Numbers in the following paragraph refer to numbers in the Fig. 1. When the temperature difference between the top of the SHC 1 and the pit 9 is at least 3°C and the heat pump 6 does not work, the valve 2 is opened between P and B ports. The warm brine circulates between SHC, preaheater for the domestic hot water tank 7a, the ground heat exchanger 9, backflush valve 11, flowmeter 10 and circulation pump 4. When the temperature difference falls to less than 2°C, the pump 4 stops the brine circulation When thermostats, controlling the hot water or room temperature activate the heat pump, the valve 2 opens the path P to A and the circulation pump 4, as long as the atmospheric temperature is higher than the temperature of the cold brine, starts to work.. The brine flows then between SHC 1, valve 2, flat heat exchanger 3 and through pump 4 back to SHC. The brine is cooled down in the heat exchanger 3 to low temperature. At the cold surface of the SHC condenses the water vapour in the air and releases both the sensible heat of the air and the latent heat of the water vapour. The expanded, cold (< -5°C) heat transfer fluid in the heat pump circuit receives the heat from the brine and transports it to the loop 5 in the pit. Depending on the temperature difference between the fluid and the soil surrounding the loop, the fluid either loads the storage magazine or consumes the stored heat.

The heat capacity of the pit is dependent on the moisture content of the soil. Therefore all rainwater from the roof of the house is lead to the drainage tube system 12.

2. Results and discussion.

An existing family house in the northern or middle Europe does not need to consume more than 10000 kWh/year. Solar energy distribution and energy requirements in such a house over one year can be seen in the Fig. 2.

Fig 2. Energy distribution during one year.

The solar energy values in the diagram are calculated from measurements of solar irradiation, air temperatures and humidity The values are based on the SHC area 12 m2 and -5°C cold surface.

The air volume passing the collector during each night is calculated to 500m3. The electrical current consumption was daily recorded from the wattmeter of the house. The diagram shows, that energy demand of the house can be covered by solar energy between weeks 11 and 43. Between

weeks 1-10 ( 40 % of the year energy consumption) and 44-52 (26 % of the total energy consumption, totally 3400 kWh), has the energy for heating of the house be retrieved from a heat storage magazine.

About 1200 kWh electric energy consumes the heat pump for transfer of the energy from the pit to the house. The heating season begins in Stockholm area (59° 27’N) in the middle of September. The temperature of the moist soil, 0,5 m below the surface (Fig. 3) was at that time 12°C and the water content of the soil was 50%. The calculated heat capacity of the pit was in the middle of September 50 kWh/m3, inclusive the latent heat of the soil humidity.

Fig. 3. Temperature in the pit during the heating season 2006/2007.

The heating system was not in the full drift during this winter season. The soil layer was only 50 cm above the heat collecting loop and the SHC was not yet functioning. The mean air temperature between the 10th and 28th February was -4,7°C with the minimum -6,1°C between the 22nd and 25th February. The relatively thin layer of the soil above the heat collecting loop did not prevent measurable influence of the air temperature on the temperature at the bottom of the pit. See Fig. 4. That explains the temperature depression in the pit during the last third of February. In spite of that, the heat pump could still keep the adjusted room temperature.

The effect of the latent heat of high soil humidity could be observed in the Fig.3 from the horizontal parts of the diagram. In January 2007, the most of energy for the heat pump was delivered by the freezing water. And in April, the sun radiation supplied heat for melting of the ice. The 25th of April melted the last ice crystal in the pit. On May 14th, the bottom of the pit reached the reference temperature of the ground. According to our measurements, the average insolation in April 2007 was 175 W/m2 and the sun radiation time was 348 hours. This corresponds to 1948 kWh energy theoretically absorbed by the dark surface of the pit.

The disadvantage of the shallow soil layer is the dependence of the soil temperature in the pit on the temperature of the air and hence the energy losses in the cold weather. The advantage is, that the ice in the ground, which originates from the short distance between parallel sections of the heat collection loop, does not result in permanently frozen pit bottom.

During the autumn 2007 the pit was filled with the soil up to 1 m depth. The SHC remained still closed, so that all energy for heating of the house was taken from the humid soil. The results can be observed in the fig. 5.

Fig. 5 Temperatures in the pit during the heating season 2007/2008

10 —

Reference soil temperature

Bottom temperature of the pit

0 —

From the midle of December to 10th of February remains the temperature in the pit within temperature +1 to -1°C. The average air temperature in this period is 1,1°C. The lowest temperature in the storage magazine coincides with the lowest air temperature in spring, -7,8°C on March 24th. The regression analysis of the correlation between the air temperature and the temperature at the bottom of the pit between 10th of March and the end of the month returned the magnitude of the proportional term 0,086°C/°C and the regression coefficient R2 = 0,494. The

direct influence of the outside temperature is therefore minimal. But the heat pump was running with full capacity and the magazine was almost empty.

The minimal influence of the air temperature on the temperature at the bottom of the pit kept the soil frozen even at the end of May. The ice in the pit started to melt first after the start of the brine circulation through the ground heating loop. The tight placement of the energy collection loop could not be used without active heating of the frozen soil.

4. Conclusions.

1. Cooling of the brine flowing through the SHC with the cold heat transfer fluid in the heat pump circuit makes it possible utilization of the SHC even during nights and cloudy, cold days.

2. Using of energy wells as heat source for the heat pumps is possible only in landscapes, where the soil depth is only few meters. An alternative solution, the digging of the loop for energy collection into the soil, required so far large ground areas, because the formation of permanent groundfrost has to be avoided. The coupling of the SHC to the heat pump, described in this contribution demonstrated, that the ground area needed for the energy storage can be diminished to 10-12 m2/ kWh. The expected energy content of the storage volume can be equal or higher than 40 kWh/m3.

5. Literature References:

[1]George A. Olah et al: Beyond Oil and Gas: The Methanol Economy; Wiley-VCH Verlag & Co. KGaA.

[2] http://www. parc. com/research/publications/files/5706.pdf.

[3] http://ep. espacenet. com

[4]M. Semadeni, Energy Storage as an Essential Part of Sustainable Energy Systems. Working Paper No 24, May 2003; CEPE ETH Zentrum WEC Zurich;

(www. cepe. ethz. ch)

[5]Simone Raoux and Matthias Wuttig Editors, Phase Change Materials: Science and Applications, Amazon N. Y.

[6] www. texsun. se

[7] www. megatherm. se

[8] Model VV from EMS Brno, CzechRepublic; www. emsbrno. cz

[9] www. kippzonen. com