Geothermal HVAC Systems - The Basics and Applications (Draft) George Hu, P.E., LEED AP Air Water Energy Engineers, Inc. Updated 11/27/2012
The Geothermal Basics Geothermal Map of the U.S.A.: Temperature > 212 F Temperature < 212 F Map source: Geo-heat Center, Oregon Institute of Technology Terminology: Geothermal vs. GSHP Geothermal Resources Classification per DOE High temperature geothermal resources: Underground reservoirs of steam, hot water and dry hot rocks (e.g., electricity generation via steam turbines) Moderate-to-low temperature geothermal resources: Direct use applications such as for space heating Lower temperature, shallow ground geothermal resources: Used by geothermal heat pumps for space heating and cooling
On-site Renewable Energy Classification per USGBC Deep-earth water or steam sources: Geothermal energy systems using deep-earth water or steam sources (but not vapor compression systems for heat transfer). These types of systems are eligible for renewable energy credits for LEED. Examples include Geothermal Heating Systems, Geothermal Electric Systems. Geoexchange / Ground Source Heat Pump / Geothermal Heat Pump systems: Systems that use vapor compression cycles for heat transfer and do not obtain significant quantities of deep earth heat. These types of systems are ineligible for on-site renewable energy credits for LEED. Ground Source Heat Pump Systems (aka Geothermal Heat Pump Systems) for Building HVAC Water-to-Air Ground Source Heat Pump: (Refrigerant) Vapor Compression Cycles Water-to-Water Ground Source Heat Pump: (Refrigerant) Vapor Compression Cycles
System Types by Ground Construction Horizontal Closed System: Buried HDPE pipes Pump circulating water / glycol in ground loop Ground is heat storage device: Heat rejected to ground in cooling season Heat extracted from ground in heating season System: HDPE U-tubes in vertical boreholes Pump circulating water / glycol in ground loop Ground is heat storage device: Heat rejected to ground in cooling season Heat extracted from ground in heating season
Standing Column Well System: Emergency bleed to prevent heat pump freezing or loop temperature too high Well Pump Open (no glycol) Lake Source Closed : Lake, River or Other Water Body Coil or heat exchanger isolates heat transfer fluid and source water Lake Source Open : Lake, River or Other Water Body Direct use of source water
Ground Water Temperatures in the U.S.A. Example underground Temperatures: Boston: 50 o F Philadelphia: 55 o F Miami: 78 o F Constant underground temperatures: - Warm during heating season; - Cool during cooling season - High energy efficiency for heat pumps due to the physics of heat transfer Data source: ASHRAE Design of Geothermal Systems for Commercial and Institutional Buildings, 1997
High Energy Efficiency of Geothermal Heat Pump Systems Heat from the Ground For example, in heating application, for a heat pump with COP = 4.0, for every BTU of heat delivered to space, ¼, or 0.25 BTU comes from electricity, the other 3/4 BTU comes from the ground. The water pump also consumes certain amount of electricity, but compared with the heat delivered to the space, that amount of electricity is very small, therefore not substantially affecting the overall system COP. (COP = Coefficient of Performance = heat output / electricity input) With oil or gas-fired heating system, for every BTU of heat to be delivered to the space, it requires 1 BTU of oil or gas to be burnt. Considering the fact that the fuel burning device (e.g., boiler) is not 100% efficient, it actually requires more than 1 BTU of fuel to be burnt (for boiler efficiency of 85%, it requires 1.18 BTU to be burnt). During cooling seasons, the geothermal heat pump system is also more energy efficient than conventional air-cooled systems, because the ground is cooler than the air, making it easier for the heat to be ejected to the ground than to the air. Efficiency from Heat Recovery Heating Cooling Cooling Cooling Heating Rejected heat from cooling units gets utilized by heating units in the same ground loop
Geothermal Heat Pump System Applications Heating, Ventilating and Air-Conditioning (HVAC) HVAC with geothermal heat pump systems can be applied to all geographic areas in the U.S. However, application to certain climate areas where either heating or cooling is dominant require careful consideration of the specific building and its requirement. In heating or cooling dominant climate areas, ground heat imbalance could be a problem, but it depends on the specific building project and its requirement. Proper systems could be engineered to address ground heat imbalance concerns or even take advantage of it to the project s benefit. Contact your geothermal engineer to assess the suitability of geothermal for your project. Domestic Hot Water Heating The high energy efficiency of geothermal heat pump systems can be taken advantage to produce domestic hot water. For residential and light commercial applications, the temperature of hot water produced by residential type geothermal heat pumps is usually limited to 120 o F, just high enough to meet most residential domestic use. With new technology available today, large commercial HVAC equipment manufacturers are making geothermal heat pump products that are capable of producing hot water temperatures up to 170 to 180 o F, suitable for large commercial projects. Snow-melting Snow melting is an energy intensive activity, which makes geothermal heat pump systems appealing due to their high energy efficiency. Snow melting systems may be a requirement in places such as emergency access areas of hospitals, airports and other critical commercial or industrial facilities. Snow melting systems also found their use in residential driveways especially in luxury houses. District Heating and Cooling Sources for Campuses or Group of Buildings Geothermal Heat Pump Systems are increasingly being used as district heating and cooling energy sources for campuses and group of buildings, due to their high energy efficiency and reduced environmental impact such as reduced greenhouse gas emissions. Sometimes these district energy systems are built to replace existing dirty coal-fired energy plants in university campuses. Geothermal heat pump systems have also been successfully built for large residential projects that involve multiple houses or apartment buildings. Wide Variety of Building Types Potential Candidates for Geothermal Systems Geothermal heat pump systems do not use any fuel but electricity, which almost any building project has access to. In theory, any building which needs heating and/or cooling can utilize geothermal heat pump systems. Practically, geothermal application may be limited by available land area to lay out horizontal ground loops or to drill the required number of vertical boreholes; or, it may be limited by the lack of water rights or strict environmental regulations if it is a lake or river type system in question. Other practical concerns include higher upfront costs, which involves the calculations of economics or return of investment period to verify geothermal feasibility.
Because of geothermal systems high energy efficiency and the fact that they don t require boilers and chillers / cooling towers, and the fact that the ground loop, once built, is invisible, there are certain types of building projects are particularly suited for geothermal systems, such as: Renovation of historical buildings Lack of room to install major mechanical equipment such as chillers and cooling towers Lack of room to run major ductwork and other mechanical components Aesthetics is a primary concern Building projects that have high energy performance goals and/or high LEED certification goals High energy efficiency of geothermal system helps to achieve the goal Buildings that target for net-zero energy performance High energy consumption reduction by geothermal system make net-zero energy goal easier to achieve Buildings that are highly energy intensive (such as hospitals, research labs etc.) The economics are easier to work out to favor geothermal systems when the building is highly energy intensive payback time would be shorter Buildings that lack access to natural gas It s an opportunity to go with the clean, high energy efficiency geothermal system Buildings that is close to available water bodies such as a river or a lake This is particularly true for many existing mill buildings which used to rely on hydro power from the rivers to drive the machines inside the buildings Buildings owned by institutions Institutional buildings are usually owned by the same owner for long period of time, the owner reap the operational savings from the geothermal systems Buildings that want to project image of sustainability conscientiousness The building itself could be a good educational tool for its occupants regarding sustainability. Having a geothermal system enhances that image There are many factors to consider for potential geothermal systems for a particular building project. That is especially true for existing buildings. For suitability of geothermal for your specific project, solicit advice from your geothermal HVAC engineer.
A Glance of Reported Geothermal HVAC Projects Project Size Location Geothermal System Type Eureka Lodge Retrofit (2008) King s Mill Hospital (2011) Cerom Grain Research Center (2009) Chinese National Stadium (The Bird Nest) (2008) Liberty Island Retail Pavilion (2010) Springhill Suites Hotel (2008) Mistissini Community Center (2009) NewBridge on the Charles Hebrew Senior Life Community (2009) Whitmore Lake High School (2006) Ball State University West Chester University Private Residence (2009) Driveway Snow-melting System (2010) 28-room, 11,000 SF 920-bed, 1.4m SF 25,600 SF Main Bldg. + (3) 1,700 SF Greenhouses 91,000-seat Energy Savings Carbon Reduct ion Payback Time Comments Eureka, CO Open $1,650 /month Geothermal enabled building/business to operate year-round, not limited by unavailability of propane during snow season Mansfield, England Montreal, Canada Beijing, China 7,000 SF Liberty Island, NY 80,145 SF Pensacola Beach, FL 74,809 SF North Quebec, Canada 1m SF Dedham, MA 150,000 SF Whitmore Lake, MI 5.8m SF, 40+ buildings Muncie, IN Lake Source Closed Standing Column Well ; Hybrid System Horizontal + Lake Source Closed 1.8m SF West Chester, PA 15,000 SF Rye, NH ~7,000 SF driveway Rye, NH 126,500 /Yr 2,078 Tons /Yr $63,000 / Yr 347 Tons / Yr 4.3 Years Geothermal heating & cooling for athletic suites, media rooms, underground venues Fuel oil expensive on the island; Unsightly mechanical equipment not desired due to location Energy intensity 43% lower than comparison w/ conventional system Payback not in question system cost less than conventional system $153,600 /Yr 4.6 Years Facility has ice arena that requires simultaneous heating & cooling $325,000 / Yr 8,000 Tons / Yr 35% savings 7.5 Years $2m / Yr 8.8 Years Convert entire campus to geothermal, 1 st phase started in 2009. 70% + savings for heating; 40% savings for cooling 66 88% savings compared with 2008 & 2009