Special to Licensed Architect Ground Source Heat Pumps - Conditioning a Building Near You

Transcription

1 Special to Licensed Architect Ground Source Heat Pumps - Conditioning a Building Near You By Garret W. Graaskamp, P.G., A.I.; American Ground Water Trust Learning Objectives: After taking this course, the reader will have a better understanding of: 1. What geothermal energy is and its technology 2. The common types of heat pumps used to collect the earth s energy 3. The cost benefits of ground source heat pumps 4. How to properly design and install GSHP systems In 2008, approximately one out of every 38 new homes included a ground source heat pump (GSHP) to condition the interior space. According to Air-conditioning Heating and Refrigeration Institute (AHRI) statistics, the number of shipped GSHP units more than doubled from 33,000 in 2005 to over 71,000 units in Shipment volume (40,417 units) was 35% ahead of the 2008 pace (29, 894 units) through June The building industry has taken notice of the environmental efficiency and low operating and life-cycle costs of GSHP technology. GSHP technology has (unfairly) been the ugly stepsister to solar and wind "renewable energy" for decades because it does not produce electricity directly. The 24/7 reliability of GSHPs and their use of the sun s renewable energy has been overlooked for years. The American Recovery and Reinvest-ment Act of 2009, signed into law this past February, finally recognized the value of GSHPs as a significant technology in the nation s effort to reduce our reliance on foreign fossil fuel supplies and global climate change impacts. The Act authorizes, among other incentives, a 30 percent tax credit for residential installations and a 10 percent credit plus accelerated depreciation for commercial installations through The technology is not new. Nearly every home in the nation uses it 24 hours per day to preserve food in refrigerators. William Cullen at the University of Glasgow first demonstrated in 1748 the vapor compression refrigeration process that is fundamental to GSHPs. It was not until 1834 when American Jacob Perkins, while living in London, first put the vapor compression refrigerator into practical use. However, refrigerators did not become a common consumer product until after the invention of Freon in Freon replaced hazardous refrigerants then in use such as methyl chloride, ammonia and sulfur dioxide. In particular, methyl chloride units had been involved with several deaths in the early 1920 s and people started putting their refrigerators out in the backyard. The thought of conditioning the air in a building using GSHPs was first mentioned in the commercial press on October 25th 1948 in Modern Living Section of Life magazine in an article entitled "Fireless Furnace." Although General Electric, among other companies, ramped up production and marketing of food refrigerators to the nation in the early 1950s, it took until the late 1970s before GSHPs began to establish a foothold in the HVAC marketplace. EARTH ENERGY GSHPs harvest "low temperature" geothermal heat energy from the ground beneath our feet. The source of the energy is renewable everyday from the sun and occurs everywhere across the world. Forty-seven percent of the energy radiating into the earth's atmosphere is absorbed into the ground and, on a sunny day, this is enough energy to replenish all the energy used in the world on an annual basis (5 x 1020 Joules [U.S. Energy Information Agency]) in about 90 minutes. This geothermal (little g) energy is different from the more commonly thought of "hot rocks" Geothermal energy ("Big G Geothermal") the uses steam to create electricity. Big G is associated with volcanoes and geysers with high temperature ground water above 300 F (150 degrees Celsius). Big G energy is derived from heat dissipating from hot magma (liquid rock) that has risen from very deep portions of the earth (the earth s mantle) to within 3 to 5 miles of the earth s surface. Big G energy is only found in limited areas around the world associated with current or former regional volcanism. The western United States has famous examples including volcanoes Mount Rainier, Mount St Helens and Yellowstone Figure 1: Map of the ground water temperatures in degrees Fahrenheit across the United States, which approximate the deep earth temperatures at about 30 feet below ground surface. (Figure courtesy of McQuay International; Application Guide AG ) 34 LICENSED ARCHITECT VOL 13 NO. 3 FALL 2009

2 National Park. However, in total Big G is limited in the United States to only portions of the states of Washington, Oregon, California, Arizona, New Mexico, Nevada, Utah, Idaho, Montana and Wyoming. There are no Big G locations east of the Mississippi River capable of producing electricity. Outside the Big G areas of the United States and indeed the world, the deep earth temperature at depth s between 30 to 500 feet is constant and equal to the average annual air temperature plus about 2 F (about 1oC) for a particular region. Figure 1 is a map of the ground water temperatures found around the United States, which approximate the ground temperatures at about 30 feet. Below about 500 feet in non-volcanic regions (that is, most everywhere), the earth s temperature begins to rise again at a rate of about 1.5 F (about 1 C) per 100 feet of depth due to heat flowing by conduc-tion out from the earth s interior. There is enough variation in the deep earth temperature that the temperature should be confirmed with a Thermal Conductivity (TC) test as part of designing a GSHP system for commercial structures (> 25 tons [a "ton" is equal to 12,000 Btus per hour; the amount of heat needed to melt a 2,000 pound block of ice at 32 F in 24 hours. ])). TC test data are necessary to optimize the operating efficiency of a large GSHP system. COLLECTING THE EARTH S ENERGY There are four common types of heat pumps including air-to-air, air-to-water, water-to-water and water-to-air. Air-to-air heat pumps have been popular for several decades, especially in regions with moderate air temperatures throughout the year. Air-to-water units are typically used in water heating activities and not space conditioning applications. Water-to-water units are an effective technology for radiant heat applications where seasonal cooling is not necessary or is accomplished with an air-to-air system. Water-to-air systems, which comprise most of the GSHPs being installed today, are the focus of the remainder of this article. Ground source heat pumps have been, and continue to be, called by several names including water source HPs, ground source HPs, geoexchange HPs, Geothermal HPs and earthcoupled HPs to name a few. Ground source heat pump (GSHP) is used here to emphasize that the source of the energy is the ground verses an airsourced system. The name also underscores a GSHP s capability to gather and reject heat energy to the ground regardless of whether there is groundwater present. The most common GSHP system consists of three "loops" including the indoor refrigerant loop, outdoor ground loop and the indoor air distribution loop. An optional fourth loop for domestic hot water (DHW) is a common easy addition to the system. Design of the air distribution loop including fresh air Figure 2: GSHP system (closed loop) in heating mode. (Figure courtesy of the International Ground Source Heat Pump Association) exchange, is important to any properly functioning space conditioning system, but is beyond the scope of this article. The refrigerant loop operates like any you will find in a refrigerator, except that the flow path can be reversed to accommodate a heating cycle in the winter and a cooling cycle in the summer. During the winter, heating season (Figure 2) the liquid refrigerant in the GSHP unit collects heat from the ground loop and evaporates as it accepts the heat energy at a constant temperature and pressure. The refrigerant gas continues to the GSHP compressor where the pressure and temperature of the refrigerant are increased to a superheated gas as it passes through the compressor. The superheated refrigerant travels to a second heat exchanger (e.g., fan coil) where interior return air passes over the refrigerant loop causing the refrigerant to condense to a liquid. The heat released during the condensation process warms the return air, which flows back to the living space. The refrigerant then passes through an expansion valve that serves to control the flow through the refrigerant loop and further cools the refrigerant. The cool refrigerant now starts the process again. In the summer, the reversing valve is switched and refrigerant flows in the opposite direction allowing heat from the interior air to be absorbed by the refrigerant and rejected back into the ground via the ground loop. THE GROUND LOOP There are two types of ground loop designs. One is an "open system" where ground water is pumped directly as the energy transfer fluid. The second is a "closed loop" system in which a piping circuit, usually made of High Density Polyethylene (HDPE) pipe, is used to circulate water or an (Continued on page 36) LICENSED ARCHITECT VOL 13 NO. 3 FALL

3 Association of (Continued from page 35) antifreeze solution between the ground and the refrigerant loop in the GSHP. A closed loop system is self-contained and there is no direct contact with the closed loop fluid and groundwater. A less common closed loop system known as direct exchange (DX) eliminates the water circuit and runs the refrigerant loop through copper piping directly one aquifer with another). "Pump and Dump" is the common term for water discharged to a surface location (Figure 3). This arrangement frequently requires state or federal discharge permits related to wetland concerns and water quality and quantity control issues. It is also extremely important to conduct an aquifer pump test at the design phase to ensure that there is enough ground water in the aquifer to satisfy the water demand from the GSHP system plus all other water needs for the structure. This is true whether one well or multiple supply wells are used on the property. The GSHP system will need between 1.5 to 3 gallons per minute per ton of conditioning load. An average home with a four ton conditioning load would require 1.6 million to 3.1 million gallons per year if the GSHP unit were operating only half-time throughout the year. Knowing the ground water quality is important for open systems. Mineralized water (Iron is a common problem) can precipitate and foul the well screens, reducing the flow and efficiency of the GSHP system. Damage or failure of the refrigerant-to-water heat exchanger inside the GSHP is also a Figure 3: "Pump and Dump" open loop systems pump groundwater from a water well and release it to a surface water body or second (discharge) well. (Courtesy of ClimateMaster Inc.) into the ground and then back to the GSHP unit to exchange with the air distribution loop. There are several types of open loop systems. Groundwater pumped through an open system may be returned to an aquifer (i.e., zone of underground water storage) after exchanging its heat with the GSHP unit refrigerant or it may be discharged to a surface water body such as a pond, lake or river. In the first case, most state regulatory agencies require that the groundwater be returned to the same aquifer from which it was obtained to maintain the water balance (out volume = returned volume) and to protect water quality (i.e., not mixing water from Figure 4: Horizontal ground loop systems are installed in shallow trenches or excavations. They are a type of closed loop system. (Courtesy of ClimateMaster Inc.) 36 LICENSED ARCHITECT VOL 13 NO. 3 FALL 2009 Figure 5: A "pond loop" collects heat energy from the surface water bodies and is an example of a closed loop system. (Courtesy of ClimateMaster Inc.) Figure 6: A vertical loop system can be installed on very small lots and does not require groundwater to be effective although the system must be designed to accommodate "dry" conditions if ground water is deep and not in contact with the loop pipe. (Courtesy of ClimateMaster Inc.)

4 consideration. The system design must address these concerns and include mitigating operating and/or treatment procedures. Closed loop systems can be designed as shallow (typically 4 to 8 feet deep) horizontal piping configurations (figure 4), submerged loops in ponds and lakes (figure 5), or in deep vertical boreholes (Figure 6). The choice of design depends on geology (local soil type and thickness) and the availability of open land space or a large surface water body. Vertical ground loops can be installed on "near zero" footprint lots but horizontal and pond loops require significantly larger footprint space. Loop spacing (separation) and piping length are critical components of any ground loop design. Less is never more. In 1954, professors at the University of Wisconsin-Madison demonstrated that a cubic foot of "soil" contained 40 Btus of energy in the form of heat. We now know that this quantity will vary widely depending on the geology and moisture content of the ground. In any case, a certain volume of ground will hold a certain quantity of heat energy just as a certain size gas tank holds a fixed number of gallons of gasoline (fixed amount of energy). If the ground loop spacing is too close and/or the total pipe loop length is too short for the actual energy loads to be conditioned in the structure, then the GSHP will not operate efficiently and will not be able to meet adequately the space conditioning demand and may ultimately fail. The inefficiency and demand response will become progressively worse with time. However, if the GSHP system is designed to access enough ground energy and the energy gathered from the ground is in balance with the energy returned to the ground, a GSHP system can satisfy the space conditioning needs of a building indefinitely. GSHP EFFICIENCY In 1993, the Environmental Protection Agency called GSHPs "the most energy-efficient, environmentally clean, and cost-effective spaceconditioning system" available. This fact has not changed. The basis for this statement lies with a GSHP s ability to move the energy stored as moderate year-round deep earth temperatures into (or out of) a structure as opposed to (1) air-to-air heat pump systems that exchange energy with seasonally-extreme outdoor air temperatures or (2) burning fossil fuels in on-site in relatively inefficient boilers and furnaces to create "new" heat energy. Traditional fossil fuel heating, ventilation and airconditioning (HVAC) systems are typically only 80 to 95 percent efficient when new or just after a maintenance treatment. Air-to-air systems may reach efficiencies of 300 percent, but not when you need them most in the depths of winter s cold or during the hottest days of summer. Current GSHP technology routinely operates at efficiencies of 400 to 500 percent. For each dollar spent on electricity to run a GSHP system, four to five dollars of heating or cooling is produced. Figure 7 presents a comparison of Btus per dollar spent on energy based on 2009 April/May fuel pricing information for Illinois compiled by the U.S. Energy Information Agency (EIA). Maintenance of a GSHP system is minimal compared to other traditional HVAC systems providing a clear operating cost advantage for GSHPs. Figure 7: Properly designed and installed GSHP systems in Illinois have the lowest operating costs per 100,000 Btu produced. They have a low carbon footprint impact compared to burning fuel oil or natural gas in an on-site furnace/boiler because 48 percent of the electricity consumed in Illinois is from non-carbon emitting renewable energy sources or nuclear power plants. (Data from the Energy Information Agency, August 2009) A FINAL CONSIDERATION "Rules of Thumb" are currently very common chatter among many new entrants into the GSHP design and installation industry. One must be extremely careful with these "rules" in order not to become thumbless as the result of an improperly designed or installed GSHP system. GSHPs are the most efficient space conditioning HVAC method available today. However, in order to earn this honor, the GSHP systems must be designed and installed properly by trained and experienced professionals. A team approach is usually the best strategy. When qualified in-house expertise is not available (especially for a large home or commercial project), it is recommended that the Architect should seek out an experienced International Ground Source Heat Pump Association (IGSHPA) Certified Geothermal Designer who is also a professional engineer, an IGSHPA or manufacturer accredited [ground loop] Installer and an appropriately licensed and experienced HVAC contractor to install the indoor equipment, piping and air distribution system. The buildings you design today will be in service for the next 50 to 100 years. Your HVAC choice will have an immediate and long-lasting impact on energy consumption and the carbon footprint created by operating your building creations. Please choose wisely. LICENSED ARCHITECT VOL 13 NO. 3 FALL

5 ALA Questionnaire - Ground Source Heat Pumps - Conditioning a Building Near You Learning Objectives: After taking this course, the reader will have a better understanding of: 1. What geothermal energy is and its technology 2. The common types of heat pumps used to collect the earth s energy 3. The cost benefits of ground source heat pumps 4. How to properly design and install GSHP systems Program Title: Licensed Architect - Ground Source Heat Pumps - Conditioning a Building Near You ALA/CEP Credit: This article qualifies for 1.0 HSW LU of State Required Learning Units and may qualify for other LU requirements. (Valid through November 2010.) Instructions: Read the article using the learning objectives provided. Answer the questions. Fill in your contact information. Check whether logging of ALA/CEP credit (ALA members with logging privileges only) or certificate of completion is desired. Sign the certification. Submit questions with answers, contact information and payment to ALA by mail or fax to receive credit. Article and tests are also available online: QUIZ QUESTIONS 1. Which type of GSHP is the most commonly installed version today? a) steam-to-air heat pump b) water-to-air heat pump c) air-to-air heat pump d) water-to-steam heat pump 2. The undisturbed deep earth temperature of the ground at a depth of 50 feet beneath a property is approximately equal to: a) the average annual air temperature plus 2 F b) the average annual water temperature in the nearest surface water body to the property c) the average annual summertime air temperature divided by four. d) one degree Fahrenheit per foot times the depth in feet of the vertical ground loop borehole 3. The most common GSHP system, without a domestic hot water option, consists of how many heat exchange loops? a) one b) two c) three d) four 4. During evaporation of the refrigerant in the refrigerant loop, heat is released by the refrigerant. a) True b) False 5. The most common installation depth for the pipe in a horizontal ground loop configuration is. a) 4 to 8 feet b) 8 to 12 feet c) 12 to 16 feet d) 16 to 20 feet 6. Ground source heat pumps can move the renewable energy from the sun that is stored beneath the ground to heat interior spaces of a building. a) True b) False 7. What percentage of the sun s energy is absorbed by the ground on an annual basis? a) 37 percent b) 41 percent c) 47 percent d) 53 percent 8. Standard ground source heat pumps cannot be used to produce electricity. a) True b) False 9. The vapor compression refrigeration process is fundamental to the operation of GSHPs. It was first demonstrated in what year? a) 1748 b) 1834 c) 1928 d) Which statement below is true? a) A closed loop system exchanges ground water with the refrigerant loop to transfer energy and an open loop system dumps ground water to either a surface water body or second well. b) A shallow horizontal closed loop confriguration uses groundwater to exchange heat energy with the GSHP refrigerant loop. c) An open ground loop system can pump groundwater directly to the refrigerant loop to exchange energy but a closed loop system pumps water or an antifreeze solution to the refrigerant loop. d) A "Pump and Dump" loop is a common description for an outdoor vertical closed loop configuration. Contact Information: Last Name: First Name: Firm Name: Middle Initial: PAYMENT: ALA/CEP Credit or Certificate of Completion: Cost: $15 (ALA Members) $20 (non-members) Check or Credit Card Please send me a certificate of completion (required by certain states & organizations) that I may submit. Please log me for ALA LU credit (ALA members with logging privileges only). Your test will be scored. Those scoring 80% or higher will receive 1 LU HSW Credit. Address: City: State: Zip: Tel.: Credit Card No: Expiration Date: (VISA or MASTERCARD only) Fax: Address: Association of, 22N159 Pepper Road, Ste. 2N, Barrington, IL Attn: ALA/CEP Credit Certification: (Read and sign below) I hereby certify that the above information is true and accurate to the best of my knowledge and that I have complied with the ALA Continuing Education Guidelines for the reported period. Signature: Date: 38 LICENSED ARCHITECT VOL 13 NO. 3 FALL 2009