North Seattle Community College HVAC Program Instructor – Mark T. Weber, M.Ed. Geothermal Heat Pumps

Objectives After studying this unit, you should be able to: –Describe an open- and closed-loop geothermal heat pump system –Explain how water quality affects an open- loop geothermal heat pump –Describe different ground-loop configurations for closed loop geothermal heat pump systems

Objectives (cont’d.) –Explain the advantages and disadvantages of series- and parallel-flow configurations in geothermal heat pump systems –Explain the different system fluids and heat exchanger materials –Describe different geothermal well types and water sources for heat pumps

Objectives (cont’d.) –Explain some of the most common service problems with geothermal heat pump systems –List and explain the governing formulas that calculate the amount of heat rejected or absorbed by the water-side of a geothermal heat pump –Describe a direct geothermal heat pump system

Reverse-Cycle Refrigeration Geothermal heat pumps use the earth, or water in the earth, as heat sources and heat sinks –Heat pumps use the energy stored in the earth’s crust for heating Air-conditioning loads transferred to the earth –Can be used for space heating and cooling Uses four major system components, four-way reversing valve

Geothermal Heat Pump Classifications Open-loop –Water source heat pumps –Water is used as the heat transfer medium –The water is then expelled back to the earth –Typically use a well, lake, or pond

Geothermal Heat Pump Classifications (cont’d.) Closed-loop –Earth-coupled system –The same heat transfer fluid is reused –The fluid is circulated in buried plastic pipes –Used primarily where there is not enough water to support an open-loop system

Open-Loop Systems Heat is transferred between a water source and the air from the space –Water is then expelled back to the earth –Heating mode Heat is absorbed from the water source and transferred to the air in the space –Cooling mode Heat is absorbed from the space and transferred to the water source

Figure 44–3 An open-loop, water-source heat pump with boiler and cooling tower to maintain the loop temperature

Water Quality Water flow must be able to handle required capacities The temperature of the water must be within desired range Water temperature determines heat transfer capability The water must be clean

Water Quality (cont’d.) Water and refrigerant piping is configured in a counter-flow design The heat exchanger is a coaxial tube-in- tube type Heat exchangers are usually made of copper alloys to extend the service life

Closed-Loop Systems Utilize ground loops or water loops –Many yards of buried plastic pipe –Loops are completely sealed –Water or antifreeze solution is circulated through the loops –A low-wattage centrifugal pump is used to circulate the liquid

Closed-Loop Systems (cont’d.) Figure 44–7 A ground loop showing a series-vertical configuration in the heating mode

Closed-Loop Systems (cont’d.) Figure 44–8 A ground loop showing a parallel-vertical configuration in the cooling mode

Closed-Loop Systems (cont’d.) The circulating fluid exchanges its heat with the refrigerant loop –Heat exchange takes place within the heat pump’s cabinet –The heat exchanger will not get fouled The air loop is used to distribute conditioned air

Closed-Loop Systems (cont’d.) Domestic water can be heated by compressor discharge gas –Requires a separate heat exchanger –Domestic water is circulated by a pump –Uses a counterflow tube-in-tube heat exchanger –The hot gas is desuperheated while the water is heated

Ground-Loop Configurations and Flows Vertical systems – used when there is a shortage of land Horizontal systems – used when land is available without hard rock Slinky loop –Designed to reduce trench length –Can be installed in lakes or ponds

Ground-Loop Configurations and Flows (cont’d.) Series flow –Only one path for the fluid to flow –Trapped air can be removed easily –Have a high rate of heat transfer per foot of pipe –Larger diameter plastic pipe is needed –Installation costs are higher –Larger pressure drops

Ground-Loop Configurations and Flows (cont’d.) Parallel flow –Use smaller diameter plastic pipe –Installation costs are lower –Air is difficult to remove from the system –Requires less antifreeze than series systems –Water flow balancing is difficult

Figure 44–17 Different flow paths in ground loops

System Materials and Heat Exchange Fluids Buried pipe usually made of polyethylene or polybutylene If there is not threat of freezing, pure water can be used in the ground loops Antifreeze solutions –Salts –Glycols –Alcohols

System Materials and Heat Exchange Fluids (cont’d.) System components must be chosen carefully when they are to be used with salts, glycols, or alcohols New pre-mixed geothermal loop fluids have great antifreeze, anticorrosive, and heat transfer properties R-410A is the leading alternative to replace R-22 in new equipment

Geothermal Wells and Water Sources Drilled wells –Equipped with submersible well water pumps –Water is pumped to the individual units and is then discharged –Discharge water can be directed to lakes or streams –Most wells are grouted to prevent water contamination and rusting

Geothermal Wells and Water Sources (cont’d.) Return wells –Return the discharged water back to the ground –Supply and return wells should be located far enough apart to prevent the supply and return water from mixing –Supply and return wells should be at least 100ft apart

Figure 44–21 A return well system

Geothermal Wells and Water Sources (cont’d.) Slow closing solenoids in the return line –Prevent water hammering –Keeps heat exchanger and pressure tank pressure equal –Helps keep minerals dissolved in the water

Geothermal Wells and Water Sources (cont’d.) Dedicated geothermal wells –Closed-loop system –Uses only one well –Supply water comes from top of the well –Return water is introduced at the bottom of the well –Used when there is not enough water for other standard well systems

Geothermal Wells and Water Sources (cont’d.) Dry wells –Used for the discharge water in an open-loop system –Basically large reservoirs filled with gravel and sand –Water is filtered as it seeps through the gravel –Water then returns to the underground aquifer

Geothermal Wells and Water Sources (cont’d.) Pressure tanks –Used on well systems and open-loop geothermal heat pumps –Pressurized tank for water storage –Prevents the well pump from short cycling –The well pump fills the pressure tank to a predetermined pressure

Geothermal Wells and Water Sources (cont’d.) Pressure tanks (cont’d.) –When the tank pressure drops to a predetermined pressure, the pump comes on again to fill the tank –The tank should be sized so that the pump comes on about once every 10 minutes

Geothermal Wells and Water Sources (cont’d.) Figure 44–24 The operation of a well system’s pressure tank

Water-to-Water Heat Pumps Utilize two coaxial heat exchangers Configured as either open-loop or closed-loop system Common to see a buffer tank installed on the condenser water side to prevent high head pressure and to function as the water supply tank for the radiant heating system

Water-to-Water Heat Pumps (cont’d.) Figure 44–26 Heat exchanger configuration on a water-to- water heat pump system Figure 44–27 Buffer tank location on a water-to-water heat pump system

Troubleshooting Similar to the methods that are used for air-to-air heat pumps Ground loop pressure and temperature readings are needed A temperature probe measures the temperature difference between the inlet and outlet of the water’s heat exchanger

Troubleshooting (cont’d.) Pressure gauge –Used to determine the pressure drop across the heat exchanger –Helps to determine the flow rate through the heat exchanger Troubleshooting the refrigerant and electrical circuits of geothermal heat pumps is similar to other refrigeration systems

Troubleshooting (cont’d.) Ground loop (water loop) provides means for a water-to-refrigerant heat exchanger Amount of heat transferred = gpm x temp differential x 500 Temperature differential: temperature difference between the water entering and leaving the heat exchanger

Troubleshooting (cont’d.) A conversion chart from pressure drop to gpm may be needed Low antifreeze flow rate be caused by: –Defective circulating pump –Air restriction in the piping –Contaminated or kinked pipe in closed-loop –Low water supply pressure in open-loop

Troubleshooting (cont’d.) Symptoms – Reduced antifreeze flow (heating) Low suction pressure, large temperature differential –Reduced antifreeze flow (cooling) High head pressure, large temperature differential –Mineral deposits in heat exchanger (open-loop) Lower-than-usual temperature differential High head pressure in cooling mode Low suction pressure in heating mode

Direct Geothermal Heat Pump Systems Direct geothermal systems –Refrigerant lines buried in the ground Refrigerant loop acts as the evaporator in the heating mode –In the cooling mode, conventional air- cooled condenser is used No coaxial heat exchangers or centrifugal pumps are used –Can be used as first-stage heating

Direct Geothermal Heat Pump Systems (cont’d.) Installation and refrigerant-loop piping –Installation costs are lower than a stand alone geothermal system –Existing condensing unit acts as the pump and heat generator Copper loops are buried 3 to 4 feet; no buried joints underground Loops are connected by brazing to a header –Existing refrigerant lines are tapped for connections

Direct Geothermal Heat Pump Systems (cont’d.) The earth loop (the refrigerant loop), may be three different configurations: Diagonal, Vertical, Horizontal Connected to a refrigerant distributor or manifold, which divides the refrigerant flow equally to each loop Manifold is connected to the heat pump’s compressor unit

Direct Geothermal Heat Pump Systems (cont’d.) Heating mode –Heat is transferred from the warmer earth into the refrigerant loop Cooling mode –Refrigerant temperature entering the refrigerant (earth) loop is higher than that of the earth and will now be transferred to the earth

Refrigerant Management System System consists of two components: – Liquid Flow Control, Active Charge Control Three main objectives: –Improve system efficiency, reliability, and serviceability –Continuously return lubricating oil back to the compressor without returning liquid refrigerant –Stabilize liquid and vapor refrigerant flow in long refrigerant

Refrigerant Management System (cont’d.) Liquid Flow Control –Regulates the rate of liquid refrigerant flowing from the condenser to the evaporator by responding directly to the amount of vapor bubbles arriving at the control from the condenser’s outlet –End result is a larger condenser with lower condensing pressures, lower compression ratios, and higher system efficiencies

Refrigerant Management System (cont’d.) Active Charge Control (ACC) –A thermally insulated reservoir replaces the standard accumulator –Purpose is to constantly deliver refrigerant vapor and oil to the compressor in the optimum conditions and quantities –Determines when the system is properly charged without using gauges, wet and dry bulb readings, or charging charts

Summary Energy is transferred daily to and from the earth by solar radiation, rainfall, and wind Geothermal heat pumps use the earth, or water in the earth, for their heat source and heat sink

Summary (cont’d.) Because the earth’s underground temperature in the summer is cooler than the outside air, heat loads from summer air conditioning can be rejected underground more efficiently Geothermal heat pumps are very similar to air source heat pumps in that they both use reverse-cycle refrigeration

Summary (cont’d.) Geothermal heat pumps are classified as either open- or closed-loop systems Water quality is one of the most important considerations in the design of an open-loop geothermal heat pump system Open-loop systems usually use well water as their heat source and heat sink

Summary (cont’d.) Heat exchanger fouling can be a problem if water quality is poor Water sources for open-loop systems may be an existing well or a new well Pressure tanks are used in conjunction with wells in open-loop systems Closed-loop heat pump systems recirculate the same antifreeze fluid

Summary (cont’d.) Closed-loop or earth-coupled systems are used where there is insufficient water quality or quantity Loops can have series or parallel fluid flows The buried piping or underground heat exchanger is usually either polyethylene or polybutylene pipe

Summary (cont’d.) The antifreeze solutions inside the buried piping are used to prevent freezing of the heat pump heat exchanger and for heat transfer purposes Water-to-water heat pump systems often use a buffer tank to store the heated water until it is needed by the heating circuits

Summary (cont’d.) Waterless heat pump systems utilize buried refrigerant lines instead of buried water lines Waterless heat pump systems transfer heat into and out of the refrigerant by using the ground as the heat source in the winter and as the heat sink in the summer