • PVT will be next research - low-temperature. Accelerative Effect: Take note of this: the TIME factor contributes more than thinking merely of annual input and output figures. Immediate restoration of energy level takes place after a heating cycle. Modelling: Useful exercise, but where next? • Understanding natural recharge rate has helped. • Use Degree Day and PV data from typical year • Adapt the algorithm to read typical year data and forecast solar panel area based on different housesize, GSHP capacity and borehole size. • Adapt it to clustered boreholes. Read all about it! Website is: http://


Conclusions: ONE. The Vacuum tubes were tried and found to be less effective than the Sunbox. That is to be the subject of another paper, later in the year. TWO. The improvement in heat pump performance from the expected 15% to 40% may be due to the Time factor - immediate restoration of energy levels occur immediately after a heating cycle by the heat pump. THREE. The Energy Volume Modelling has been useful in developing a methodology, using past data on the existing house, borehole and solar systems. For the author, the method for calculating the daily rate of natural recharge was a pleasing discovery. Someone else in this room may also have discovered this. Please email the author. The next phase is to make it applicable to another houses, boreholes and solar systems, by using annual weather statistics from a typical previous year. Degree Days could be used to forecast heat pump demand, and PV records could be used to make an index of ‘sunniness’. This would help to estimate the required capacity of the heat pump, borehole and area of solar panels for future installation. Please read the website for more news of this project. If you have been, thankyou for listening.

DNC SET paper Vancouver 2012  

The Powerpoint for DNC's paper for the SET conference in Vancouver 4-7 September 2012.

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