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Old 04-26-14, 12:29 PM   #1
buffalobillpatrick
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Default Solar with HP study

http://www.task51.iea-shc.org/data/s...D_in_press.pdf

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.
Acknowledgement
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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