Wednesday, July 11, 2012

Accelerative effect of Solar Earth Charging

10 July 2012: Having worked on the thermal simulation for the interaction of GSHP and borehole, I still have a question. The solar augmentation undoubtedly lifts the energy levels above the line representing the best summer peak, but we are getting a greater than expected advantage from it - which is why I use the word 'accelerative'.

Metered figures compared
The Sunbox puts down approx 3,000 kWh, but the house needs about 9,000-10,000 kWh annually, there is still the majority, perhaps 6-7,000 kWh to come from natural recharging. These figures, and the orange line in the diagram below suggest that the improvement is welcome, but not much more than 15%-20%. The true results, as recorded by meter, is that the annual electrical consumption was cut by about 40%, from more than 5,000 kWh to less than 3,000 kWh. These figures vary slightly with seasons, but they represent the typical figure achieved. Why is the effect of solar augmentation accelerative?

I have explained this some months ago in a posting, but I want to explain it again in the light of the recent thermal modelling efforts.

Local zone of warmth or coolth
Looking at the plan of the boreholes, it's clear that the zone immediately around the borehole pipes is the most active zone. We have a global annual radius of about 3.6 metres which is the distance that energy is likely to move in an out from, seasonally, but on a daily basis, it is probably less than a meter. It cools most quickly when the GSHP is working, but it is also replenished most quickly by the Sunbox.
   Also, with twin boreholes, we have a beneficial warm zone between the boreholes that nurses the solar heat put down by the Sunbox. If there was no Sunbox, this zone would be malicious, as it would nurse 'coolth' and make natural recharging more difficult.

Seasonal behaviour
There are more concepts to consider besides COP (coefficient of performance), and one of these is the SPF (seasonal performance factor). COP can be measured at any single moment, and varies according to conditions. When a heat pump starts work in the autumn, it's finding plenty of source energy and had a good COP, and in January after the heat source is exhausted, the COP is very bad. SPF shows how the GSHP has performed over the seasons. If the COP is frequently improved by solar energy, then over the length of a year, the SPF will be improved.
• During the Summer, the added solar heat energy must move outwards and fill the larger space (until it meets the energy being pulled in by natural recharging) and after that equilibrium, whatever else we can add is a bonus. If we add too much, there would be leakage.
• In the Equinox periods, the zone is more active because you have mild and sunny days, but still you have cold evenings. Therefore, daytime solar energy is put down, and raises the energy level of earth immediately around the pipes, and the heat pump gets this back 3-6 hours later, having had no time to escape, even a metre from the pipe.
• During the Winter, we are reliant on long term stored energy, but it is surprising how many bright clear but cold days there are, when the GSHP is busy working, but the Sun is shining onto the panels and giving a temporary boost to the zone around the pipe.

If during Equinox, the energy levels lead the HP to perform as if it is Summer, and if during Winter, the HP performs as if it is Equinox, then that will show up at the end of the year as a very good SPF.

Volumetric difference between model and reality
My thermal model treats the energy bulb as one large cylindrical mass, and assumes uniform conductivity. Because conductivity is slower in the real world, the warmth put down stays near the pipe for longer, and returns more willingly. Also, the real world model has the benefit of the 'nursed zone' between the boreholes. Hence, the improvement seems greater than one would expect from the annual figures - hence, accelerative.

Another accelerative effect: restoration
Time plays another important role: another clear example of accelerative effect is the restoration of energy level immediately after the GSHP completes a heating cycle. The ground has briefly been cooled, there is a good delta-T, and for a period of time, perhaps 20-30mins, the Surya system is pumping warmer liquid down, to restore the energy level. I can see this happening after each heating cycle, the little green light glimmering as warmer glycol is buried. Since the sensors were re-arranged, this has worked better. The upper one is attached to the surface of the black collector, and the lower one is attached to the up-coming liquid.

Horizontal systems are possible
This accelerative effect means that one can apply solar charging to horizontal systems. The short term return would work just as well, and in winter, it would reduce the risk of freezing the ground - defrosting it after each heating cycle if there is a glimmer of a bright sky. In summer, the interseasonal charging would not work well because it could overheat the upper layer of soil and dry out the grass.  If an insulation layer is buried in the ground above the slinkies, there could be an element of solar charging inter seasonally, and this would lessen the risk of drying out the grass. David Maritan (in Pavia Italy) has done this successfully with the 'Compact Collector', a vertical grid of pipes.

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