Farzin M. Rad, PEng
ASHRAE Member
Alan S. Fung, PEng, PhD
ASHRAE Member
Wey H. Leong, PEng, PhD
Not an ASHRAE Member
ABSTRACT
This document demonstrates that hybrid ground source heat pump system (GHSP) combined with solar thermal collectors is a feasible choice for space conditioning for heating dominated houses. It was shown that the solar thermal energy storage in the ground could reduce a large amount of ground loop heat exchanger (GHX) length. Combining three solar thermal collectors with total area of 6.81m2 [73.3ft2] to the GSHP system will reduce GHX length by 15% (from 222m [728ft] to 188m [617ft]). The system malfunctioning in the cooling season was also detected and options for fixing the problem were presented. Sensitivity analysis was carried out for different cities of Canada and results were demonstrated that Vancouver, with mildest climate compared to other cities, was the best candidate for the proposed solar hybrid GSHP system with 7.64m/m2 [2.33ft/ft2] GHX length reduction to solar collector area ratio. Overall system economic viability was also evaluated using a 20-year life-cycle cost analysis. The analysis showed that there is small economic benefit in comparing to conventional GSHP. The net present value of the proposed hybrid system and conventional GSHP system were estimated to be $44,834 and $41,406 respectively.
In addition, a sensitivity analysis has been carried out to determine the relationship between the solar collector area and the ground loop heat exchanger (GHX) length. It was shown that the ratio of GHX length reduction to solar panel area of 4.7m/m2 [1.43ft/ft2], rendering the optimum ratio which corresponded to 32m [105ft] GHX length reduction with 6.81m2 [73.7ft2] solar collectors area.
INTRODUCTION
Geothermal applications for buildings are mostly limited to full dependence on ground soil temperature for 100% of the heating and cooling energy. Although there are the advantages of low energy and maintenance costs in favor of this approach, space limitations and high initial costs may restrict a full geothermal installation. Restrictive regulations such as mandating a minimum borehole size, grouting materials, wage rates and heat exchange method, generally increase the cost of such system. The initial cost may put the project above the budget, and in some cases, the drilling conditions may prevent the use of a large conventional closed-loop borehole field.
In many buildings, annually, the amounts of heat extracted from and injected into the ground are not balanced. The vertical closed loop is a common type of earth coupling mostly used in buildings that have limited land areas. In designing this type of system, consideration must be made of the thermal response of the ground throughout the expected project life (i.e., 25 years). An annual imbalance in ground load will lead to lower heat pump entering fluid temperatures (EFT) in heating-dominated buildings or higher heat pump EFTs in cooling-dominated buildings, to a point where equipment capacity may be compromised if the ground loop heat exchanger is not long enough. This imbalance requires either a very large ground loop heat exchanger or some mechanism for assisting the system by supplementing deficit heat or rejecting excess heat. Because the cost of installing a very large round loop heat exchanger may be excessive, there are a number of alternate ways to assist a GSHP. These include solar collector, which injects additional heat into the ground for heating dominated buildings, and cooling tower, which rejects excess heat into the atmosphere for cooling dominated buildings.
Systems that incorporate both a ground heat exchanger and an aboveground heat exchanger are commonly referred to as hybrid GSHP. In hybrid systems, the peak heat pump EFT from year-to-year should be approximately equal. In this study, the system utilizes a solar thermal collector as an aboveground heat exchanger, called a solar assisted ground source heat pump (SAGSHP).
The purpose of this study was to evaluate the performance and viability of hybrid geothermal heat pump systems with solar thermal collectors for residential house applications in Canada. The main objective was to perform a system simulation approach to assess the feasibility of this kind of hybrid system in heating dominated buildings. An actual residential building was modeled and the results compared to actual data collected by monitoring the related operation of equipments through specific months. It would be ideal if this study attracts the interest of researchers and contractors and provides valuable information for designing and installing this kind of hybrid system in heating dominated buildings in Canada.
HOUSE MODEL DESCRIPTION
The house selected for the proposed study is located in the City of Milton, Ontario. The house was one of two energy efficient demonstration houses built by a local builder in 2005. It is a detached two-storey building having 5,360 ft2 (498 m2) of heated area including the basement.
As per builder specifications, the house temperature is set at 21°C [70°F] and 24°C [75°F] in the heating and cooling periods, respectively. Air leakage at 50 Pa [0.015 inHg] is 1.41ACH (518 l/s). A continuous ventilation of 0.3ACH (110 l/s) through heat recovery ventilation system (HRV) is also considered. The sensible internal heat gain from occupants is set to be 2.4 kWh/day [8.2kBtu/day]. The occupancy of the house is two adults and two children for 50% of the time with a hot water consumption of 225 liters/day. The base loads are considered to be 22 kWh/day including interior and exterior lighting, appliances use and others. TRNBuild, a component of the TRNSYS simulation software, was used to generate the house load profile.
The climate of Toronto, Ontario (which is about 60km [37mi] east of Milton) was chosen for this study.
SAGSHP MODEL DESCRTIPTION
The ground loop heat exchanger (GHX) system consists of four vertical closed loop circuits, joined in parallel. Each borehole has 0.25m [10 in] diameter and 55m [180ft] length. They are located 3.6m [12ft] apart from each other in the backyard and merge in a 1.8m [6ft] below grade area. Figure 1 shows this arrangement.
Figure 2 shows a schematic of the solar assisted ground source heat pump (SAGSHP) system.
The GHX loop is connected in parallel to the solar thermal collectors. The solar collectors receive a percentage of the total flow from the ground loop exchanger. Two circulation pumps form part of the heat pump system and they are located upstream and down-stream of the GHX flow. A solenoid valve and a control valve control the flow rate to the solar collectors.
The TRNSYS modeling environment (studio) was used to construct the system using standard and nonstandard component models. The component models used are as below:
Heat Pump Model
Ground Loop Heat Exchanger Model
Solar Collector Model
Water Tank Model
In-Floor Radiant Heating Model 6- Gray Water Heat Recovery Model
Ventilation Model
Flow Control and Pump Component Model
Heat Pump Equipment Control and Scheduling
DISCUSSION AND RESULTS
Optimum Flow to Solar Thermal Collectors
In the as-built system, the percentage of the ground loop flow rate divert to the solar collectors was studied using three different flow quantities. From the equipment manufacturer, the maximum acceptable flow rates to the heat pump and the three solar panels were determined to be 1173 kg/hr [7gpm] and 120 kg/hr [0.71gpm], respectively. This means that the total maximum flow rate in the ground loop exchanger was the sum of the two, i.e., 1293 kg/hr [7.7gpm]. The maximum flow diversion from the ground loop was determined to be 10% of the total flow. This study showed that by increasing the fluid flow to the solar panels from zero to 10% of the heat pump flow rate, the overall system energy consumption in the heating mode decreases, whereas, in the cooling mode the system energy consumption increases. This trend is in favor of the system performance in the heating season and against it in the cooling season. This indicates that for Toronto weather conditions, a residential house with a hybrid GSHP system can benefit from higher flow diversion to solar thermal collectors in the heating season, however, the reverse is true in the cooling season.
Sensitivity Analysis of the Ground Thermal Conductivity
Thermal conductivity is one of the physical properties needed for sizing GSHP. As the exact soil thermal conductivity for the site of the house was unknown, the house was modeled with different soil thermal conductivities and the effects recorded. Changing the soil thermal conductivity leads to a change in the entering fluid temperature (EFT) to the heat pump. This is a very important parameter for the efficiency of the heat pump. The heat pump of the house is designed to work between 0°C[32°F] and 35°C[95°F]. Analysis was carried out with four different soil types in the range of 1.47 W/mK[0.85Btu/hr-°F-ft] to 2.5 W/mK[1.44Btu/hr-°F-ft]. The result showed that, in the heating season, the lower thermal conductivity leads to a lower EFT to the heat pump. Soil types with a thermal conductivity lower than 2 W/mK[1.16Btu/hr-°F-ft] lead to an EFT lower than 0°C and cause a malfunction of the heat pump. On the other hand, a higher soil thermal conductivity leads to higher EFT in heating season and more efficient heat pump operation. In conclusion, based on overall regional soil type, soil with 2 W/mK[1.16Btu/hr-°F-ft] is a reasonable selection, and this was the value used in the rest of the studies. Having better heat transfer in the vicinity of the ground heat exchanger loop is ideal for the system whereas having lower thermal conductivity in the backfill volume would be good for overall thermal storage. The relationship between solar collector area and ground loop length was one of the important aspects of this study.
Solar Collector Area and Ground Loop Heat Exchanger Length Relation
In the heating season, the results of this study showed that for this specific house located in Toronto region, three solar panels in the system helped to reduce the total ground loop heat exchanger (GLHE) length by 15% compared to the system that only had a conventional heat pump. Increasing the number of solar panels from three to six did not double the GLHE length reduction whereas its trend would be in the range of 8% to 13% after that. An optimum relationship of three solar panels with the reduced GLHE length of 15%. Figure 3, shows percentage ground loop length reduction versus number of solar panels. In the cooling season, adding solar panels to the system would have a negative impact on the system performance and an increase in the heat pump energy consumption. Therefore in heating dominated places where the cooling season is short this incremental energy consumption for space cooling would not be significant compared to the potential savings in the heating season. Figure 4 show entering fluid temperatures (EFTs) to the heat pump in a typical heating and cooling season, respectively.
System Cost Analysis
A 20-year life-cycle analysis of the system showed only small economic benefit for the hybrid system compared to the system with only a GSHP. This was due to the low borehole drilling cost of $33/m[$10/ft]. At the time of study the borehole drilling costs were estimated to be in the range of $29/m[$8.84/ft] to $39/m[$12/ft] for different ground conditions. However, for the case of higher drilling costs the economic benefits would be considerable, because of the 15% reduction of GLHE length due to the three solar collectors.
Table 1 shows the Net Present Value (NPV) of hybrid solar-ground source heat pump system considering borehole cost of $33.00/m [$10/ft], solar collectors cost of $125/m2 [$11.6/ft2], electricity cost of $0.10/kWh and interest rate of 6%. During the system life cycle of 20 years, the SAGSHP system energy consumption increases slightly year over year. This effect corresponds to the reduction of a 2°C [3.6°F] in the minimum EFT to the HP from the first to 20th year. In the case of GSHP system without solar collectors there will be higher energy consumption due to a near 4°C [7.2°F] reduction of minimum EFT to the heat pump from the first to 20th year. This effect was not considered in this study, as it was beyond the scope of this work.
Field Study and Verification
For this study, there was limited field data available to validate the simulation results. For heat pump energy consumption, there were only 42 days of data in the heating season and no considerable data in the cooling season.
However, there was almost eight months of data available for the EFT to the heat pump in 2007. The comparison of the simulation results with the field data showed a 2.7% to 6% deviation in energy consumptions. The source of this deviation was partly due to the weather data (TMY2) used in the simulation. By adjusting the simulation results with the actual weather data derived degree-days for 2007 for Toronto this deviation was reduced to 0.01 to 2.7%.
Solutions to the Problem of the As-built System
The existing system had problems functioning properly in the cooling season. The analysis results showed that the system was not properly sized for the cooling season as the EFTs to the heat pump were exceeding the allowable EFT defined by the manufacturer. This happened from June to August, almost the entire cooling season. Simulated solutions include: (a) stopping the flow to the solar panel in cooling season; (b) selecting a heat pump with a modified cooling capacity and specification; and (c) increasing the GLHE length. All three solutions are applicable with case (a) being the simplest and cost least to implement. This problem could have been preventable if the borehole lengths were increased by 35%. This solution could be justified as the system would perform better in the heating season in spite of extra borehole cost. Figure 5 shows, the existing heat pump EFT limits in system with three solar panels. It shows that in almost entire cooling season, the EFTs will fall above the heat pump capacity limit (35°C[95°F]) and system stops functioning.
System Viability in Different Cities of Canada
By considering the same house characteristics, the effects of different climates in Canada were investigated. For this purpose, six Canadian cities in different regions were studied. The results are tabulated in Table 2. It is shown that in different cities, in general, as the ratio of annual ground heating load to annual ground cooling load of the house decreases the reduced GLHE length to solar collector area ratio decreases. This ratio was 7.64 m/m2 [2.32 ft/ft2] for Vancouver with an annual heating load to annual cooling load ratio of 1.54. For Edmonton with annual heating load to cooling load ratio of 3.8 resulted in reduced GLHE length to solar collector area ratio of 2.93 m/m2 [0.89ft/ft2]. A higher reduction ratio, Vancouver, indicates better viability of the hybrid GSHP system. This would not be an absolute conclusion as other parameters such as ratio of heating degree days to cooling degree days in each city also affects the conclusion.
CONCLUSION
In this study, overall system viability was evaluated and existing system problems were detected through the dedicated
modeling and simulation of the installed solar assisted GSHP system.
Viability of System
The result of this study have shown that the hybrid GSHP system combined with solar thermal collectors could be a feasible choice for space conditioning for heating-dominated houses. For the house in this study, the seasonal solar thermal energy storage in the ground in the hybrid system was sufficient to offset large amount of GLHE length that would have been required in conventional GSHP systems. The economic benefit of such system depends on climate, as well as borehole drilling cost.
System Simulation Approach
This study demonstrates the value in utilizing a system simulation approach to evaluating alternatives in complex systems. The hourly time step simulation for the implementation of complex control and operation strategies enabled the assessment of transient system responses. This study will be further enhanced by examining and analyzing: 1) different configuration and control strategies; 2) the interaction of different components; and 3) potential benefits in broader geographical areas.
REFERENCES
Kavanaugh, S.P., “Analysis and Development of a Design Method for Hybrid Geothermal Heat Pumps,” Draft, University of Alabama, Tuscaloosa, March 1997.
Singh, J.B. “Advantage of Using the Hybrid Geothermal Option,” Letter to contractor, June 1996.
Bose, J.E., Smith, M.D., and Spitler, J.D. “Advances in Ground Source Heat Pump System- An International Over view”, Proceedings of the seventh International Energy Agency Heat Pump Conference. 1:313-324. Beijing May 2002.
TRNbuild, and Multi-zone Building modeling, TRNSYS 16 manual, Volume 6, 2004.
Essential Innovation Technology Corp., EI Geo- Exchange system, Surrey, BC, Canada , 2005. http://www.eitechcorp.com
TRNSYS 16 Manual, TESS library Ver.2, 2004.
ASHRAE Standard 93-2003 “Methods of testing to determine the performance of solar collectors”, ASHRAE, Atlanta, 2003.
Toronto Hydro Electric, Residential rates, 2007. http://www.torontohydro.com/electricsystem/residential/rates/index.cfm
Bank of Canada, Average loan rates, http://www.bankofcanada.ca, 2007.
Rad Farzin, “Viability of hybrid ground source heat pump system with solar thermal collectors”, Ryerson University, Master thesis, Toronto, July 2009
Rad F., Fung A.S., Leong W. “Combined Solar Thermal and Ground Source Heat Pump System”, 11th IBPSA Conference, Glasgow, U.K, July 2009
Wang, Enyu, Fung, Alan S., Qi, Chengying, and Leong, Wey H., "Performance prediction of a hybrid solar ground-source heat pump system", Energy and Buildings, In Press. 2012. doi:10.1016/j.enbuild.2011.12.035.
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