Borehole storage coupled with heat pump for domestic heating system and space free cooling ----- The analysis of temperature traces of the boreholes and system SPF Bo He, PhD; David Kroon, Msc. Nibe AB, Box 14 Järnvägsgatan 40 285 21 Markaryd, Sweden E-mail: bo.he@nibe.se; david.kroon@nibe.se Abstract Ground source heat pump system represents an energy efficient technology due to the relatively high and stable heat source and so that borehole storage coupled with heat pumps is increasingly applied for domestic heating system. However, the temperature of ground could become colder and colder during operating period and this temperature level would influence the coefficient of performance (COP) of heat pump. Therefore, it becomes very important to maintain the temperature level for the ground. This paper presents a project concerning boreholes storage coupled with ground source heat pumps for domestic heating and free cooling from boreholes for space cooling. The temperature traces of boreholes during an operating period have also been presented here. The results indicate that free cooling from boreholes storage used for space cooling increased the brine return temperature to the boreholes and kept the temperature balance in the boreholes. Through heat pumps COP or HSPF (BTU/W h) discussion, it indicates that ground source heat pumps coupled with boreholes storage shows a primary potential for energy saving and increasing renewable energy using. 1. Introduction Europe s 2020 vision: a long-term strategy on energy policy, aimed at leading the world in the fight against global warming, was agreed by European Union heads of state. The key points of Europe s 2020 vision are: At least of 20% energy used in the EU will come from renewable source by 2020 EU greenhouse gas emissions will be reduced by 20% below 1990 levels by 2020 At least 10% of the fuels used in transport will be biofuels by 2020. For renewable energy production, the 11,5 % increases would be needed to meet the 20% by 2020. This target should be allocated as specific targets for each Member State. Energy efficiency will be an important factor to achieve a 20% reduction in greenhouse gas emissions (New Scientist). There are three factors: electricity, heating/cooling and transport, which are affected by renewable energy and energy saving. According to a report from Swedish Energy in 2007, Fig. 1 shows the total energy use in Sweden from 1970 to 2006 (www.energimyndigheten.se). It has been found that the biggest potential for energy saving could be in the residential, commercial and industry sector. Heat pump technology has a great potential to contribute to energy saving in both building and industry sectors.
Geothermal energies are defined as renewable energy in an EU proposal for promotion of using energy from renewable sources, which proposal was published on December 2008 in Brussels (17086/08). So ground source heat pumps will play an important role in the future for renewable energy using. Fig. 1 The total energy use in Sweden 1970-2006 (Conversion losses in the production sector are allocated to end user) In this paper, one project of boreholes storage system coupled with heat pumps for domestic hot water, space heating and free cooling is presented. The temperatures from boreholes have been traced during the whole operating period; because this temperature is one of the most important factors to influence the heat pumps COP. Through this project, it has been indicated that the system of ground thermal energy storage coupled with heat pumps has shown a primary potential for saving energy and meeting the increased requirement of renewable energy using. 2. The system and borehole temperature traces 2.1) The system of borehole storage coupled with heat pumps The system includes u-type boreholes and ground source heat pump units. It produces hot water and is used for space heating in the winter time. The free cooling from boreholes is used for space cooling when indoor temperature reaches a certain uncomfortable level. The system is shown in Fig. 2. The boreholes connect with 6 heat pump units, some of the heat pump units are used for space heating and some of them are used for hot water production. The boreholes are also connected with a ventilation system, which is used for space cooling. In this system, the numbers of boreholes are 10 and the depth is 165m. The total heat capacity of the heat pump units is 140 kw.
Readiator Hot water Free cooling T T ventilation Boreholes Fig. 2 The principle scheme of system of the boreholes storage coupled with heat pumps 2.2). Borehole temperature traces The above mentioned system is installed at the company Nibe headquarters office building, which is located at the south Sweden Markaryd. The climate in Markaryd shows quite a big difference in outdoor temperature in winter time compared to summer time. Winter time the temperature could go down to -20 C and the corresponding summer temperatures could be 30 C or even higher. The annual average outdoor temperature in Markaryd is around 7 C. Based on this, there is normally a need for both heating and cooling in many buildings in the area of Markaryd, especially in office buildings. The geological assumptions for Markaryd are also good regarding to the use of ground source heat pumps. The distance to the rock is normally 5 to 10 m and the thermal conductivity of the rock (gneissic granite) is between 3-4 W/mK. During the operation, all heat pump units could run at the same time or stop at the same time. It could also be that some of the heat pump units are running and some of them are stopped at the same time. Free cooling system and heat pump units could be running at the same time. Brine from boreholes is parallel connected with brine inlet pipe for each heat pump and all brine outlet pipes from each heat pump are collected together before going back to the boreholes. The measured points of boreholes inlet and outlet are as shown in the Fig. 2. The different parameters from all heat pump units and boreholes have been recorded continual by Nibe developed RCU (Communicate Control Unit) system since winter 2005 and The RCU is controlled via internet. The analysis of all recorded data from this system has been conducted. The Fig. 3 shows the average brine temperature from and back to boreholes during 2007. Form Fig. 3 it can be seen that the coldest days were in February during 2007. At this month the average brine temperatures from boreholes to heat pump was lower than around -1 C. The average brine temperatures from heat pump back to boreholes reached around -3 to -4.5 C. From the beginning of March 2007, the outdoor temperature started increasing and the brine temperatures back
to the boreholes rose. It means that the energy requirement from heat pump decreased and resulted in the brine temperature back to boreholes increased. It also could be seen that the average temperature difference between brine outlet and inlet from boreholes kept at a quite constant value. The average brine temperature from and back to boreholes during 2007 Temperature from boreholes Temperature return to borehols 12,00 11,00 10,00 9,00 8,00 7,00 Temperature ( C) 6,00 5,00 4,00 3,00 2,00 1,00 0,00-1,00-2,00-3,00-4,00-5,00 2006-12-29 2007-01-28 2007-02-27 2007-03-29 2007-04-28 2007-05-28 2007-06-27 2007-07-27 2007-08-26 2007-09-25 2007-10-25 2007-11-24 2007-12-24 Date Fig. 3 The average brine temperature from and back to the boreholes during 2007 At the end of April, free cooling ventilation system started operating and the brine return temperature to the boreholes reached a positive peak value at the beginning of May. At Fig. 3 it shows that free cooling operation was from April to the end of August during 2007. From September heat pumps ran mainly for heating application, the average brine return temperature to boreholes continued decreasing and the brine temperature from boreholes decreased also but a constant temperature difference between brine from and return to the boreholes was kept. Around end of November, suddenly the brine return temperature to boreholes reached -3 C, but the brine temperature from boreholes was still higher than 0 C and resulted in the temperature difference between outlet and inlet of the boreholes being higher than 3 K. This phenomenon indicated that borehole storage function well and the temperature in the boreholes did not fall down as much as the brine return temperatures did. According the minimum brine temperature from and return to boreholes during 2007 it indicated that the lowest brine temperature from boreholes was around -3 C when the brine return temperature reached around -5 C at a very short period. At the most times of 2007, the minimum brine temperatures from boreholes were at a positive value (above 0 C). It can also be seen that the frequencies and the differences of temperature peak value from top to bottom of the brine return to boreholes were much sharper than the brine temperatures from boreholes. It means that the brine temperatures from boreholes were changing much smoother than brine return temperatures to boreholes. It is due to the boreholes storage keeping an almost constant temperature. The data analysis from 2008 has also been conducted. The coldest days during 2008 were between January to March. Fig. 4 shows the borehole temperature traces from the end of March to April. In the figures, the outdoor temperature traces are also shown and it can be seen that the lowest brine
temperature from boreholes was around -1 C when brine return temperatures reached around the lowest point -4.5 C. The average brine temperatures from boreholes were above -0.5 C when brine return temperatures were at the lowest level. The Fig. 4 indicates a very stable operating period and the brine temperatures from boreholes kept at a constant level. The situation in 2008 was the same as 2007, from the beginning of April the air temperature increased and free cooling ventilation system started operating and it resulted in that the borehole temperature rose progressively. The hottest days during 2008 were in July and Fig. 5 shows the boreholes temperature traces during the summary from July to August. It can be seen from Fig.5 that during these hottest days, the boreholes storage was utilized mainly for space cooling so that the brine return temperatures to the boreholes were higher than the brine temperatures from boreholes. After the hottest period the brine temperature from boreholes remained at a constant level and the average value was around 9 C. It became colder and colder from September, boreholes storage switched to mainly be used for space heating and hot water production. Therefore the brine temperatures back to the boreholes fell down. Due to the colder brine returned to the boreholes it resulted in the boreholes temperature decreasing. However, the brine temperatures from boreholes decreased step by step. At October the average brine temperatures from boreholes were around 7 C and at the end of November it changed to 5 to 4 C. The Fig. 6 shows the brine temperature from and back to the boreholes during December and the brine temperatures from boreholes were rather stable. 2008-03-2008-04 Outdoor temperature Temperature from boreholes Temperature return to boreholes Temperature ( C) 15,00 14,00 13,00 12,00 11,00 10,00 9,00 8,00 7,00 6,00 5,00 4,00 3,00 2,00 1,00 0,00-1,00-2,00-3,00-4,00-5,00 2008-03-22 2008-03-23 2008-03-24 2008-03-25 2008-03-26 2008-03-27 2008-03-28 2008-03-29 2008-03-30 2008-03-31 2008-04-01 2008-04-02 Date Fig. 4 The brine temperatures from and return to the boreholes during the end of March to the beginning of April, 2008
Boreholes temperature during 2008 7-8 Outdoor temperature Temperature from boreholes Temperature return to boreholes 32,00 30,00 28,00 26,00 24,00 Temperature ( C) 22,00 20,00 18,00 16,00 14,00 12,00 10,00 8,00 6,00 4,00 2008-07-27 2008-07-29 2008-07-31 2008-08-02 2008-08-04 2008-08-06 2008-08-08 2008-08-10 2008-08-12 2008-08-14 2008-08-16 Date Fig. 5 The brine temperatures from and back to boreholes during the end of July to the middle of August 2008-12 Outdoor temperature Temperature from boreholes Temperature return to boreholes 11,00 10,00 9,00 8,00 7,00 Temperature C 6,00 5,00 4,00 3,00 2,00 1,00 0,00-1,00-2,00-3,00 2008-11-28 2008-11-30 2008-12-02 2008-12-04 2008-12-06 2008-12-08 2008-12-10 2008-12-12 2008-12-14 2008-12-16 2008-12-18 2008-12-20 Date Fig. 6 the brine temperatures from and back to boreholes during December
3. Discussions The above mentioned boreholes storage temperature traces should be related to heat pump efficiency. The most common heat pump efficiency measurement in heating model is called the Coefficient of Performance (COP). It is defined as the ratio of heat delivered by heat pump and the electricity supplied to the compressor. The higher the COP, the higher efficiency. The variation of heat pump s performance COP depends on the climate, the temperatures of the heat source and how much supplementary heat is required (E. Granryd 2002). The COP for a heat pump varies with the heat source temperatures (brine temperature from boreholes) and the required energy (water supplying temperature). A larger temperature difference between condenser and evaporator results in a lower COP. For example, when the brine temperature is 10 C and the water supplying temperature 35 C for space heating, the COP value could be 5.0. However, when the brine temperature is -5 and water supplying temperature 60, the COP would be 2,5. Therefore, a more realistic measurement is calculated on a seasonal basis. The operating performance of heat pump over a season is called the Seasonal Performance Factor (SPF). A heat pump is operating in heating model; the performance is referred to as the Heating Season Performance Factor (HSPF) (Wikipedia). Usually, HSPF is the estimated seasonal heating output in BTUs divided by the seasonal power consumption in watts (HSPF=BTU/Wh). An HSPF of 6.8 corresponds roughly with an average COP of 2. New heat pumps manufactured after 2006 are required to have an HSPF of at least 7.7. An HSPF >= 9.0 is considered high efficiency and the most efficient heat pumps have an HSPF of 10 (http://www.furnacecompare.com). According the above analysis of the temperature traces in the boreholes during the year 2007 and 2008, the average seasonal values of brine temperatures from boreholes could be determined. So the average COP and HSPF of the system boreholes storage coupled with heat pumps could be obtained and indicated in the below tables. The seasons were divided according to the boreholes temperature being at a closer value. From these tables it can be seen that the average COP was between 3.5 to 4.3 and it means that the heat pump system delivered around three and half to four times more energy than they consumed. It indicates also that the system of boreholes storage coupled with heat pumps is a high energy efficiency system. Table 1 Average COP and HSPF during 2007 Season Date Average brine C Average COP HSPF (BTU/W h) 1 January -1 3.53 12.0 February 0 3.63 12.4 2 March 3 April, May 4 3.93 13.4 4 June, July, 9 4.27 14.6 August 5 September, 6 4.07 13.9 October 6 November 3 3.83 13.1 December Annual Year 2007 4,3 3.93 13.4
Table 2 Average COP and HSPF during 2008 Season Date Average brine C Average COP HSPF (BTU/W h) January, 1,5 3.73 12.7 1 February 2 March, April, 0 3,63 12.4 3 May, June, 9 4,27 14.6 July, August 4 September, 7 4,13 14.1 October 5 November 3 3.83 13.06 December Year 2008 4,9 4.03 13.8 The cooling COP can be defined as the ratio of the cooling delivered from system and the electricity input to the system. Usually, the Seasonal Energy Efficiency Ratio (SEER) (Wikipedia) is a measure of the seasonal cooling efficiency. Usually, SEER is the number of BTUs of cooling provided per watt of electricity energy consumed. In the presented system of borehole storage coupled with heat pumps, the free cooling is from boreholes and the average cooling COP is calculated by the cooling capacity divided electricity used for fans and brine circulating pumps. The average cooling COP could reach 16 and SEER could be 55 (BTU/Wh). The free cooling from boreholes could also be applied in single family house if a ground source heat pump has been installed. This concept has already been developed by company Nibe to combine ground source heat pump with passive cooling or ventilation recovery unit. Here, brine from boreholes goes through the fan connector and free cooling is used for space cooling. Or brine from boreholes goes through the ventilation recovery and the indoor air coming from the house transfers energy to brine. The purposes are: 1) To increase the brine temperature and increase the heat pumps COP and HSPF. 2) If the heat pump is not running; the brine with a higher temperature goes back to boreholes and it could rise the boreholes storage temperature. 3) If a cooling ventilation system has been installed in the single family house, the free cooling from boreholes could be used for space cooling. By using measured value it has been calculated that the average cooling COP of the passive cooling unit could reach 7.2 to 7.6 and the SEER could be 24.5 to 26 (BTU/W h) (MOS GB 0734-2). 4. Conclusions The challenge of energy saving in the future is clear. New technologies to reduce energy demand and increase renewable energy need to be developed and expanded. The technologies of boreholes thermal energy storage coupled with heat pumps for domestic heating and space cooling can play an important role as it provides great potential for improved energy efficiency and the efficient utilization of renewable energy. In this paper, the system of boreholes storage coupled with heat pumps for domestic heating and free cooling from boreholes for space cooling has been presented. The different parameters from boreholes and heat pumps have been recorded by a RCU system. The data from 2007 to 2008 have been analysed and the brine temperature traces from and back to the boreholes are obtained. The Coefficient of Performance (COP) and Heat Season Performance Factor (HSPF) of the heat pump are presented. From the brine temperature traces it can be seen that the brine temperatures were mainly influenced by the outdoor temperature and it depended also on the energy requirement from the system. The brine temperature traces shows that the temperatures from boreholes varied very smooth even when the
brine temperature back to the boreholes fluctuated up and down with a sharper peak. At the coldest days during 2007 and 2008, the brine temperatures back to boreholes could reach -5 C, however, the brine temperature from boreholes to heat pumps kept around -1 C and it was not following the brine return temperature decrease. This phenomenon indicated that the borehole storage functions well. The stable borehole temperatures will provide stable COP and HSPF values. When outdoor and indoor temperature increased the cooling ventilation system started operating and free cooling from boreholes was used for space cooling. The purposes of utilization free cooling are to keep room temperature at a comfortable temperature level and it could also raise the boreholes storage system temperature. This is due to that the heat exchange was carried out between brine and warm air and it resulted in brine with a higher temperature back to boreholes. The temperature different between boreholes and ground or rock would cause energy interaction and thermal energy was transferred to ground or rock. Therefore the ground or rock temperature was recovered and the temperature balance was kept. The COP of ground source heat pumps mainly depends on the brine temperature from boreholes and required supplying temperature. From presented average COP of heating and cooling or HSPF and SEER values it is proved that the system of boreholes storage coupled with heat pump for domestic heating has promising potential to save energy and increase the renewable energy using. The free cooling combined with boreholes storage increases the efficiency of energy utilization. In the future, this technology should be improved to reach an even higher COP level. 5. References New Scientist Environment: http://www.nwescientist.com/article/dn11343 Energy in Sweden 2007: http://www.energimyndigheten.se Climate-energy Legislative package Proposal for a Directive of the the European Parliament and of the Council on the promotion of the use of energy from renewable sources. COUNCIL of the EUROPEAN UNION, Brussels, 11 December, 2008. 17086/08 E. Granryd, I. Ekroth, P. Lundqvist, Å. Melinder, B. Palm and P. Rohlin, 2002 Refrigerating Engineering. FuranceCompare: http://www.furnacecompare.com/faq/definitions/hspf.html Wikipedia: http://en.wikipedia.org/wiki/hspf Wikipedia: http://en.wikipedia.org/wiki/seer MOS GB 0734-2: http://www.nibe.se