Key Performance Indicators

Key performance indicators are: 
  1. Ground Temperature in borehole
  2. GSHP workload (relative to weather)
  3. Annual Energy figures of house and heat pump and PV
  4. Energy levels in borehole, as estimated from dynamic model
The heat pump was commissioned in March 2007. Although it performed well enough, no formal metering records were kept in its first 2.5 years. In August 2009, this project started, and since then, meter records have been taken daily. As the time rolls on, it is possible to review old records and generate graphs of performance, and consider annual performance, comparing with annual weather records.
     One question is: Has the Coefficient of Performance (COP) improved? I had a datalogger1, but I returned it because of the difficulty of reading back data. Without a datalogger, I cannot attach thermal sensors to individual components in the heat pump interior, but I have used metering records instead. Using meter readings in a spreadsheet, I can review the performance daily, weekly, monthly, annually. These can be averaged over 2 yrs, compared with previous records and from this, I can derive ideas on the improvement in performance. COP is only one of the factors, and is one that engineers want to know. Most readers of this blog and I want to ask: Are the bigger ideas2 working, such as the performance of the ground and the 'building'3?

The graphs below will be updated at regular intervals.

1. Ground Temperatures
  • 1. Ground Temperatures - one of the big ideas here is to inject solar heat directly underground, to defrost the ground and provide a larger thermal energy store. Since the Surya Sunbox was installed, the ground hasn't fallen below 10º at the lowest points of this recent winter or last winter. Notice the differences in the angles of the slopes, in both entering and exitting a winter or summer peak. (Installation of sun boxes was mid March 2010).
       In a typical year, in Spring, there is a long slow climb out of the Winter trough, levelling off during the Summer, with a late peak in September, and then there is a steep drop off from October to December, starting its recover in February.
       The ground temp curve illustrates the weather patterns. In 2012, there were months of cold weather and rain through the whole summer season, combined with the Sunboxes being out of action for a whole month Mar-Apr due to a reconfiguration of the plumbing layout. By the end of Summer, the ground temperature recovered and reached 13.7ºC, more than a degree higher than the natural deep ground temperature if there was zero solar charging. After three years of recording temperatures it seems clear that the deep ground temperature does not go above 14ºC except during the actual process of charging (it might be over 20º during the day). When tested at midnight it settle back to just below a peak of 14ºC. The performance in the following winter suggests that the ground has greater 'resilience' after a good summer, indicating that the energy reserves are good.
      Where does the heat go to? When you have a heat store of infinite size, one can only guess that it moves outwards. As the annual rate of pulling heat up from the borehole is greater than the rate of putting it down, it's simple arithmetic that this heat that moved out comes back in winter.
  • 2. Workload of the Heat Pump4 (this is for engineers who wish it to be reduced by improving the COP). But it is important to relate the workload to Weather conditions because a mild winter can mislead one into thinking that the heat pump is getting more efficient simply because it uses less power. These two lines show that the performance is still improving because the lines are moving closer to each other.
3. Summary of annual performances
  • 3. Summary of Annual performances, comparing house electrical import with electricity consumed by the GSHP and that generated by the PV roof. There was a major rise in consumption in Autumn 2010, but during 2011 the trend was for reducing consumption. The regular 2,000 difference between house and GSHP shows that we can't do much more to our lifestyle (lighting, cooking, power, vent) than we are doing already. The downward trend is consistent to falling GSHP consumption. These GSHP figures include DHW. We have reached a lowest level after 2011, the GSHP consumption bottomed out at 2,580 kWh/ annum, half of its annual consumption in 2007-2008 and 2008-2009. Both lines have risen as a result of the cold rainy weather that predominated in 2012. PV generation peaked in early 2012, but the grey conditions have since reduced the PV. Considering biennial figures (not on the above chart), the PV is higher than the GSHP consumption.
4. Energy levels in borehole
  • 4. Energy levels in borehole, as estimated by a dynamic thermal model based on data collected since Sept 2009. The blue graph models the effect on the borehole of withdrawing energy rapidly with a GSHP during the heating season and hoping that the ground will recover each summer. It does recover, but not to the level originally hoped for. The orange graph shows that with a modest amount of solar augmentation, the energy level is restored, and will be maintained year after year - with good and bad summers there are small variations, but the general level is higher. The GSHP is therefore running more efficiently. Although the largest proportion of energy is coming from the natural surroundings, the effect of augmentation is to lift the lows and higher to higher than before. These curves are based on actual data recorded daily from Oct'09-Dec'12. the blue curve is not exactly as the GSHP would produce because this is based on real data, and the real power consumption of the GSHP was reduced by solar charging - hence, the assumption about the withdrawal of thermal energy is slightly wrong in the blue curve. It is the orange curve that is most significant.
  • How is the model calculated? You can read more details on the page called:
  • You can see my Powerpoint presentation for CIBSE Tech Symposium Liverpool April 2013
This graph overlays 2009, 2010, 2011 and 2012 over each other with colour codes
to make sense of the differences.

1 Datalogger note
I no longer have a datalogger on the system as there are other research projects taking place in the department which need them, and our datalogger was never as good as my daily metering records. I could never get useful data out of them, and there were regular problems of 'memory full'.
2 Big ideas note
There are more big ideas, for example whether this can be scaled up to groups of housing, apartment blocks, major buildings. 
3 Building note
 In this case, the 'building' implies the entire System equation - the physical building (a system of materials formed into an envelope which loses and gains heat), the Occupants (people who need heat, who turn lights on and off, use hot water etc), and the PV system (stores electricity in the Grid), the Heating system (GSHP supplying an underfloor circuit), the Hot water system, and how these all balance.)
4 Workload versus Weather
Energy used by the heat pump is in kilowatt hours, and degree days are an index of temperature and time that indicates the heating requirement of a building in this region. 15.5ºC is the base for calculation. Although they are not the same units, they share one parameter, that of Time. If it is colder for one week, the GSHP must work harder, and vice versa. 

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