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Old 04-26-14, 01:29 PM   #1
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Default Solar with HP study

Conclusions pg6

There are three major reasons for adding solar collectors in a ground-source heat pump system; one is to decrease the use of electricity and the others are to raise the temperature in the borehole or to decrease the net heat extraction.
If the reason is to decrease the use of electricity, the best energy saving is in a system where the solar collectors produce domestic hot water during summertime and recharge the borehole during winter. Natural recharging from the surroundings is large during summertime, when the heat demand in the building is low. If the installation is not undersized, the temperature in the borehole increases naturally during summertime and this is enough. It is more energy efficient to use solar heat during summer for domestic hot water, compared to recharging the borehole during summer. Recharging during summertime gives a higher temperature in the borehole, but it gives almost no annual savings in electricity if the system is well-sized. One reason for this is that the temperature increase is soon lost in the ground and gives more or less only advantages in close timing with the extraction. The sooner the recharged heat is used – the better. Note that even without recharge there will be an increase in the borehole temperature when the heat pump is not in operation, but when it starts the temperature immediately decreases.
During wintertime, the temperature in the borehole is much lower as the heat extraction rate is large, and normally the coldest month in the borehole is February (or early March). In November– February there is low irradiation and the solar collectors normally deliver a very small part to the domestic hot water system. The actual date for shifting the operating mode of the solar collectors may differ according to the latitude, weather conditions andsystem, but for the simulated system, the best efficiency was to change between domestic hot water and recharging respectively in November 1st and February 28th. The recharging during winter has most impact, since the temperature in the borehole is low and the heat pump is working during the worst conditions. The solar collector can produce heat at a low temperature, which is more efficient compared to the high temperatures required for domestic hot water. The advantage or savings in electricity is although dependent on the sizing of the system. If there is almost no need for auxiliary electric heat, the advantage of recharging is low.
For systems with too short boreholes, the savings in electricity may be large, especially if the solar heat can replace electricity from the auxiliary electrical heater. If the boreholes are deeper than the normal sizing, there are no savings at all with recharging. However, solar heat for domestic hot water is always replacing electricity as the operating time of the heat pump decreases irrespective of the sizing of the borehole.
If the reason to introduce solar heat in the system was due to a low temperature in the borehole, the system is probably under- sized and recharging with solar heat may be very efficient. Recharging will also decrease the net extraction from the ground and the problem with long-term cooling of the ground can be diminished. The sizing of the borehole depends on the actual conditions regarding heat extraction and possible thermal influ- ence from other boreholes. The sizing can also change during life- time due to the enlarged load from a pool or an extension of the house. In this case solar collectors can be interesting.
One disadvantage of recharging solar heat with long operating times is the increased demand of electricity to the circulation pumps. Longer operation time may be accepted with high- efficiency pumps. The advantage is dependent on the sizing of the system, especially if the recharged heat is replacing auxiliary heat. The use of electricity to the circulation pumps can easily exceed the decreased use of electricity in the heat pump, if the circulation in the solar collector system and the borehole system is allowed to run whenever there is possible solar heat to charge in the borehole.
In single family dwellings in Sweden, the bulk of the heat demand occurs during wintertime and the performance of the system during this time is of decisive importance for the seasonal performance. When evaluating the merits of different system
alternatives, it is important to compare the whole year perfor- mance and to include all the demands of electricity, especially to all circulation pumps.
The coefficient of performance (COP) for the heat pump can momentarily be very high in systems with solar heat, when the temperature to the evaporator is elevated. However, the seasonal performance factor (SPF) for the system (on an annual basis) was low in systems with undersized boreholes. Reporting COP values obtained during periods with favourable operational conditions is misleading if the overall annual system performance (SPF) is not mentioned in the same context.
As systems with ground-source coupled heat pumps and solar collectors are very complex, it is hard to give general results or design rules. If the system is well-designed regarding heat pump capacity, borehole depth, and building load and presuming all subsystems are working well, the best use of solar heat is for producing domestic hot water during summer (March–October) and recharging the borehole during winter (November–February). The optimum periods for this control strategy depend on the size of the heat load and domestic hot water.
A careful design of the system is important in order to minimize the use of electricity.
The authors are grateful to Formas, the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning, for funding this work.

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Old 04-27-14, 07:46 AM   #2
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A very nice write up, but I believe the basic concept is not applicable for many.

The basic idea is that stored BTU's (in the form of solar heat) can be stored in the ground and that this heat remains in place to later be extracted.

In many (most?) locations, migrating ground water will move this delivered heat away from the introduced heat site.

However, if the geology is stable (rock), with little ground water movement (or even a water table), this concept of stored heat can work.

But I suspect that this concept of heat storage/extraction will work in only a fraction of locations.

If ground water is available within 50-60 feet of surface (15-20 M), then I doubt it will work as lateral water movement is surprisingly fast.


consulting on geothermal heating/cooling & rational energy use since 1990
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buffalobillpatrick (01-13-15)
Old 04-27-14, 10:54 AM   #3
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Default Passive Annual Heat Storage...

There has been quite a bit of work already done on this and it's called Passive Annual Heat Storage, or PAHS for short.

There is information about it:
Some people are having success making this thing work.

I'm not an HVAC technician. In fact, I'm barely even a hacker...

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Old 01-13-15, 08:57 PM   #4
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This is NOT about PAHS.

This is about Solar charging the field in Winter for use in Winter.
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Old 01-13-15, 11:20 PM   #5
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I have a GSHP and solar thermal but I would not consider putting solar heat into the ground loop to save electricity in winter. Instead, I use the solar heat to heat my house directly. There is no energy storage involved, and hence no storage losses. I can't see how, having collected the solar energy once, it would be efficient to move the heat into the ground and then try and collect it again with my ground loop.

The suggestion that this approach allows the collection of solar energy at a lower temperature is a red herring I think. I have tried doing this to heat (de-ice) my front steps, thus using the low-grade heat that would otherwise be lost. It doesn't work well because the panels either produce nothing or can produce hot water which is usable as such. There is relatively little wasted low grade heat, and any savings from running the panels at a lower temperature would be minor compared to the likely losses involved in heating the ground loop.

The only time I can see this approach helping is if the ground loop is too short, but then it is a poor solution to that problem. It would be better to extend the ground loop.
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Old 01-14-15, 09:48 AM   #6
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I think there are more pressing issues then heating the earth so your ground loop gets extra warmth.

I suspect this measure would be one to do after you have all other bases covered , as a icing on the cake more then a direct line of fire. If one has finished his other Solar heating projects then adding this method to your arsenal can only improve efficiency.

it would tie up that loose end.
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Old 01-15-15, 01:22 PM   #7
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Solar panels are much more efficient with cool water entering, lower cost un-glazed panels are a perfect match for recharging ground loops.

See graph below

Preliminary Review of Geothermal Solar Assisted Heat Pumps - Appropedia: The sustainability wiki

"Geothermal heat pump systems are commercially available in a variety of arrangements to be used in the heating and cooling applications of buildings. A major drawback of such systems is the long term operational consequence of decreasing ground temperatures, resulting in lower system performance. In recent developments so called “hybrid-systems” have been examined to address this issue. In these systems an independent energy source, potentially solar, is used to accommodate the unbalanced annual loads that often exist under certain climatic or load conditions "

" In heating only or heating-*dominated applications,
the yearly load imbalance leads to ground cooling
over the years. The borehole length must be
increased to cope with that fact, especially in larger
systems with multiple boreholes that thermally
interact with each other. Supplemental heat sources
such as solar collectors have the potential to reduce
the required length of ground heat exchanger, hence
the capital cost. "

" The benefit of using solar heat in the system with ground-source heat pump depends on the performance of the actual system. If the bore holes are too short for the system and heat demand, the best savings of electricity is to use the solar heat during summer time for domestic hot water and during winter timefor recharging the borehole.
The benefit of recharging during summertime is very limited. "

" In the near future, for ground source heat pumps,
compact ground heat exchangers should probably be
an answer for small gardens, without expensive
borehole drillings. Solar re-charging of ground loops
is today a field of research in the geothermal heat
pumps industry. This article illustrates the case of a
residential 6kW geothermal heat pump system
installed end of 2011 in the north of Italy, with a soil-water heat pump and
compact ground collectors with solar re-charge. Data from the system monitoring are
here presented and discussed. "
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Old 01-15-15, 03:23 PM   #8
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Geothermal heat pump system at Mt Best

"The crucial part of a heat pump system is the source of heat. In our case it is 10,000 litres of water stored in pressurised underground tank.

In winter, we take heat energy from this water (we cool it down) and deliver this energy at a suitable temperature to the house interior.

Water in the tank is heated by two sources: the Earth interior and our custom made low-temperature polyethylene solar collectors.

Why is this system energy efficient?

To begin with, the system is designed (with Dr Mirek Piechowski from MP Energy Consulting, who holds a PhD in thermal sciences) to operate at very low heat source temperatures (4-10°C).

In this temperature range, the planetary interior actually heats up the water in the undergound tank, because underground soil temperatures are higher. We turned heat storage loss into heat gain. Isn't it intelligent choice?

Secondly, during sunny winter days our solar collectors warm up our 5°C water to higher temperatures, even on partly cloudy days.

Since solar collectors operate at very low water temperatures (5-10°C), heat losses due to convection are minimal. On a sunny winter day we once managed to capture 57kWh from our 24 m˛ collectors. Who needs fossil fuels?

Thirdly, our heat pump has COP (coefficient of performance) between 4 and 6 in the temperature range of our heat source.

It means that for every 1 kWh of electricity we use to run the heat pump, we transport 3 to 5 kWh of heat from the underground water tank to the house interior."
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Old 01-15-15, 05:09 PM   #9
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" The use of ground-source heat pumps for heating buildings and domestic hot
water in dwellings is increasing rapidly in Sweden. The heat pump extracts
heat from the ground by a U-pipe in a vertical borehole. In order to reduce the
electricity demand in the system, the combination with solar collectors is introduced.
This system may be designed in many ways and the advantages differ a lot.
Solar heat can also be used for recharging of boreholes when
neighbouring boreholes are thermally influencing each other. "
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Old 01-17-15, 06:09 PM   #10
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This approach seems as if it is an industrial continuous improvement project. In theory, if you have enough solar heat gain and run your ground source close enough to a certain bare minimum, you could save some money by deliberately undermining your ground loop. The sun would then provide the free heat gain that the loop cannot. On a good day in the swing season, you might eveeven heat up the ground.

To me, this sounds rather like suicide. Why not oversize your ground loop? The savings would quickly pay for the extra borehole or spool of pipe. More importantly, there would never be a chance of freezing the loop field.

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