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Economic applicability evaluation of heat pump systems in buildings with different insulation levels.

INTRODUCTION

In recent decades, experts have debated the economic feasibility of heat pump systems for room heating in residential houses. The use of this technology has increased despite of the lack of consensus among experts as to whether these systems are economically feasible in all cases. The US Environmental Protection Agency (EPA) has estimated that the use of a geothermal heat pump system has the potential to reduce the energy demand for room heating by up to 72% compared to conventional heating and air conditioning systems. The economic questions are, however, still being debated.

Because of the relatively constant temperature in the ground, geothermal heat pump systems typically have a better performance than heat pump systems with external air as a primary heat source. However, the implementation of geothermal heat pumps in locations with relatively low heating requirements might not be economically feasible, since the capital costs of these systems are significantly higher than air-to-air heat pump systems. Air-to-air heat pump systems have a smaller investment cost than conventional heating systems, but these heat pumps have in general lower performance values than the geothermal water-to-air heat pumps in colder climates because of cold external temperatures. Therefore, air-to-air heat pump systems may be economically feasible only in milder climates with shorter heating seasons.

One important piece of information for evaluating the operation performance of heat pump systems is the seasonal coefficient of performance (SCOP), or heating season performance factor (HSPF), the metric used in the United States (U.S.). The SCOP (or HSPF) describes the seasonal performance of a heat pump system.

The insulation level of the building envelope, along with the climate, influences the SCOP (or HSPF) and, therefore, the economic feasibility of heat pump systems. A building with a higher insulation level has a shorter specific heating season than a building with a lower insulation level, which reduces the cost for building operation. Also, a building with more insulation has smaller heating loads and can be operated with smaller supply temperatures at the condenser than a building with less insulation, which is another cost reduction factor. Finally, smaller heating load values permit shorter vertical geothermal heat exchangers, which are significantly less costly than longer heat exchangers. Thus, an economic evaluation of heat pump systems will benefit from the inclusion of information about the building envelope and its effects on the SCOP values.

Currently, the SCOP (or HSPF) is often characterized by a single generalized value. This value is determined at specific test conditions in a laboratory. The use of these generalized values for the SCOP can lead to errors in the economic evaluation of heat pump systems because the SCOP (HSPF) depends on the building and climate specific heating season and its characteristic heating load values.

Spiter et al. (2012) analyzed the feasibility of foundation heat exchangers for residential ground source heat pumps in several climate zones in the U.S.. The researchers evaluated this special geothermal heat pump system in a modeled reference residential building with different quantities of insulation. Their research shows that heat pumps based on foundation heat exchangers are technically feasible in milder climates of the U.S.. The economic question is not discussed in their research work. But again, it is clear that economical benefits may occur as a result of the lower operation cost of such systems.

Self et al. (2013) compared two heat pump systems, one with a vertical geothermal heat exchanger and one with external air as the primary heat source, with conventional heating systems in the cold climate of Canada. The researchers conclude that the geothermal heat pump technology is economically feasible in all regions studied. In this study, no climate and building specific analysis for the specific SCOP (HSPF) value of the heat pump systems was done. The addition of a more detailed analysis of these SCOP values would make the economic evaluation possible.

No previous work has focused on the comparative economic feasibility of geothermal and air-to-air heat pump technologies in residential buildings with different quantities of insulation located in different climate regions. The goal of this study is to illustrate the economic feasibility of ground source water to an air heat pump system and an air-to-air heat pump system for residential buildings with three different insulation standards. Another goal is to clarify the significance of the consideration of insulation levels and climate for the definition of SCOP values. A typical single-family residential building is chosen as a test case. The feasibility is predicted for a large number (238) of locations in the United States. The SCOP (HSPF) of the heat pump system for the use in a representative residential building is calculated in a dynamic calculation process with three different qualities of the building envelope.

This study uses the thermal building simulation program TRNSYS 17. The computer simulation uses a bin method calculation to derive the SCOP (HSPF) values. A Benchmark analysis is used to compare the two heat pump systems to a conventional furnace as the conventional heating unit. Evaluation criterion in this study is the Life Cycle Cost (LCC) analysis, which includes the net present value (NPV).

METHODS

Description of the Heating Systems

Three heating systems are compared in the study. Two heat pumps, an air-to-air heat pump system, and a geothermal heat pump system, are compared with a conventional furnace system as a benchmark. For the air-to-air heat pump, the external air is taken as a primary heat source. The SCOP is dynamically calculated for each of the three buildings and each climate. The geothermal heat pump system is equipped with a vertical heat exchanger. The length of the vertical heat exchanger and the SCOP is calculated for each of the three buildings and each climate. The conventional system, a gas furnace, has an efficiency of 95%.

Simulation Model of the Heat Pump

To obtain the SCOP, the hourly coefficient of performance of the two heat pump systems is calculated in the simulation process. On the basis of the primary heat source temperature and the temperature of the heating coil in the mechanical ventilation, the COP is calculated by using the data in fig.1. The SCOP is the annual average of the hourly COP values. Only time periods with heating demand and warm water demand, which depends on heating season and climate, are considered for the calculation.

The heating energy demand is calculated by assuming a constant air supply temperature of 50 C (122 F). A variable air volume flow is assumed in the heating energy demand calculation in each time step. The dynamic calculation of the SCOP value is adjusted to accommodate the domestic warm water demand. The performance of all three heating systems is calculated to cover both space heating and domestic warm water heating, the two usual cases. The building is assumed to have four occupants with a warm water demand of 200 L (53 Gal) per day. The make up temperature of the warm water is 60 C (140 F). The building is assumed to have the same domestic warm water demand for each day of the year.

In this study, the defrosting at the evaporator coil is considered in the calculation of the COP and SCOP values. Frost formation on the evaporator coil of an air-to-air heat pump system occurs when the external air drops below the dew point.

Dimension of Geothermal Vertical Heat Exchanger

Dimensioning the length of the vertical heat exchanger's borehole is an important parameter for defining the initial cost of the geothermal heat pump system. The borehole should be as long as possible to maximize ground heat load coverage. The designer of the system must find a balance between increased length of the vertical heat exchanger and its investment. To determine the optimal length of the borehole, the designer must consider the thermal resistance of the ground material, maximal heating load, fluid flow rate in the pipe, and borehole diameter.

In this study, the length of the borehole is calculated with the sizing equation for total borehole length estimations by Bernier (2006). Their calculation method was applied to all climates and all building insulation levels separately. The ground loads are calculated in a pre-simulation process, where the building specific hourly heating loads are calculated in the dynamic simulation software TRNSYS 17. The thermal behavior is simulated in the software component specialized for building simulations (Type 56). The actual COP is calculated for each time step with heating demand.

A maximal length of 100 meters (328 ft) for a single borehole is defined for the vertical heat exchanger. Additional vertical loops are used in cases where the maximal length would exceed 100 m. A single loop configuration is chosen as the vertical ground heat exchanger. The pipe diameter is 19 mm (3/4 inch). The geothermal heat pump system is a closed loop system, where the circuit of the ground heat exchanger is separated from the heat pump circuit.

A one-dimensional finite volume calculation model, introduced by Yavuzturk and Spitter (1999), is used to predict the heat transfer and primary temperature for the geothermal water-to-air heat pump system. The model calculates the temperature distribution around a vertical borehole by combining analytical and numerical solution techniques. The monthly heating energy demand is used for the calculation.

Building Properties

The building has a floor area of 186 [m.sup.2] (2000 [ft.sup.2]) and is assumed to be a timber construction. The window-to-wall ratio at the south oriented facade is set to 25%, 15% at the north oriented facade, and 20% at the east- and west-oriented facade. The internal heat gains are assumed to be maximal 5 W/[m.sup.2] for the artificial light and 9 W/[m.sup.2] for the electrical appliances. The set-points for the thermostat of the space heating system is set to 21 C (70 F) in the heating operation.

Three different insulation levels were considered in the analysis. The heating energy demand and heating load is predicted for each combination of location and insulation level. The building with low insulation has no insulation material in its external walls and has a single glazing. Older, not renovated residential buildings built before the 1940's often have this construction, even when built in cold climate zones with prevailing heating demand.

The medium insulation level refers to a building with 2 inches of insulation in the building envelope. This insulation level can often be found in residential buildings built after the 1950s in the cold climates. This insulation level is often used for building renovation. High performance buildings in the warm climates with prevailing cooling energy demand might have this insulation level. The high insulation level is referred to a good practice design level and has 4 inches of insulation in the building envelope. The physical properties of the three buildings are shown in Table 1.

Climate Zones

For this study, 238 locations in the United States have been chosen. This large number of data has been chosen to derive detailed information about the general feasibility of heat pump systems in different climate zones. The goal is to classify the heat pump systems according to the ASHRAE climate data classification (2003). The 238 climate data taken for this study is in the TMY2 data format and provided by the US Department of Energy.

Calculation of Life Cycle Cost

The Net Present Value of the Life-Cycle Cost (LCC) is the evaluation criterion used in the present study and is calculated by following equation:

LCC = [I.sub.initial] + [t.sup.*]([Q.sub.site, heat] x [P.sub.heat] * 1/[(1 + [P.sub.heat]).sup.t]) (1)

Where [I.sub.initial] is the initial investment of the system, [Q.sub.site, heat] is the site heating energy demand, [P.sub.heat] is the present energy cost for heating (year 2012), and t is the calculated life-time period for the LCC. Maintenance cost and residual cost are not included in this study. The average increase of present energy cost for natural gas is taken from the energy price indices of the National Institute of Standards and Technology (NIST).

Investment Cost

The initial cost of a heat pump system depends on the maximal performance of the system. Robert and Gosselin (2013) described a model how to calculate the initial cost of heat pump units including labor cost. This model is taken for this study and is adjusted to the United States. Typical drilling costs range from $40 to $72/m ($12 to $22/ft). For this study, the mean drilling cost of $56/m ($17/ft) is taken for the calculation. The cost of the air-to-air heat pump system and the conventional furnace system also depend on the maximal capacity of the system and is derived from the RS means dataset (2013).

Heat Pump Service Life

Typically, heat pump systems have a warranty of 20 to 25 years. The service time of the heat pump systems in the LCC in this study is set to 10 years. Lovvorn and Hiller (2002) did a survey about the estimated service time and found the average expected lifetime is 10 years. Even with longer expected life time periods, this data can still be used as a pessimistic assumption for this study.

RESULTS

Seasonal Coefficient of Performance depending on Climate and Insulation Level

Fig. 1a-f show maps of the United States where the SCOP for each heat pump system in each simulated location is illustrated. The SCOP of the air-to-air heat pump is shown in Fig.1. The map shows that the SCOP is below 1.5 in all locations for the building with a low insulation level. A higher SCOP is calculated for locations in the southern part of the U.S. for the building with the medium insulation level. The SCOP is, in general, low even for the locations in the warm climate zones. Fig.1c shows that the SCOP for the air-to-air heat pump can be increased, when the insulation level of the building envelope is increased. Most of the SCOP values in the northern and colder climates are between 2 and 2.5 for the building with a relatively high insulation level. In general, the SCOP increases as the building is located in the warmer south.

Fig.1d-f shows the SCOP values for the geothermal heat pump. Fig. 1d illustrates that most of the locations have a SCOP below 1.5 when the building has a low insulation level of the building envelope. Only a view locations in southern California, Texas, and Florida have a higher SCOP than 1.5.

In general, the SCOP of the geothermal heat pump significantly rises at all locations when the insulation level of the building envelope is increased. Fig. 1e-f shows that the SCOP values for the building with a medium and high insulation level decreases for locations with a warmer climate. The SCOP is within 4-4.5 for relatively cold northern climates, which is relatively high for that climate. A comparison between the maps illustrating the air-to-air heat pump and the geothermal heat pump shows that the SCOP for the geothermal heat pump is, in general, higher for the geothermal heat pump.

Comparison of SCOP values with ASHRAE Climate Zones

The SCOP is, in general, higher in warmer climates when the SCOP data is compared between the test buildings with the three different insulation levels and the ASHRAE climate data map. The SCOP of the air-to-air heat pump for the building with the medium insulation level is below 1.5 for climate regions 4 to 7. In climate zone 3, the SCOP is between 1.5 and 2. The SCOP is between 1.5 and 2 in climate zones 5-7 for the building with a high insulation level.

A clear correlation is seen when the predicted SCOP values of the geothermal heat pump is compared with the ASHRAE climate zones. Most of the calculated SCOP values in climate zones 6-7 are between 1.5 and 2 in the building with a medium heat transfer. In climate zone 5, most SCOP values are between 2 and 2.5 and, in climate zone 4, the SCOP is between 2.5 and 3. The SCOP is larger than 2.5 in most of the locations in climate zones 1 to 3.

For the building with a high insulation level, the calculated SCOP values cannot as clearly compared to the ASHRAE climate zones like for the building with a medium insulation level. The general tendency of having higher SCOP values in warmer climate regions is also visible for this building.

Benchmark Comparison between Furnace and Heat Pump System

Table 2 lists the LCC of all systems and the SCOP for seven locations. Figure 2a-c shows the results of the comparison between the conventional furnace and the air-to-air heat pump systems. A specific location is marked with a red cross when the LCC of the furnace is smaller than the LCC of the air-to-air heat pump. The air-to-air heat pump is recommended when its LCC is smaller than that of the furnace system. In this case, the location is marked with a circle.

Figure 2a shows that the air-to-air heat pump system is recommended only in some coastal locations in southern California, Texas, and Florida when compared to the furnace system. The comparison in fig.2b of the medium insulation building shows that the air-to-air heat pump is recommended at most of the locations. Some locations exist in the far north of the country where the LCC of the furnace is smaller than that of the air-to-air heat pump. The air-to-air heat pump is also not recommended in warm locations in the far south of Texas and Florida.

Fig. 2c shows that the air-to-air heat pump is recommended in most of the climates in the US, when the building has a relatively high insulation level. Compared to figure 3a and b, the air-to-air heat pump is not recommended in more climates in the south part of the US in fig 2c. It seems that the heating energy demand is too small to justify the more expensive air-to-air heat pump system. The comparison of the results in fig.2a-c shows that the air-to-air heat pump is at more locations recommended above the furnace in the test building with higher insulation level.

Fig.2d-f shows the maps with the comparison of the geothermal heat pump and the furnace system. Like for the air-to-air heat pump, the geothermal heat pump system is not recommended in most of the climates when the building has a low insulation level.

Figure 2b shows that the geothermal heat pump is also not recommended in most of the climates when the building has a medium heat transfer. The geothermal heat pump has a lower LCC in most locations located in the coastal north west of the US. A direct comparison of the air-to-air heat pump (fig.2b) and the geothermal heat pump (fig.2e) shows that the air-to-air heat pump is recommended in much more locations in the building with the medium heat transfer.

A comparison of fig. 2e and 2f shows that the number of locations where a geothermal heat pump is recommended is significantly increased when the building insulation level of the building is improved. The geothermal heat pump system has a smaller LCC than the furnace system in most of the northern climates with prevailing heating energy demand. The geothermal heat pump is not recommended in most climates in the southern part of the US.

Sensibility Analysis

A sensibility study is done to verify the general tendency of this research work. A variation of the initial cost and the COP for both heat pump systems is provided. Both parameters are assumed to be 10% higher or lower than the values described in the method section. For each climate zone, the LCC of the heat pump systems is calculated with the above explained variation of the initial cost or COP and is compared to the LCC of the conventional furnace system. The number of locations where the heat pump system is recommended is compared to the results of the calculations in the above mentioned benchmark study. The deviation is smaller than 6% in most of the analyzed cases. A maximal deviation of 20% is found when the initial cost of the air-to-air heat pump is assumed to be 10% smaller. A higher deviation is seen when the COP of both heat pump systems is increased. The heat pump system is maximal 33% more often recommended than in the benchmark study. However, the same correlation and conclusion like in the benchmark study can be observed for all cases of the sensibility study.

CONCLUSION

The study shows that the SCOP (or HSPF) of both investigated heat pump systems is significantly increased in the buildings with higher insulation levels. The results of the benchmark analysis show that the insulation level of the heat transfer of the building envelope has a significant influence on the feasibility of both investigated heat pump systems. Both heat pump systems are economically feasible in more locations when the insulation level of the building envelope is reduced.

The air-to-air heat pump and the ground source heat pump are not economically feasible in most of the locations when the building has a relatively low insulation level. The same heat pump system is feasible in the cold climate zones when the building has a higher insulation levels. The geothermal water-to-air heat pump is at non of the evaluated locations economic feasible, when the insulation level of the building is low. Because of the relatively high initial cost, the geothermal heat pump is economically feasible only in the northern climates in the buildings with a medium and high insulation level.

The results of the research work show that increased insulation levels and local climate conditions have a huge influence on the economic feasibility of heat pump systems. Increased insulation levels lead to significantly more locations where air-to-air and geothermal heat pump systems are economically feasible. SCOP levels for heat pump systems should be presented depending on both the climate and the insulation level of the building.

REFERENCES

Bernier M., 2006, Closed-loop ground coupled heat pump systems, ASHRAE Journal 48(9), 12-19

Briggs, R.S., Lucas, R.G., and Taylor, T, 2003, Climate Classification for Building Energy Codes and Standards: Part 2-Zone Definitions, Maps and Comparisons, Technical and Symposium Papers, ASHRAE Winter Meeting, Chicago, IL

Claeeson J., Eskilson P., 1988, Conductive heat extraction to a deep borehole: thermal analyses and dimensioning roles. Energy, 13, 509-27

Florides G., Kalogirou S., 2007, Ground heat exchangers--a review of systems, models and applications. Renevable Energy, 32, 2461-78

Francisco P.W., Davis B., Baylon D., Palmiter L., 2004, Heat Pump System Performance in Northern Climates, ASHRAE Transactions, 110, 442-451

Hendron R., Engebrecht C., 2010, Building America House Simulation Protocols, National Renewable Energy Laboratory

Kacanaough S.P., Rafferty K., 1997, Ground-source heat pumps: design of geothermal systems for commercial and institutional buildings, Atlanta, ASHRAE Transaction

Lovvorn N.C., Hiller C.C, 2002, Heat Pump Life Revisited, ASHRAE Transactions, 108

Peippo K., Lund P.D.,1999, Vartiainen E., Multivariable optimization of design trade offs for solar low energy buildings, Energy and Buildings, 29(2);189-205.

R. S. Means, 2013. R.S. Means Residential Cost Data, 24th Annual Edition. Kingston, MA: RS Means Construction Publishers & Consultants.

Self S.J., Reddy B.V., Rosen M.A.,2013, Geothermal heat pump systems: Status review and comparison with other heating options, Applied Energy 101, 341-348

Spitler J.D., Fisher D.E., 2012, Feasibility of Foundation Heat Exchangers for Residential Ground Source Heat Pump Systems in the United States, ASHRAE Transactions, 27, 1039-1048

Robert F., Gosselin L.,2013, New methodology to design ground coupled heat pump systems based on total cost optimization, Applied Thermal Engineering 62, 481-491

Yavuzturk C., Spitler J.D., 1999, A short time step response factor model for vertical ground loop heat exchangers, ASHRAE Transactions 105 (2): 475-485

Lars Junghans, PhD

Associate Member ASHRAE

Table 1 Insulation Level

Insu-     U-value SI W/m2K (IP R-value)                        infil-
lation                                                         tration
level        Wall         Roof         Floor        Window       ACH

low        1 (5.7)      1 (5.7)      1.5 (3.7)    5.2 (1.10)    0.42
medium     0.4 (14)     0.4 (14)    0.5 (11.3)    1.8 (3.1)     0.20
high      0.25 (23)    0.25 (23)     0.3 (19)     1.5 (3.8)     0.15

Table 2. Comparison of LCC and SCOP values

                          low insulation level

                 Power    furnace    HP air           HP geo

                            LCC       LCC     SCOP     LCC     SCOP

                  [kW]       m         m        H       m        H

San_Francisco     20.6      45.4      38.4     1.8     51.0     2.7
Boulder           50.7     108.0     168.9     0.9    179.0     1.5
Miami             15.8      2.4       2.7      2.7     17.1     3.7
Adanta            34.4      50.7      62.3     1.2     86.3     1.9
Chicago           48.1     101.7     161.4     0.9    182.3     1.5
Pordand           50.8     130.0     214.8     0.9    218.8     1.5
Houston           28.2      19.1      19.2     1.6     43.3     2.4

                 medium insulation level

                 Power    furnace    HP air          HP geo

                            LCC       LCC    SCOP     LCC    SCOP

                  [kW]       m         m       H       m       H

San_Francisco     6.8       14.5       m      3.5    13.2     5.1
Boulder           15.5      31.8     27.3     1.8    34.2     3.2
Miami             4.2       1.1       1.9     4.8     5.9     6.1
Adanta            10.7      16.4     11.2     2.5    19.7     3.9
Chicago           14.4      30.6     26.6     1.8    34.5     3.1
Pordand           16.0      42.3     39.0     1.7    44.1     2.9
Houston           7.7       4.5       3.6     3.2    10.5     5.1

                 high insulation level

                 Power    furnace HP air          HP geo

                           LCC     LCC    SCOP     LCC    SCOP

                  [kW]      m       m       H       m       H

San_Francisco     3.0      6.4     3.6     4.7     6.1     6.7
Boulder           6.0     12.1     8.1     2.7    11.4     5.3
Miami             1.2      0.9     1.9     4.5     2.4     6.1
Adanta            4.3      7.2     4.4     3.7     7.9     6.0
Chicago           5.6     12.1     8.1     2.7    11.4     5.2
Pordand           6.8     19.3    13.1     2.4    15.8     4.6
Houston           2.2      1.4     2.1     4.0     3.8     6.6
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Author:Junghans, Lars
Publication:ASHRAE Transactions
Article Type:Report
Date:Jul 1, 2014
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