Earth-Coupled Air Conditioning Unit Design Analysis Jassmine Duron, Manuel Monroy, M. A. Rafe Biswas Mechanical Engineering Department The University of Texas at Tyler jduron@patriots.uttyler.edu, mmonroy@patriots.uttyler.edu, mbiswas@uttyler.edu Abstract As residential energy costs continue to rise, homeowners seek methods of reducing their electricity consumption. An existing technology, like the Earth-Coupled Air Conditioning (ECAC) unit, offers homeowners an alternative method of replacing their energy guzzling conventional AC unit. The ECAC Unit uses the earth's constant temperature. The ECAC Unit removes the need of high electricity consumption components that are present in the standard AC unit and instead, uses the earth s constant temperature to provide space heating and cooling and compensate for the absence of such energy consuming components. The ECAC unit may use a U-bend or a spiral coil pipe line that is buried underground and to run a hydronic heat exchanger fluid. The approach taken for the design analysis of the U-bend and the spiral coil was to develop which system reduced the final cost of installation. The results indicated that by using a spiral coil instead of a U-bend coil reduced the depth of the borehole needed for the process from 92 ft to 46 ft. This reduction in depth led to a reduction of total hydraulic head required by the system from 320 feet for the U-bend to 90 ft for the spiral coil. By using the spiral coil instead of the U-bend, the costs were reduced for the unit. Introduction A major problem that has faced the current world is the increase in the cost of energy. From a residential point of view, the highest percentage of electricity consumption is accounted by the process of cooling and heating of space and air conditioning. Total energy consumption of Texas has risen by an average of 2.2 percent annually since 1960. Residential and commercial consumption both increased gradually, while the demand for transportation of fuel increased even more rapidly; a trend reflecting a growing population and an expanding economy. Industrial consumption fell by 13.3 percent from 2003 to 2005, due to higher energy prices and greater investments in efficiency 1. Energy efficiency can help meet the energy needs of the population by reducing the demand for energy. Better power plants, advanced automatic technology, and energy-saving lighting and appliances have proven that economic growth can be achieved with lower energy consumption. Air conditioning and water heating units have been proven to consume approximately two-thirds of the electricity used in a month by the average U.S. and Texan household, therefore the energy efficiency of these two applications is considered to be a significant factor in the process of lowering the electricity expenditure 2. The typical household family would be able to have the same results of using the needed energy at a lower electricity cost. The reason for the lowered cost derives from the decrease of energy usage, this system would move the heat from one medium to another instead of generating the thermal energy from electric energy. Technologies involving the usage of the Earth's near constant subterranean temperature to cool air
for residential homes have been explored in order to find a solution to the continuous rise in usage of electricity. The ECAC system consists of an underground heat exchanger that captures heat from the fluid passing through the tubes and dissipates heat to the ground. These systems still use the basic heat pump design of moving heat from one source to another, but also take advantage of the relatively constant ground temperature. At the depths of about 30 ft the earth remains at a constant temperature, reflecting the average air temperature of the area. For example, in Tyler, Texas, this temperature will stay steady between 68 and 71 F 3. This ECAC unit is a relatively simple system that requires low maintenance with the lower cost compared to conventional space heating and cooling systems. This paper will present and analyze a design of an ECAC unit for a research house located at the University of Texas at Tyler. The materials of construction of the unit were chosen through extensive research and calculations. The key analysis and design selection focused on the comparison of the underground loop piping of the geothermal system for two different layouts: a set of 3 U-bend loops and a spiral coil loop 4. Finally, a cost analysis for the ECAC unit compared initial cost, operation cost and energy consumption cost to a standard AC unit. Background Types of ECAC There are five types of earth-coupled systems available for use in the U.S. under the two categories of open and closed loops. There is only one type for an open loop system as shown in Fig. 1, while a close loop system, shown in Fig. 2, can be set up as four types: horizontal, vertical, slink and pond setting 4, 5. Fig 1: Open loop ECAC system The open loop system uses groundwater as a heat source or sink. Water is pumped from a well and flows through a heat exchanger, and then is discharged back to the environment. The primary problems found in this type of system include the availability of an adequate amount of water source to meet the needs of the home and the heat pump, and the quality of the water obtained. Water may contain chemicals and minerals that tend to foul the system by depositing scale or sludge on the heat exchanger.
In contrast, the close loop system is designed for space and water heating to circulate the liquid through a length of buried pipe to capture the heat in the soil while the warm liquid is returned to the heat exchanger to be cooled and continue circulation for several cycles as needed. The closed loop does not need the supply of groundwater while there is no need to pump out of or recharge back into the ground. This means the system is fully sealed to avoid the previously mentioned problems of the open loop system. Fig 2: Closed loop ECAC systems From the four types of closed loop system, a vertically installed pipe in Fig. 3 was chosen for its simple design 6. A vertical coupled system consists of one or more vertical boreholes through which water flows in the pipe. A distinct advantage of a vertical system over a horizontal system is the requirement of less surface area. A vertical closed loop uses approximately 150 ft of borehole per ton 2. The ECAC unit runs a line of piping underground in a loop. The soil temperature where the unit will be installed becomes an important factor to consider for the ECAC unit because it will transfer heat from the earth to the system or vice-versa. Soil Survey The temperature of the soil below 30 ft of the earth's surface remains relatively constant throughout the year. The average annual soil temperature for Tyler, Texas at a depth of 30 ft is around 68 to 71 o F 3. It is important for this temperature to be taken at this depth to remain constant, which will lead to greater pump efficiency for the system. The temperature must also be assessed when calculating the length of the vertical loops. Different soils depending on their soil properties like moisture content and dry density offer a variation in heat-flow resistance. For dry organic soils, the pipe length required to pick up sufficient heat may be 2 to 3 times the length required for a wet, granular soil. Fig. 4 shows a graph of the variation in heat flow resistance of soil with moisture content and soil type 7. The heat transfer rate also increases as soil texture becomes finer while loam mixes have transitional
values between sand and clay. In Tyler, Texas shown in the map in Fig. 5, the area consists of the East Texas Timberland soil 3. This area consists of upland soils but most are deep, light-colored, acid sands and loams over loamy and clayey subsoils. Deep sands and red clays are in scattered areas. The Bottomland soils are mostly brown to dark-gray, acid loams and some clays. The area mainly consists of clay and loams for the soil type. The thermal conductivity is 0.64 Btu/(ft hr o F) for clay and is 0.52 Btu/(ft hr o F) for loam 8. Once the soil properties and ambient temperatures were analyzed, the piping material for the underground coil was studied by comparing many different materials. It was determined that the International Ground Source Heat Pump Association (IGSHPA) recommends High Density Polyethylene (HDPE) pipe material for underground applications 9. Fig 3: Schematic of close vertical loop ECAC unit
Fig 4: Variation in heat flow resistance of soil Fig 5: Soil map of Texas
Design Specifications and Selection An initial comparison between materials of construction for both a standard AC unit and an ECAC unit showed the differences and similarities between the units as illustrated in Table 1. The units are compared for the university research house, which is a single-family, 1-story home with 3 bedrooms, 2 bathrooms and a garage 11. A total of 600 ft of borehole would be typically installed given a 4-ton unit in the research house 11. Thus, the pipe layouts for the U-bend and spiral coil loops, and pump and heat exchanger sizes are designed and evaluated accordingly. An overall cost analysis and comparison can be done on the two AC units based on the results obtained. Table 1: Materials of Construction Standard AC Unit Earth Coupled AC Unit Evaporator Evaporator Compressor Compressor 2 One Way Valves 2 One Way Valves Capillary Tube Capillary Tube * Condenser * Double Piped Heat Exchanger * 2 Fans * Fan * Pump & Motor Piping System The plastic material HDPE has been used repeatedly in many successful applications of geothermal heat exchangers. In retrospect, a metallic tube with a higher thermal conductivity would be ideal for efficient transfer heat. Such a tube would reduce the length of the underground tube used to remove from the system or add heat to the system. The pipes in the underground loop would also need to withstand deformations caused by the surrounding soil and environmental effects. Although different metals like stainless steel or copper could be used, the high cost is a significant deterrent and thus a cheaper material like HDPE is chosen. The HDPE has a low thermal conductivity of 0.4 W/(m K), which is significantly less than the thermal conductivity of any metal tubing. To compensate for the low thermal efficiency of HDPE, the surface area of the pipe is increased by selecting a bigger diameter. The increase in diameter means that more of the Polyethylene pipe will come in contact with the soil thus increase heat transfer. One of the tasks required by the project was to analyze the depth of the U-bend layout. The depth depends on the total length required to satisfy the 48,000 BTU/hr needed by the system. Louisiana State University managed to test a U-bend setup that included the 1-1/4 in. polyethylene pipe line 10. To meet the 48,000 BTU/hr heat transfer rate, the length required L is calculated from: Q = UΔTL (1) where Q is the rate of heat transfer of the heat exchanger in Btu/hr, L is the length of the heat exchanger in ft, U is the average conductance rate for heat transfer from the circulating fluid to the earth in Btu/(ft hr o F) and ΔT is described as
ΔT = (T EX + T IN)/2 - T E (2) where TEX is fluid exit temperature, TIN is fluid entry temperature and TE is the earth temperature. All temperatures are measured in o F 10. Table 3 illustrates the assumed temperature values used for the calculations of the pipe length. Table 3: Temperature values of the coolant loop Temperature o F TE 69 TIN 194 TEX 72 ΔT 64 For a single U-bend pipe layout, the total length required calculated is 552 ft. This results in a depth of 266 ft, which include a 20 ft horizontal length. For a 3 U-bend pipe layout similar to the vertical loop in Fig 2., each U-bend pipe length has to be 204 ft with a depth of 92 ft. The pipe diameter is evaluated given the research house located in Tyler, Texas that has the attic located 16.5 ft above ground and has a 3 vertical U-bend closed loop installed where each U-bend has a depth of 275 ft and horizontal spacing of 20 ft 11. The total length for the pipe was analyzed to be 1809 ft. To have the coolant loop link the outlet of the heat exchanger located in the attic to the underground pipe and connections of the other end of the underground pipe to the pump that connects into the inlet of the heat exchanger, the fittings are to be 3 NPT 90 degree elbows, 3 SW 90 degree elbows, 4 NPT unions and 4 sets of compressed coupling with a nut. Since HDPE pipe are commonly rated at a maximum of 250 psi, the nominal diameters ranging from 1/2 to 2 in. are employed to determine the total pressure change ΔP in the system ΔP = ρgδz + ρv 2 (fl T/D H+ΣK)/2 (3) where ρ is the fluid density in lbm/ft 3, g is the gravitational acceleration constant in ft/s 2, Δz is difference in height between the underground pipes and the house attic in ft, v is the fluid velocity in ft/s, f is the dimensionless friction factor in the pipe, LT is the length of the pipe in ft, DH is the diameter of the pipe in ft, and ΣK is the sum of dimensionless minor loss coefficients for all fitting and bends. Table 5 shows the results of the analysis for range of nominal diameters based on cost per length and change in pressure. The 1/2 and 3/4 in. nominal diameters have to be neglected because they exceed the rated maximum pressure. In addition, the 1-1/2 and the 2 in. nominal diameters are also undesirable due to high cost. Therefore, the 1 and 1-1/4 in. nominal diameters would be better options for the system.
Table 5: Cost analysis of pipe diameter Nominal diameter (in.) $/ft Total Cost ΔP (psi) 1/2 1.10 $ 1,890 665 3/4 1.50 $ 2,862 266 1 1.82 $ 3,293 170 1-1/4 3.14 $ 5,680 138 1-1/2 4.55 $ 8,231 132 2 4.91 $ 8,882 128 Pump Sizing After the length of the pipe layout has been calculated, a pump is selected to consistently provide the underground pipe line with the required pressure to move the enclosed fluid and is one of the most important components of the ECAC unit. For a proper pump selection, the calculations of the total hydraulic head of the existing system is obtained from the calculations of the total pressure change. The 1-1/4 inch pipe diameter required the system to have a pump that provided 320 ft of head. Based on the pump curve shown in Fig. 7, the E51G-T51G is selected for the system 12. Fig. 7: E51-T51 series catalog proposed performance pump curve The E51G-T51G pump estimates to provide the head of 320 ft at a capacity of 10 GPM for an efficiency of 38 percent. This pump comes equipped with a motor, and has a total length of 2 ft. The height of pump is 7 in., which includes the base plate. However, the pipe diameter has to be resized to a 1-1/2 in. nominal diameter Schedule 40 pipe to ensure proper connection to the T51G pump. Instead of installing a reducer fitting, the same diameter should be used in the system to prevent undesired head loss. Fig. 8 illustrates the layout of the ECAC unit with the fundamental components. Alternate Pipe Layout - Spiral loop An alternate pipe layout to the U-bend loop system investigated is the spiral loop system. The spiral loop coil configuration can minimize the vertical depth of the piping by circulating a length of the pipe around N number of times. The coil will wrap around the circumference of a cylindrical form with a diameter Dspiral. At each loop, the hypotenuse of the triangle is a distance P (pitch) along the
cylinder from the previous loop 13. The vertical depth, which must be a minimum of 30 ft to ensure the earth's constant temperature is reached, can be determined by h spiral = NP (4) where N is defined as 13 N = L T/(π 2 D spiral2 +P 2 ) 0.5 (5) where LT has been determined to be 1809 ft, Dspiral is selected to be 10 ft, and P is to be 0.277 ft. Fig. 8: Component layout of the ECAC U-bend unit The resulting calculations yield the number of turns N to be 58. The vertical depth of the spiral coil comes out to be 16 ft, which is added to the minimum displacement of 30 ft. Thus, the total vertical displacement becomes 46 ft. Both the horizontal and vertical lengths are reduced in the spiral coil compared to the set of 3 U-bend loops. Fig. 10 shows the final spiral coil layout. The advantages of the spiral coils are they offer a high heat transfer efficiency and reduce pipe connection complexity. The foundation piles for the spiral coil are much thicker in diameter and shorter in length. The U-bend piping loop is initially more costly to install because of the large
borehole compared to the smaller borehole for the spiral coil. In addition, for the U-bend loops, air choking may become a problem at the turning tips of the pipes to restrict flow of the fluid. On the other hand, the spiral coil loops can overcome this problem since the bends are replaced with smooth turns. Fig. 9: Spiral coil layout of the ECAC U-bend unit Heat Exchanger Sizing The overall ground heat exchanger design procedure includes the selection of the heat exchanger configurations, plastic pipe selection, load calculations and heat pump sizing. Double pipe heat exchangers are the most common type used in geothermal heating and cooling systems. These heat exchangers are affordable for both design and maintenance, making them a good choice for small applications. Double pipe heat exchangers are made by installing one or more tubes inside a package or root tube, and form inner channels and ring channels between the inside and outside tube. The fluids in the two channels exchange heat through the inner tube wall. The geothermal heating and air conditioning industry uses virtually the same standard heat exchangers commonly used in the chemical industry, and thus readily available in the market 16. Among the desirable characteristics of the heat exchanger, ease of maintenance and cleaning is needed to ensure optimal efficiency without interruptions in regular operation. The high corrosion potential usually presented in the fluid is a
critical factor taking place into the selection of the heat exchanger material and the length between the intervals when maintenance is need to be performed on the system. A large surface area of heat transfer is required for relatively low fluid temperatures and temperature differences like in a residential AC unit. In AC systems with a liquid heat source, the double tube heat exchangers are the main piece of equipment. Geothermal heat exchangers are typically made of a PVC shell and flexi-metal base. For the selection of the size of the heat exchanger, the house size and requirements are taken into consideration. Systems are generally sized in units of tons; an average home may require 3 tons, while a larger home may require 6 tons. Geothermal heat pumps can come in all standard sizes that require 220 volts 14. The geothermal system is designed for the 4-ton sized research home. The house utilizing the geothermal system has a heating load of 43,000 BTUH for a winter temperature of 38 F, and a cooling load of 36,000 BTUH for a summer temperature of 89 F. The load may increase due to the loss in efficiency, which can be a result of partial load operation when the equipment cycles off and on. This cycling increases the run time for the equipment. This adjustment is called the partial load factor (PLF) and is defined as the theoretical run time divided by the actual run time. Studies have resulted in an analytical expression for the PLF, which uses a "degradation" factor Cd 15. Moreover, ground load calculations for the design of the heat exchanger require information about the amount of heat transferred to or from the ground during specific periods of time. The entering water temperature of the ground-source heat pump is approximated from the average ground temperature of the area and can be related to the outdoor air temperature. Typically the ground water temperature will vary from the minimum during the cold months to the maximum in late summer. During the spring and fall, the ground temperature is near its mean value and the entering water will be about the same because the heat pump operates infrequently. Taking into consideration of all relevant factors, the double pipe heat exchanger used of the research house is to be made of a 2 in. nominal diameter Schedule 40 PVC outer tube and a 1-1/4 in. nominal diameter 316 stainless steel inner tube. The PVC tube costs approximately $7.53 for 10 ft of pipe. The 1-1/4 in. 316 stainless steel pipe is about $60.30 for 6 ft of pipe. Given the thermal loads and the heat exchanger tube properties, the effect of the overall length of the double pipe heat exchangers on heat transfer between fluids can be determined. The selected fluids in the heat exchanger are the water and the refrigerant R410a, which is also the refrigerant of choice in the majority of standard and earth coupled commercial and residential AC units. Assuming the inlet temperatures of the ground water and the refrigerant R410a and given the subsequent thermodynamic properties, the outlet temperatures and the heat transfer rates have been evaluated over various pipe lengths shown in Table 6. Item Table 6: Double Pipe Heat Exchanger Analysis of Various Lengths Units Length ft 1 4 8 12 16 TOUT - R410a o F 97.48 91.52 86.04 82.27 79.56 TOUT - Water o F 71.20 74.01 76.65 78.45 79.74 Heat Transfer - R410a Ton 0.50 1.68 2.76 3.51 4.05
Heat Transfer - Water Ton -0.50-1.68-2.76-3.51-4.05 Overall Selection In the U.S., the average cost of a geothermal heating and cooling system is about $14,000 based on figures from the year 2008 16. The majority of the cost is due to the high intensive labor associated with the installation of the system. The individual components of the system however are relatively economical. Based on the requirements of the research house system, a commercially available geothermal cooling package manufactured by GeoCool is selected. The GeoCool CFX041 4-ton geothermal heat pump package includes a Copeland scroll compressor, variable speed fan motor, Cupro-Nickel coil, safety high and low pressure switches and geothermal heat exchanger utilizing chlorine free and non-ozone depleting refrigerant R410a for maximum efficiency 17. GeoCool heat pumps also have a lifetime compressor and 10 year parts warranty to insure the customer with maximum efficiency for several years. The package includes a digital diagnostic center allowing for quick and easy troubleshooting and maintenance. The GeoCool heat pump package is also enclosed in a compact stainless steel cabinet allowing for extra protection of the heat pump system as well as minimizing the overall size of the system. After the system has been properly installed, the equipment needs to be properly operated and periodically maintained during its service life. The rate of fluid flow through the loop system should be measured by using one of the two common methods: usage of a calibrated flow meter or gauge measurement of the pressure drop. Air filters should be inspected every three months. The condensate pans should be checked for proper drainage and algae growth every three months, too; in the presence of algae a proper chemical treatment should be conducted. Service should be performed to the heat pump system at least once a year, including a visual inspection, refrigerant leak test, cleaning the evaporator coil, and a check of the compressor amperage, fan and pump motors. A log book can be used to record such values so a deteriorating condition can be detected before component failure. The refrigerant cycle should be serviced by a competent technician, preferably one who is factory-trained to service the actual equipment installed. The heat pump is priced at $2,979. The GeoCool Geothermal Heat Pump package holds the highest efficiency ratings in its class as well as qualifies for up to 30% Geothermal Tax Credit 18. Energy and Cost Comparison After the final design has been selected, the evaluation and cost analysis to compare the overall costs of operating the ECAC unit to the standard AC unit. For operational cost calculations, the average unit is assumed to runs 12 hr/day for an average electricity rate in Texas of approximately $0.12/kWh 2. Given the specifications of GeoCool heat pump, the power input is estimated to be about 3.27 kw 17. Thus, the daily and annual operational costs are $4.71 and $1719, respectively. On the other hand, the average standard air conditioning unit requires approximately 4.12 kw. Thus, the daily and annual operational costs are higher values of $5.93 and $2165, respectively. The ECAC unit is capable of providing upto $4468 in operational savings over 10 years. These savings only consider the cost of electricity to operate the units. Although the ECAC unit costs over $1,500 more to install than the standard AC unit, the overall electricity savings as well as the lower annual maintenance cost allows ECAC unit to provide savings of over $3,000 over a 10-year period as can be seen in Table 7. The preliminary savings are based strictly on initial cost and operation;
they do not include the potential savings of the 30% Geothermal Tax Credit. Table 7: Overall Cost Comparison Component Earth Coupled AC Component Standard AC Unit Unit Heat Exchanger $2,979.00 Condenser $1,609.00 Compressor Included Compressor $778.70 Coils Included Coils $500.00 Fans Included Fan/Motor $300.00 Pump $1,173.00 Fan/Motor $300.00 Fittings $40.00 Fittings $15.00 Piping $855.90 Piping $138.48 Annual Maintenance $240.00 Annual Maintenance $300.00 Annual Operation $1718.72 Annual Operation $2165.40 Initial Cost $5,047.90 Initial Cost $3,641.18 10 Year Cost $24,935.10 10 Year Cost $28,295.18 Conclusions The implementation of an ECAC unit on a research house has the potential of providing significant energy and savings over the span of less than a decade. The ECAC unit utilizes the temperature gradient between the outdoor air and the ground well beneath the earth s surface to reduce the high use of electricity associated with standard AC units. An initial investment of over $1500 more compared to a standard AC unit is required for the selected ECAC unit manufactured by GeoCool after completing the design analysis. This additional investment is due to the significant increase in the overall length of piping as well as the pump required to operate the geothermal heat pump. Although the initial cost of manufacturing and installing the ECAC unit is higher than that of the standard AC unit, the operational and maintenance savings allows the ECAC unit to reduce overall costs by more than $3,000 over a period of a decade. Thus, ECAC units have the great potential to resolve the growing residential energy demand while mitigate the effects of the increasing cost of energy. References 1. Wagers, H., Wagers, M., 1985, The Earth-Coupled or Geothermal Heat Pump Air Conditioning System, Proceedings of the Second Symposium on Improving Building Systems in Hot and Humid Climates, College Station, TX, September 24-26, 1985. 2. U.S. Energy Information Administration, State Energy Profiles Texas Quick Facts, Available online at http://www.eia.gov/state/?sid=tx. Accessed [9/24/2014]. 3. Steptoe, L., 2008, Soil Survey of Tyler County, Texas, Natural Resources Conservation Service, United States Department of Agriculture. Web Soil Survey. Available online at http://websoilsurvey.nrcs.usda.gov/. Accessed [11/9/2014]. 4. Specialty Heating & Cooling, Geothermal heat pumps The Earth saves you Money, Available online at http://www.specialtyheating.com/green-solutions/. Accessed [9/24/2014]. 5. Brenneman Well Drilling, Inc., Geothermal Heating & Cooling, Available online at http://www.brennemanwelldrilling.com/geothermal.html. Accessed [9/24/2014]. 6. Janna, W. S., 2015, Design of Fluid Thermal Systems, 4th ed., Cengage Learning, Stamford. 7. Lund, J., 1989, Geothermal Heat Pumps - Trends and Comparisons, Geo-Heat Center Bulletin, 12(1). Available online at http://geoheat.oit.edu. Accessed [9/24/2014].
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