GHP Efficiency
Content
Edition No. 22
April 2005
ENGINEERING
S Y S T E M S O L U T I O N S
I
n previous issues of Engineering System Solutions, we have demonstrated the importance of using energy analysis to determine optimal HVAC solutions for a building. This newsletter provides the same analysis for geothermal water source heat pump systems. We have taken a typical HVAC system for small office and school applications and analyzed the energy savings and life cycle costs of implementing and optimizing a geothermal system. The result is a system that is significantly more efficient than alternative systems, with lower life cycle costs and a favorable payback to justify possible installed cost premiums. Bob Koschka, our senior applications engineer for water source heat pumps, provided the technical expertise in the development of this newsletter. More information about geothermal water source heat pump systems and how to apply them can be found in our Geothermal Heat Pump Design Manual (AG31-008). For a copy, contact your local McQuay representative or visit www.mcquay.com. Carol Marriott, P.Eng. Applications Manager McQuay International
Optimizing Geothermal Heat Pump Systems For Higher Efficiency, Maximum LEED Points and Lower Installed Costs
Why are Geothermal Heat Pumps So Efficient? Geothermal heat pumps are significantly more efficient than traditional water source heat pump systems. Traditional systems use a boiler and a tower to maintain their loop temperature. When the loop temperature rises above the set point, the closed circuit cooler works to lower the temperature. When the loop temperature drops below the set point, a boiler is used to raise the loop temperature. Loop temperatures are often maintained between 60°F and 90°F. Geothermal systems use the ground, a pond or well water to maintain their loop temperature. As a result, no fossil fuel is expended, significantly reducing the energy use of the system. Loop temperatures can range from 35°F to 100°F. The lower loop temperatures provide more efficient cooling than traditional systems, particularly at part load. Because the majority of the operating hours in most commercial applications are devoted to cooling at part load, the geothermal system will be significantly more efficient. For example, a traditional system maintaining a loop temperature above 60°F might have a performance of 22 EER. A geothermal system can have a performance as high as 36 EER. The remaining portions of this newsletter demonstrate how these efficiencies can pay off for the building owner over the life of their geothermal HVAC system. Set A Baseline Design And Compare Efficiencies The first step in demonstrating the efficiency of a geothermal system is to set a baseline design. To do this, we followed the Informative Appendix G, Performance Rating Method, from ASHRAE Standard 90.1. Our baseline is a 5-story, 100,000 ft2 office building with standard office hours, a common application for water source heat pumps and several other HVAC systems. Using McQuay Energy Analyzer™ software, we can do a quick analysis to determine the energy savings and life cycle payback of the geothermal water source heat pump system. Our baseline HVAC system is a packaged VAV rooftop system that meets the minimum requirements of ASHRAE 90.1-2004. With the exception of the HVAC system, all other design parameters are kept the same to provide an equal comparison of the systems. The results for five different cities are shown in Table 1. The energy rates used in this analysis are average state energy rates in accordance with the ASHRAE Standard 90.1 Informative Appendix G. These are published by the Energy Information Administration of the Department of Energy (DOE). They are updated annually and can be found at http://www.eia.doe.gov.
Conventional Boiler/Tower System
Table 1. Comparison of Baseline VAV Rooftop System and Geothermal Water Source Heat Pump System in a 5-story, 100,000 ft2 Office Building.1
City Philadelphia, PA Minneapolis, MN Helena, MO Denver, CO Chicago, IL Climate Zone 4A 6A 6B 5B 5A Baseline Energy Cost (USD) 136,379 103,246 99,157 102,873 117,256 Geothermal Energy Cost (USD) 114,816 89,954 84,073 94,857 106,229 Percent Savings 15.80% 12.90% 15.20% 7.80% 9.40%
1Actual results may vary based on occupancy and occupied hours.
Geothermal Systems
In addition to energy rates, the differences in savings from Table 1 result from local requirements. For example, Philadelphia is in climate zone 4A (as defined by ASHRAE Standard 90.1-2004 and DOE climate zones) and economizers are not required for its baseline design. All other baseline designs require economizers, which lower energy costs because economizer hours reduce condensing unit hours. Comparing the results in Table 1 to LEED 2.2 guidelines2 shows that the geothermal system could earn up to two points under Energy and Atmosphere, Credit 1, depending on the energy rates and the location (Table 2). It is important to note that we are not comparing optimized systems in our example above. The geothermal system represents a typical design used by McQuay. The baseline VAV system provides a method for quantifying the efficiency difference between systems that meet all prerequisites for LEED Energy and Atmosphere, Credit 1. Refer to Edition 20 of Engineering System Solutions (April 2004), LEED
What about costs? The high efficiency of a geothermal system is often perceived to come at a cost premium, primarily because geothermal systems are relatively new compared to other common HVAC systems. Like many new technologies or systems, the cost premium can depend on the experience of the designer. In Table 3, we have calculated the simple payback of the geothermal system for the five cities in North America shown in Table 1. Some assumptions are made in this calculation. First, it is generally accepted that geothermal systems cost less to maintain than standard VAV rooftop systems. In order to be fair to both systems, the maintenance cost savings versus the rooftop VAV system was given a range from $0.02/ft2 to $0.06ft2. Second, the capital cost premium of the geothermal system was given a range from $0.50/ft2 to $1.50/ft2. Table 2. LEED 2.2 Energy and Atmosphere, Credit 12
Energy Cost Savings (%) 10.5 14 17.5 21 24.5 28 31.5 35 38 42
2At
LEED Points 1 2 3 4 5 6 7 8 9 10
Energy and Atmosphere, Credit 1 – Measuring Efficiency to Maximize LEED Points, for a complete
explanation of this method. In addition, although we modeled the building energy use in this analysis, as is required by Energy and Atmosphere, Credit 1, we have only compared the energy use of two HVAC systems. We have not examined the effects of changing the lighting, envelope or miscellaneous electric loads in the building. All of these factors must be considered to maximize points for LEED Energy and Atmosphere, Credit 1.
the time of this publication, the public comment period for LEED Version 2.2 had been completed but the guideline had not been published. Visit www.usgbc.org for the current version of the LEED Certification Program.
Table 3. Life Cycle Cost Comparison and Payback.
Capital Cost Premium ($/ft2) City Annual Utility Cost Savings ($) Annual Maintenance Cost Savings ($/ft2) $0.02 Philadelphia, PA 21,563 $0.04 $0.06 $0.02 Minneapolis, MN 13,292 $0.04 $0.06 $0.02 Helena, MO 15,084 $0.04 $0.06 $0.02 Denver, CO 8,016 $0.04 $0.06 $0.02 Chicago, IL 11,027 $0.04 $0.06 $0.50 2.1 2 1.8 3.3 2.9 2.6 2.9 2.6 2.4 5 4.2 3.6 3.8 3.3 2.9 $1.00 $1.50
Payback Period (years) 4.2 6.4 3.9 3.6 6.5 5.8 5.2 5.9 5.2 4.7 10 8.3 7.1 7.7 6.7 5.9 5.9 5.4 9.8 8.7 7.8 8.8 7.9 7.1 15 12.5 10.7 11.5 10 8.8
As you can see from Table 3, the payback for the cost premium of a geothermal system ranges from less than 2 years to 15 years, depending on the utility and maintenance cost savings, and the capital cost premium. The cost premium for a geothermal system is easily overcome by cumulative energy savings in some climates and applications. Note that simple payback only looks at first year cost savings. Rising costs for natural gas and electricity in many areas of the United States can reduce the payback period.
Table 4. Energy and Cost Comparison of 85°F and 95°F loops
Capital Cost Energy Energy Loop Size Difference Energy Payback Loop Size Used 95°F Used 85°F 95°F (ft) in Loop Savings ($) (years) 85°F (ft) Loop ($) loop ($) Size ($10/ft) 113,619 89,155 83,365 93,865 105,188 57,170 44,640 38,910 44,550 51,400 114,816 89,954 84,073 94,857 106,229 45,490 37,060 32,300 35,310 41,860 116,800 75,800 66,100 92,400 95,400 1,197 799 708 992 1,041 98 95 93 93 92
City
Philadelphia, PA Minneapolis, MN Helena, MO Denver, CO Chicago, IL
Table 5. Constant Flow Versus Variable Flow Using VFDs
Capital Cost Premium ($) City Utility Cost Savings ($) $6,000 $8,000 Payback Period (years) Philadelphia, PA Minneapolis, MN Helena, MO Denver, CO Chicago, IL 10,703 7,897 7,206 8,349 9,659 0.56 0.76 0.83 0.72 0.62 0.75 1.01 1.11 0.96 0.83 0.93 1.27 1.39 1.2 1.04 $10,000
Optimizing Geothermal Systems There are three parameters that should be considered in optimizing a geothermal design: The effect of raising the loop temperature on operating and capital costs, using Variable Frequency Drives (VFDs) on the pumps, and using energy recovery ventilators (ERV) for the make-up air. Each has been incorporated in the design of our geothermal system in this newsletter. Loop Temperature Versus Operating Costs The loop temperature of a geothermal system affects its efficiency and capital cost. A smaller geothermal loop will run at higher temperatures and decrease the efficiency of the water source heat pump units. However, a smaller loop results in significant capital cost savings. Table 4 compares the energy use and capital cost of geothermal systems with loops designed at 85°F and 95°F. From this chart you can see that the energy penalty for the smaller loop is very small and may justify the capital cost savings of using a smaller loop. Adding VFDs to the Pumps The pumps serving geothermal systems are small, but they provide constant flow and run continuously. These small pumps can use a significant amount of energy over the course of a year. Adding VFDs to the system to provide variable flow can reduce this energy consumption considerably. In recent years, VFD costs have been reduced so that the payback (in energy cost savings) is worth the capital cost premium for installing a VFD (Table 5).
Energy Recovery of Ventilation Air Table 6. Energy Recovery Ventilation Payback In many parts of the United States, makeCapital Cost Premium ($/CFM) up air must be conditioned before it Utility Cost $0.50 $1.00 $1.50 City enters the building. Water source heat Savings ($) Payback Period (years) pump units are generally are not suitable for handling ventilation loads because Philadelphia, PA 3,288 2.74 5.47 8.21 they cycle on and off. In the off cycle, Minneapolis, MN 5,252 1.71 3.43 5.14 dehumidification does not occur and humid air can enter the space. We Helena, MO 4,612 1.95 3.9 5.85 modeled a standard make-up air unit and Denver, CO 1,566 5.75 11.49 17.24 an energy recovery ventilation (ERV) unit Chicago, IL 3,842 2.34 4.69 7.03 to supply ventilation air for our geothermal system. Using a range from be higher than more conventional systems, Conclusion $0.50/CFM to $1.50/CFM capital cost the payback is often very favorable for Geothermal water source heat pump premium for an ERV system, Table 6 achieving lower life cycle costs. For more systems are ideal for achieving high shows the climates that are most information on geothermal design, contact efficiency that pays back year after year in favorable for an ERV system supplying your local McQuay representative or visit energy cost savings for building owners. 18,000 cfm of outdoor air. www.mcquay.com. While the installed cost of the system can
The data and suggestions in this document are believed current and accurate at the time of publication, but they are not a substitute for trained, experienced professional service. Individual applications and site variations can significantly affect the results and effectiveness of any information. The reader must satisfy him/herself regarding the applicability of any article and seek professional evaluation of all materials. McQuay disclaims any responsibility for actions based on this document.
For comments or suggestions, please call or write: Chris Sackrison, Editor McQuay International 13600 Industrial Park Boulevard Minneapolis, MN 55441 Phone: (763) 553-5419 E-mail: [email protected] For more information on McQuay products and services, or to speak with your local representative, call (800) 432-1342, or visit our web page at www.mcquay.com. ©2005 McQuay International
April 2005
ENGINEERING
S Y S T E M S O L U T I O N S
I
n previous issues of Engineering System Solutions, we have demonstrated the importance of using energy analysis to determine optimal HVAC solutions for a building. This newsletter provides the same analysis for geothermal water source heat pump systems. We have taken a typical HVAC system for small office and school applications and analyzed the energy savings and life cycle costs of implementing and optimizing a geothermal system. The result is a system that is significantly more efficient than alternative systems, with lower life cycle costs and a favorable payback to justify possible installed cost premiums. Bob Koschka, our senior applications engineer for water source heat pumps, provided the technical expertise in the development of this newsletter. More information about geothermal water source heat pump systems and how to apply them can be found in our Geothermal Heat Pump Design Manual (AG31-008). For a copy, contact your local McQuay representative or visit www.mcquay.com. Carol Marriott, P.Eng. Applications Manager McQuay International
Optimizing Geothermal Heat Pump Systems For Higher Efficiency, Maximum LEED Points and Lower Installed Costs
Why are Geothermal Heat Pumps So Efficient? Geothermal heat pumps are significantly more efficient than traditional water source heat pump systems. Traditional systems use a boiler and a tower to maintain their loop temperature. When the loop temperature rises above the set point, the closed circuit cooler works to lower the temperature. When the loop temperature drops below the set point, a boiler is used to raise the loop temperature. Loop temperatures are often maintained between 60°F and 90°F. Geothermal systems use the ground, a pond or well water to maintain their loop temperature. As a result, no fossil fuel is expended, significantly reducing the energy use of the system. Loop temperatures can range from 35°F to 100°F. The lower loop temperatures provide more efficient cooling than traditional systems, particularly at part load. Because the majority of the operating hours in most commercial applications are devoted to cooling at part load, the geothermal system will be significantly more efficient. For example, a traditional system maintaining a loop temperature above 60°F might have a performance of 22 EER. A geothermal system can have a performance as high as 36 EER. The remaining portions of this newsletter demonstrate how these efficiencies can pay off for the building owner over the life of their geothermal HVAC system. Set A Baseline Design And Compare Efficiencies The first step in demonstrating the efficiency of a geothermal system is to set a baseline design. To do this, we followed the Informative Appendix G, Performance Rating Method, from ASHRAE Standard 90.1. Our baseline is a 5-story, 100,000 ft2 office building with standard office hours, a common application for water source heat pumps and several other HVAC systems. Using McQuay Energy Analyzer™ software, we can do a quick analysis to determine the energy savings and life cycle payback of the geothermal water source heat pump system. Our baseline HVAC system is a packaged VAV rooftop system that meets the minimum requirements of ASHRAE 90.1-2004. With the exception of the HVAC system, all other design parameters are kept the same to provide an equal comparison of the systems. The results for five different cities are shown in Table 1. The energy rates used in this analysis are average state energy rates in accordance with the ASHRAE Standard 90.1 Informative Appendix G. These are published by the Energy Information Administration of the Department of Energy (DOE). They are updated annually and can be found at http://www.eia.doe.gov.
Conventional Boiler/Tower System
Table 1. Comparison of Baseline VAV Rooftop System and Geothermal Water Source Heat Pump System in a 5-story, 100,000 ft2 Office Building.1
City Philadelphia, PA Minneapolis, MN Helena, MO Denver, CO Chicago, IL Climate Zone 4A 6A 6B 5B 5A Baseline Energy Cost (USD) 136,379 103,246 99,157 102,873 117,256 Geothermal Energy Cost (USD) 114,816 89,954 84,073 94,857 106,229 Percent Savings 15.80% 12.90% 15.20% 7.80% 9.40%
1Actual results may vary based on occupancy and occupied hours.
Geothermal Systems
In addition to energy rates, the differences in savings from Table 1 result from local requirements. For example, Philadelphia is in climate zone 4A (as defined by ASHRAE Standard 90.1-2004 and DOE climate zones) and economizers are not required for its baseline design. All other baseline designs require economizers, which lower energy costs because economizer hours reduce condensing unit hours. Comparing the results in Table 1 to LEED 2.2 guidelines2 shows that the geothermal system could earn up to two points under Energy and Atmosphere, Credit 1, depending on the energy rates and the location (Table 2). It is important to note that we are not comparing optimized systems in our example above. The geothermal system represents a typical design used by McQuay. The baseline VAV system provides a method for quantifying the efficiency difference between systems that meet all prerequisites for LEED Energy and Atmosphere, Credit 1. Refer to Edition 20 of Engineering System Solutions (April 2004), LEED
What about costs? The high efficiency of a geothermal system is often perceived to come at a cost premium, primarily because geothermal systems are relatively new compared to other common HVAC systems. Like many new technologies or systems, the cost premium can depend on the experience of the designer. In Table 3, we have calculated the simple payback of the geothermal system for the five cities in North America shown in Table 1. Some assumptions are made in this calculation. First, it is generally accepted that geothermal systems cost less to maintain than standard VAV rooftop systems. In order to be fair to both systems, the maintenance cost savings versus the rooftop VAV system was given a range from $0.02/ft2 to $0.06ft2. Second, the capital cost premium of the geothermal system was given a range from $0.50/ft2 to $1.50/ft2. Table 2. LEED 2.2 Energy and Atmosphere, Credit 12
Energy Cost Savings (%) 10.5 14 17.5 21 24.5 28 31.5 35 38 42
2At
LEED Points 1 2 3 4 5 6 7 8 9 10
Energy and Atmosphere, Credit 1 – Measuring Efficiency to Maximize LEED Points, for a complete
explanation of this method. In addition, although we modeled the building energy use in this analysis, as is required by Energy and Atmosphere, Credit 1, we have only compared the energy use of two HVAC systems. We have not examined the effects of changing the lighting, envelope or miscellaneous electric loads in the building. All of these factors must be considered to maximize points for LEED Energy and Atmosphere, Credit 1.
the time of this publication, the public comment period for LEED Version 2.2 had been completed but the guideline had not been published. Visit www.usgbc.org for the current version of the LEED Certification Program.
Table 3. Life Cycle Cost Comparison and Payback.
Capital Cost Premium ($/ft2) City Annual Utility Cost Savings ($) Annual Maintenance Cost Savings ($/ft2) $0.02 Philadelphia, PA 21,563 $0.04 $0.06 $0.02 Minneapolis, MN 13,292 $0.04 $0.06 $0.02 Helena, MO 15,084 $0.04 $0.06 $0.02 Denver, CO 8,016 $0.04 $0.06 $0.02 Chicago, IL 11,027 $0.04 $0.06 $0.50 2.1 2 1.8 3.3 2.9 2.6 2.9 2.6 2.4 5 4.2 3.6 3.8 3.3 2.9 $1.00 $1.50
Payback Period (years) 4.2 6.4 3.9 3.6 6.5 5.8 5.2 5.9 5.2 4.7 10 8.3 7.1 7.7 6.7 5.9 5.9 5.4 9.8 8.7 7.8 8.8 7.9 7.1 15 12.5 10.7 11.5 10 8.8
As you can see from Table 3, the payback for the cost premium of a geothermal system ranges from less than 2 years to 15 years, depending on the utility and maintenance cost savings, and the capital cost premium. The cost premium for a geothermal system is easily overcome by cumulative energy savings in some climates and applications. Note that simple payback only looks at first year cost savings. Rising costs for natural gas and electricity in many areas of the United States can reduce the payback period.
Table 4. Energy and Cost Comparison of 85°F and 95°F loops
Capital Cost Energy Energy Loop Size Difference Energy Payback Loop Size Used 95°F Used 85°F 95°F (ft) in Loop Savings ($) (years) 85°F (ft) Loop ($) loop ($) Size ($10/ft) 113,619 89,155 83,365 93,865 105,188 57,170 44,640 38,910 44,550 51,400 114,816 89,954 84,073 94,857 106,229 45,490 37,060 32,300 35,310 41,860 116,800 75,800 66,100 92,400 95,400 1,197 799 708 992 1,041 98 95 93 93 92
City
Philadelphia, PA Minneapolis, MN Helena, MO Denver, CO Chicago, IL
Table 5. Constant Flow Versus Variable Flow Using VFDs
Capital Cost Premium ($) City Utility Cost Savings ($) $6,000 $8,000 Payback Period (years) Philadelphia, PA Minneapolis, MN Helena, MO Denver, CO Chicago, IL 10,703 7,897 7,206 8,349 9,659 0.56 0.76 0.83 0.72 0.62 0.75 1.01 1.11 0.96 0.83 0.93 1.27 1.39 1.2 1.04 $10,000
Optimizing Geothermal Systems There are three parameters that should be considered in optimizing a geothermal design: The effect of raising the loop temperature on operating and capital costs, using Variable Frequency Drives (VFDs) on the pumps, and using energy recovery ventilators (ERV) for the make-up air. Each has been incorporated in the design of our geothermal system in this newsletter. Loop Temperature Versus Operating Costs The loop temperature of a geothermal system affects its efficiency and capital cost. A smaller geothermal loop will run at higher temperatures and decrease the efficiency of the water source heat pump units. However, a smaller loop results in significant capital cost savings. Table 4 compares the energy use and capital cost of geothermal systems with loops designed at 85°F and 95°F. From this chart you can see that the energy penalty for the smaller loop is very small and may justify the capital cost savings of using a smaller loop. Adding VFDs to the Pumps The pumps serving geothermal systems are small, but they provide constant flow and run continuously. These small pumps can use a significant amount of energy over the course of a year. Adding VFDs to the system to provide variable flow can reduce this energy consumption considerably. In recent years, VFD costs have been reduced so that the payback (in energy cost savings) is worth the capital cost premium for installing a VFD (Table 5).
Energy Recovery of Ventilation Air Table 6. Energy Recovery Ventilation Payback In many parts of the United States, makeCapital Cost Premium ($/CFM) up air must be conditioned before it Utility Cost $0.50 $1.00 $1.50 City enters the building. Water source heat Savings ($) Payback Period (years) pump units are generally are not suitable for handling ventilation loads because Philadelphia, PA 3,288 2.74 5.47 8.21 they cycle on and off. In the off cycle, Minneapolis, MN 5,252 1.71 3.43 5.14 dehumidification does not occur and humid air can enter the space. We Helena, MO 4,612 1.95 3.9 5.85 modeled a standard make-up air unit and Denver, CO 1,566 5.75 11.49 17.24 an energy recovery ventilation (ERV) unit Chicago, IL 3,842 2.34 4.69 7.03 to supply ventilation air for our geothermal system. Using a range from be higher than more conventional systems, Conclusion $0.50/CFM to $1.50/CFM capital cost the payback is often very favorable for Geothermal water source heat pump premium for an ERV system, Table 6 achieving lower life cycle costs. For more systems are ideal for achieving high shows the climates that are most information on geothermal design, contact efficiency that pays back year after year in favorable for an ERV system supplying your local McQuay representative or visit energy cost savings for building owners. 18,000 cfm of outdoor air. www.mcquay.com. While the installed cost of the system can
The data and suggestions in this document are believed current and accurate at the time of publication, but they are not a substitute for trained, experienced professional service. Individual applications and site variations can significantly affect the results and effectiveness of any information. The reader must satisfy him/herself regarding the applicability of any article and seek professional evaluation of all materials. McQuay disclaims any responsibility for actions based on this document.
For comments or suggestions, please call or write: Chris Sackrison, Editor McQuay International 13600 Industrial Park Boulevard Minneapolis, MN 55441 Phone: (763) 553-5419 E-mail: [email protected] For more information on McQuay products and services, or to speak with your local representative, call (800) 432-1342, or visit our web page at www.mcquay.com. ©2005 McQuay International
