Table of Contents
GEOTHERMAL ENERGY - THE BASICS
Many times geothermal energy is thought to be only related to volcanic activity in the forms of geysers and hot springs. Geothermal energy can be found in many places across the country and the world without the presence of volcanic activity. An energy efficient method to utilize the earth’s geothermal potential is through the use of heat pumps. These heat pump systems use the gradients in temperature between building interiors and the soil or groundwater surrounding the building. Geothermal reservoirs are not needed to capitalize on the energy contained within the ground, only the ground’s seasonal reactions to weather changes.
Proper design of a geothermal heat pump relies on several different variables, which include; soil temperature variation depending on the season, soil heat capacity/thermal conductivity, soil porosity and moisture content, average groundwater level, and the depth of the bedrock layer. The bedrock layers serves to determine certain geothermal heat pump limitations.
GEOTHERMAL HEAT PUMPS- HOW DO THEY WORK?
Geothermal heat pumps are similar to refrigerators or air conditioners. Refrigerators and air conditioners are different forms of heat pumps. A heat pump, does not makes heat, but uses existing heat and transfers it from a lower temperature location to a higher temperature location, in essence "moving" the heat at the expense of some power input. The carrying medium for this heat is known as the "working fluid" If the direction of the working fluid flow is reversed the direction of the heat flow will also be reversed. The reversing of this process allows heat pumps to both heat and cool an area effectively without the need of separate systems if careful design considerations were implemented at the start.
All heat pumps operate on the principal of the ideal gas equation:(1)
In reference to heat pump applications the values for V, n, and R remain constant and a new relation can be shown(2)
Using this simplified relation we can understand how heat pumps operate. Using Figure 1 starting at point 2 we determine the working fluid to have an initial temperature and pressure associated with it. The fluid passes through the compressor which increases the pressure of the fluid also raising its temperature above ambient air temperature at the same time due to the ideal gas law. The fluid, now at a higher temperature than before will lose heat to its surroundings via the condenser since heat flows from areas of higher temperature to areas of lower temperature. After the fluid is cooled to close to ambient temperatures it passes to the next component of the system, the expansion valve. The expansion valve slowly lets the fluid out of the condenser lowering the pressure greatly in the loop. The expansion of the fluid will cause a decrease in temperature as shown in the ideal gas law. The next component of the system, the evaporator, draws heat from its surroundings since the working fluid is now at a lower temperature than the ambient air temperature, returning the fluid back to the compressor at its initial properties.
The location of the two heat exchangers, the condenser and evaporator, determines to what areas the heat is moved to and from. The direction of flow of the working fluid may reverse essentially reversing the roles of the condenser and evaporator and also reversing the heat flow through the system. This is one of the reasons why heat pumps have been a very attractive form of climate control for residential homes since they can both heat and cool the home instead of relying on a separate system for each task. While typical air conditioners operate by removing heat from the home and exchanging it with the outside air, geothermal heat pump systems use the earth instead of the outside air as the exchange medium. The main benefit to using the soil as the exchange medium is that the temperature in the ground remains nearly constant while the temperature of the outside air can fluctuate, impacting performance of the heat pump. Various configurations for residential geothermal heat pump systems have been developed an employed.
There are many causes to the variation in soil temperatures. Soil temperature depends on the solar radiation, rainfall, seasonal temperature changes, overlying vegetation, soil type, and earth depth. As compared to air, soil has a much higher heat capacity, so the temperature patterns of soil generally lag behind the temperature variations of the air. This natural lag can help contribute to the needed temperature gradient in a geothermal heat pump system. For instance, the ground in winter is warmer than the air at this time of year, due to the lagging effect of the soil temperature, which is still containing the heat from the previous fall. This could aid in the interior heating of a building.
In general, the temperature of the soil remains constant at a depth of greater than 30 feet. This also correlates closely to the ground water temperature at 30 to 50 feet in depth. The mean earth temperature is the temperature at which the ground at below 30 feet of depth is consistently at year round, independent of seasonal variations. The mean earth temperature contour of the United States is shown in Figure 3. Figure 4 depicts the seasonal temperature change with respect to the soil depth. As shown in Figure 4, past the approximate depth of 30 feet, the soil temperature remains relatively constant independently of the surface air temperature.
Heat exchanger configurations such as a vertical closed-loop system and well-based open-looped systems install boreholes at depths of 200 to 300 feet or greater, eliminating the seasonal temperature variations in the soil. In contrast, heat exchanger configurations such as a horizontal loop system which install trenches that are usually no more than 10 feet deep rely heavily upon the seasonal variations in temperature. This makes the seasonal temperature variations extremely important when designing these types of heat exchanger systems. This is the type of consideration which must be taken into account when selecting the system of geothermal heat pump one will install. The extra cost associated with deeper trenches may be offset by the reduced burden on the heat pump units due to the ground soil having less drastic temperature changes year round. In Figure 5 the temperature of deeper soils show smaller variations in seasonal temperature, and lags further behind the seasonal temperature changes.
The deeper the installation of the geothermal heat pump trenches the construction cost may increase, but the overall cost of the life of the heat pump may decrease, ultimately saving the owner money. The determination of the optimal depth is dependent upon the seasonal change in soil temperature with relationship to depth, and the thermal properties of the soil.
SPECIFIC HEAT/THERMAL CONDUCTIVITY OF THE SOIL
The heat capacity, or specific heat, of the soil is the soil’s capability of storing heat energy. The following table displays the heat capacity of the soil and how it changes cased off of different moisture contents. The greater the moisture content of the soil, the greater the specific heat. The dryer the soil, the lower the heat capacity, and the greater the variations in temperature based off of the seasons.
|Soil Type||Heat Capacity (BTU/lb/oF)|
|Moist/Saturated Soil||0.23 – 0.25|
|Table 1: Heat capacity of soil|
The thermal conductivity is an extremely important criterion for designing geothermal heat pumps. Thermal conductivity of the underlying soil and rock determines the length of the required pipe, due to the fact that thermal conductivity is the rate heat is transferred between two media. The determination of the pipe length is critical in the determination of installation cost and energy needed to pump the geothermal heat pump fluid. The thermal conductivity of varying soil types is shown in the following Table.
|Texture Class||Thermal Conductivity (BTU/ft-hr-oF)|
|Saturated Silt or Clay||0.96|
|Table 2: Varying soil types thermal conductivity|
The amount of water in a soil greatly increases the thermal conductivity. The greater the soil porosity, the more water the soil can retain. This increases the thermal conductivity of the soil. That is why the groundwater level is very important in the design of a geothermal heat pump. Table 3 displays the impact of the soil’s thermal conductivity for a vertical closed-loop geothermal heat pump. Under dry conditions, finer soils decrease the size of the natural gaps between soil particles increasing the thermal conductivity. Soil thermal conductivity is approximately 100 times more than that of air. Water’s thermal conductivity is generally two to three times greater than soil particles. This is all the more reason to properly analyze the geography of a location of a geothermal heat pump, in order to most efficiently design the system with the geography in mind.
|Soil Thermal Conductivity (BTU/(hr-ft-oF))||# of U tubes||Depth of U-Tubes (Vertical Ft)||Total U-Tube Vertical Ft)|
|Table 3: “Thermal conductivity influence on number of boreholes and total length of the earth-coupled heat exchanger per 10 tons of load for a vertical closed-loop GHP system.”|
The thermal conductivity makes such a significant impact on the design of a geothermal heat pump, some designers may consider soaking the ground in horizontal geothermal loops to increase the thermal conductivity, especially in dryer regions. To obtain an accurate reading of the ground properties, it is best to conduct in-situ testing to arrive upon an accurate picture with the area one is working. The better the understanding of the land one will be working with a more effective and cost efficient geothermal heat pump.
The level of the groundwater table plays an important role in determining the soil’s thermal conductivity. Generally, as the seasons change there is variability in the groundwater table that must be accounted for. In addition to this, the depth of the bedrock is critical in the selection of the geothermal ground loop. In cases where standing columns wells are used, the bedrock must be close to the surface. In contrast, a vertical closed-loop system needs a bedrock depth of at least 200 to 400 feet. These approximations are made considering the soil thermal conductivity.
GEOTHERMAL HEAT PUMPS – CONFIGURATIONS/INSTALLATION
There are three main categories when dealing with geothermal heat pumps; open-loop, closed-loop, and direct-exchange. The closed-loop and direct-exchange systems can be arranged in either parallel or series configurations.
In an open-loop system, heat transfer is performed with surface or groundwater. The water is taken into heat pump units and then discharged into the environment. There are several types of open-loop configurations that can be used depending on the surrounding conditions and needs of the building.
Single-well systems rely on a lone well that serves as a water supply to the open-loop system. The water is pumped into the system and released after it is used into a drainage field or an existing body of water. An example of an open-loop single well is displayed in Figure 6. These systems provide an economical solution to a groundwater heat pump if there is a preexisting well. In residential situations, a domestic water supply well could possibly be too small to meet the water needs of the groundwater heat pump. Residential wells typically produce 300 to 400 gallons of water per day, where a groundwater heat pump for the same residence may require thousands of gallons of water per day. In addition to this, the water discharged from the system may be limited by environmental or local regulations. A slightly modified single-well system which may alleviate some of these concerns is a standing-column well.
A standing-column well uses the same concepts as a single-well system, except in a standing-column well most to all of the discharged water is dispensed into the original well source. This minimizes the amount of water discharge from the system into the environment. A standing-column well system is feasible when there is accessibility to fractured bedrock aquifers near the ground surface. A standing-column well typically consists of an installation of uncased boreholes 6 inches in diameter and at a depth of 1000 to 1500 feet. The surrounding aquifer is in contact with the borehole, which allows the formation of a standing column of water from the bottom of the well to the top of the groundwater table. An example of a standing-column well can be seen in Figure 7.
The water for the system is drawn from the bottom of the well and discharged into the top of the well. This allows for no net withdrawal from the groundwater itself. At times of higher demand for heating or cooling, this type of system can “bleed,” which means it returns only a portion of the water back to the well and the other portion is discharged into the surrounding environment. When this “bleeding” occurs a net groundwater inflow happens within the column. “This chills the standing column during periods of peak heat rejection (when building demand for cooling is the greatest) and/or warms it during peak heat extraction (when heating demand is greatest), thus reducing the required bore depth.,” [Virginia Tech Website]. In comparison to closed-loop geothermal heat pump systems, a standing-column well can save the owner significantly if it is well designed and accurately sited. The ground area required to install a standing-column well is the least of any geothermal heat pump system, making this type of system ideal for areas with limited space and the proper geological conditions.
Double-well systems consist of both a supply and discharge well. Similarly to standing-column wells, a double-well system may be used in instances where there are water discharge regulations or limits. An important design aspect of this type of open-loop system is the distance between the supply and discharge wells. A main consideration when determining the distance between the wells is the flow rate from the injection well to the production well. There may be a flow between wells, but it must be low enough so that the discharged water arriving at the production well is approximately the same temperature as the natural aquifer. Typical well spacing in a double-well system is in range of 200 to 600 feet. This is largely dependent upon the maximum system heat/cooling loads, time span of these maximum load conditions, and the natural flow rate and thickness of the aquifer. An example of a double-well system is shown in Figure 8.
SURFACE WATER SYSTEMS
A surface water system uses a larger body of water like a lake or an ocean for both the water supply and discharge points. If the water body is deep enough to have a thermocline in a source containing thermal stratification, a source of cold water that remains undisturbed is available year round. Sometimes, this colder water provided by the water supply may be enough to provide direct space cooling using water/air heat exchangers. This eliminates the need for heat pumps or refrigerant to cool the building interior. When using this direct space cooling method, the temperature of the building loop water must remain below 55 °F to provide effective dehumidification. “Data from lakes in Alabama suggest that significant thermal stratification occurs in lakes deeper than 30 feet, with bottom water temperatures between 45 and 55 °F throughout the year, even when summer surface water temperatures reach 80 to 90 °F."9
ISOLATION HEAT EXCHANGER
“Indirect open-loop systems employ an isolation heat exchanger between the building loop and the water supply. This eliminates exposure of building water loop or heat pump components to poor-quality supply water, making more sites potentially attractive for open-loop systems. The isolation heat exchanger also allows the building loop and supply water loops to be operated at different flow rates and pressures for optimal thermal and hydraulic performance.”10
EVALUATING OPEN-LOOP FEASIBILITY
- Heat exchanger is exposed to dissolved ions, suspended solids, and microorganisms from the supply well.
- The heat exchanger is prone to scaling and buildup of corrosion films and fouling.
- The thermal and hydraulic resistance to heat transfer is increased, which decreases overall efficiency.
- Water treatment is not economical.
- Required groundwater flow rate typically is between 2-3 gallons per minute per system ton.
DISCHARGE WATER PERMITTING
- Groundwater must be re-injected into the ground or drainage system.
- Must follow local water discharge regulations.
- Water is discharged at higher elevation than the intake point which represents a static pressure head. This requires more power to overcome by the circulating pump.
Closed-loop systems are the most common geothermal heat pumps. They circulate the working fluid through the pipes and do not use a water source. They work by only transfering heat through the piping networks meaning that there is no direct interaction between the working fluid and the earth. The length of required piping depends on ground thermal conductivity, ground temperature, and heating and cooling power needed, as mentioned in Geothermal Basics above.
The most common closed-loop systems are: vertical, horizontal, slinky, and pond.
Horizontal closed-loop systems are composed of pipes that run horizontally through the ground. A long horizontal trench, deeper than the frost line, is dug and U-shaped coils are placed horizontally to connect the pipes. A trench for a horizontal loop field will be similar to one seen under the slinky loop field. The width of the field is dependent on the number of pipes. Horizontal loop fields are very common and economical if there is adequate land available.
A slinky closed-loop field is installed in the horizontal orientation with an overlaying piping network. Slinky loop fields are used when there is not adequate space for a horizontal closed-loop system. Slinky closed-loop systems have easy installation.
Vertical closed-loop fields are oriented with a piping network running vertically into the ground. Holes are bored into the ground about 150-250 feet deep. There are U shaped connectors at the bottom of the hole connecting pipes. The borehole is commonly filled with a bentonite grout surrounding the pipe to provide a good thermal connection to the surrounding soil or rock to maximize the heat transfer. Vertical closed-loop fields are ideal for limited areas. During the cooling season, the local temperature rise in the bore field is influenced most by the moisture travel in the soil.
OTHER TYPES OF CLOSED-LOOP SYSTEMS
- Submerged closed-loops
- Hybrid loop with cooling pond
- Hybrid loop with cooling tower
- Hybrid loop with solar collector
DIRECT EXCHANGE (DX) LOOPS
Direct exchange systems have a refrigerant circulating through copper pipes which are drilled directly into the ground, eliminating the need for a heat exchanger between the refrigerant loop and the water loop and the water pump.
The DX loop systems have many benefits, including:
- Simple installation
- Higher efficiencies
- Shorter and smaller piping
- Lower installation costs
- Can also be used for home water heating
- Very long lifetimes
Horizontal DX Arrangement:
- Horizontal-loop DX systems require about 350 feet of copper tubing per system ton, as opposed to 450 to 500 feet per ton for polyethylene ground loops.
- Because of their shorter length, horizontal DX ground loops need only about 500 square feet of land area per system ton, considerably less than the 1,500 to 3,000 square feet needed for conventional horizontal closed-loops.16
Vertical DX Arrangement:
- Vertical DX systems require only a 3-inch diameter bores to a depth of 120 feet per ton, as opposed to 4- to 6-inch diameter bores to a depth of 200 to 300 feet per ton for polyethylene U-tubes in conventional vertical closed loops.
- Vertical DX loops need at least the same land area as their conventional counterparts, or even somewhat more.
- Vertical DX boreholes should be spaced at least 20 feet apart to minimize the possiblity of ground freezing and buckling in the heating mode or excessive warming and drying of the soil in the cooling mode.18
ADVANTAGES AND DISADVANTAGES OF SYSTEMS19
|Geothermal Loop Type||Advantages||Disadvantages|
|Open-Loop||Simpler design; lower drilling costs than for vertical closed-loop systems; more efficient performance by avoiding thermal degradation associated with heat transfer across pipe wall from ground or water body to antifreeze solution in closed-loop; lower installation cost if a supply well already exists for domestic water or grounds irrigation, with sufficient surplus production capacity to supply heat pump system.||Subject to local, state, and Federal groundwater and surface water withdrawal and discharge permitting; large water flow requirements may exceed local water availability; supply-side of heat exchangers subject to corrosive and abrasive agents, chemical scaling, and microbial fouling; main circulating pumps typically require more power in open loops than in closed loops; water discharge regulations may preclude single-well systems or constrain the design of standing-column systems; higher installation cost if a separate injection well is required for loop water discharge.|
|Horizontal Closed-Loop||Trenching costs for horizontal loops usually are much lower than well-drilling costs for vertical closed-loops, and there are more contractors with the appropriate equipment; flexible installation options depending on type of digging equipment (bulldozer, backhoe, or trencher) and number of pipe loops per trench.||Largest land area requirement; performance more affected by season, rainfall, and burial depth; drought potential (low groundwater levels) must be considered in estimating required pipe length, especially in sandy soils and elevated areas; ground-loop piping can be damaged during trench backfill; longer pipe lengths per ton than for vertical closed loops; antifreeze solution more likely to be needed to handle winter soil temperatures.|
|Slinky Closed-Loop||Slinky loops require less land area and less trenching than other horizontal-loop systems, and installation costs may be significantly less.||Greater pumping energy needed than for straight horizontal-loops; backfilling the trench while ensuring that there are no voids around the pipe coils is difficult with certain types of soil, and even more so with upright coils in narrow trenches than with coils laid flat in wide trenches.|
|Vertical Closed-Loop||Requires less total pipe length than most other closed-loop systems; requires the least amount of land area; seasonal soil temperature swings are not a concern.||Cost of drilling is usually higher than cost of horizontal trenching, and vertical-loop designs tend to be the most costly GHP systems; potential for long-term soil temperature changes if boreholes not spaced far enough apart.|
|Submerged Closed-Loop||Can require the least total pipe length and can be the least expensive of all closed-loop systems if a suitable water body is available.||Submerged loops are likely to require more regulatory permitting than buried closed-loop systems; unless properly marked, can be damaged by boat anchoring.|
|Direct-Exchange Loop||Higher thermal efficiency; no liquid/liquid heat exchangers required; less land area needed for horizontal configuration.||Soil in contact with ground loop subject to freezing; copper tubing should not be buried near large trees where growing root system could damage the coil; ground-loop leaks can lead to catastrophic loss of refrigerant; smaller supporting infrastructure in GHP industry, with greater care and higher skill needed to install and consequently higher installation costs.|
|Closed-Loop In Series||Single pipe diameter entails simpler pipe fusion joints, enabling quicker installation; single flow path enables easier purging to remove air from the loop when filling with water or antifreeze solution.||Longer flow path requires larger-diameter pipe to minimize pressure drop and maintain pump power at reasonable levels; larger diameter also entails greater antifreeze volumes; system capacity limited by total pressure drop from end to end, so not suitable for large building applications.|
|Closed-Loop In Parallel||Shorter flow paths enable smaller pipe diameter to be used, which lowers unit piping cost and requires less antifreeze; reduced pressure drop along shorter flow paths results in smaller pump power requirements.||Header lines must be larger diameter than individual loops and so require more complex pipe joining operations than series installation; special care needed to ensure complete air removal from all flow paths when purging system at start-up.|
THE BOTTOM LINE – COST EFFECTIVENESS/ENERGY REDUCTIONS/BENEFITS
- The biggest benefit of geothermal heat pumps is that they use 25%–50% less electricity than conventional heating or cooling systems.
- According to the EPA, geothermal heat pumps can reduce energy consumption—and corresponding emissions—up to 44% compared to air-source heat pumps and up to 72% compared to electric resistance heating with standard air-conditioning equipment.
- Geothermal heat pumps also improve humidity control by maintaining about 50% relative indoor humidity, making geothermal heat pumps very effective in humid areas.
- Various Geothermal heat pump loop systems allow for design flexibility and can be installed in both new and retrofit situations. Because the hardware requires less space than that needed by conventional HVAC systems, the equipment rooms can be greatly scaled down in size, freeing space for productive use.
- Geothermal heat pump systems also provide excellent "zone" space conditioning, allowing different parts of your home to be heated or cooled to different temperatures.
- Because geothermal heat pump systems have relatively few moving parts, and because those parts are sheltered inside a building, they are durable and highly reliable. The underground piping often carries warranties of 25–50 years, and the heat pumps often last 20 years or more.
- Since they usually have no outdoor compressors, geothermal heat pumps are not susceptible to vandalism on the other hand, the components in the living space are easily accessible, which increases the convenience factor and helps ensure that the upkeep is done on a timely basis.
- Because they have no outside condensing units like air conditioners, there's no concern about noise outside the home. A two-speed geothermal heat pump system is so quiet inside a house that users do not know it is operating: there are no tell-tale blasts of cold or hot air.
- Utility bills will be lowered on average 25% to 70% as compared with conventional systems.20
- Geothermal system burns no fossil fuel on-site to produce heat, it generates far fewer greenhouse gas emissions than a conventional furnace.
- Can also completely eliminates a potential source of poisonous carbon monoxide within the home or building.
- Factoring in its share of the emissions from the power plant that produces electricity to operate Geothermal systems, total emissions are far lower than that of conventional systems.
- According to data supplied by the U.S. Department of Energy (DOE) Office of Geothermal Technologies, nearly 40% of all U.S. emissions of carbon dioxide (CO2) are the result of using energy to heat, cool and provide hot water for buildings. This is about the same amount of CO2 contributed by the transportation sector.
- A typical 3-ton residential GeoExchange system produces an average of about one pound less Carbon Dioxide (CO2) per hour of use than a conventional system. To put that in perspective, over an average 20-year lifespan, 100,000 units of nominally sized residential GeoExchange systems will reduce greenhouse gas emissions by almost 1.1 million metric tons of carbon equivalents. That would be the equivalent of converting about 58,700 cars to zero-emission vehicles, or planting more than 120,000 acres of trees.
- The waste heat removed from the home's interior during the cooling season can be used to provide virtually free hot water-resulting in a total savings in hot water costs of about 30% annually, and lowering emissions even further.
- The initial cost of purchase and installation can be upwards of $20,000 before any governmental tax credits are applied. Although the lower monthly utility costs will offset this there are "payback periods" associated with the price and savings of a system which can take years.
- Some geothermal heat pump systems that utilize refrigerants can be associated with CFC's and HCFC's causing environmental concerns.
- Since the earth is used as a heat transfer medium which is typically buried, repairs in the piping loop network and be costly and time consuming.
When deciding to install a geothermal heat pump for residential heating and cooling, several factors should be considered before deciding which type. Geographical location and corresponding earth temperature variations will effect the thermal conductivity of the transfer medium. The moisture content and soil type will also effect the thermal conductivity and therefore overall system performance. By knowing the soil moisture content as well as the topography of the land the most efficient exchange loop can be chosen. Ambient air temperatures and physical size of the installation location will help determine the size of the system in terms of the amount of heat that will need to be moved during both summer and winter seasons. Local laws governing the use of refrigerants/closed/open loop systems must be consulted prior to installation of the system.
- How long will the loop pipe last? Closed-loop systems should only be installed using high density polyethylene or polybutylene pipe. Properly installed, these pipes will last for many decades. They are inert to chemicals normally found in soil and have good heat conducting properties. PVC pipe should not be used under any circumstances.
- How are the pipe sections of the loop joined? The only acceptable method for joining sections of the special pipe used for the closed-loop systems is electro fusion. Pipe connections are heated and fused together to form a joint stronger than the original pipe. Mechanical joining of pipe for an earth loop is in some restricted applications an accepted practice. The use of barbed fittings, clamps, and glue joints is certain to result in loop failure due to leaks.
- Which system is best, open- or closed-loop? The net results in operating cost and efficiency are virtually the same. Which system to choose depends mainly on whether you have an adequate groundwater supply and means of disposal. If you do, an open-loop can be used very effectively. If not, either a horizontal or vertical closed-loop system is your best choice. Over a period of years, a closed loop system will require less maintenance because it's sealed and pressurised, eliminating the possible build-up of minerals or iron deposits.
- Is the efficiency rating actual or just a manufacturer's average? All types of heating and cooling systems have a rated efficiency. Fossil fuel boilers have a percentage efficiency rating. Natural gas, propane and fuel oil boilers have efficiency ratings based on laboratory conditions. To get an accurate installed efficiency rating, factors such as flue gas heat losses, cycling losses caused by over sizing, blower fan electrical usage, etc., must be included. Geothermal heat pumps, as well as all other types of heat pumps, have efficiencies rated according to their coefficient of performance or COP. It's a scientific way of determining how much energy the system produces versus how much it uses. Most geothermal heat pump systems have COPs of 3.5 - 4.5. WaterFurnace units have typical COPs of 4 to 8. That means for every one unit of energy used to power the system; four or more units are supplied as heat. Where a fossil fuel boiler may be 50-90 percent efficient, a WaterFurnace geothermal heat pump is about 600 percent efficient. We use computer programs to accurately determine the operating efficiency of a system for your home or building.
- Are all geothermal heat pumps alike? No. There are different kinds of geothermal heat pumps designed for specific applications. Many geothermal heat pumps, for example, are intended for use only with higher temperature ground water encountered in open-loop systems. Others will operate at entering water temperatures as low as -4°C which are possible in closed-loop systems. Most goethermal heat pumps provide summer air conditioning, but a few brands are designed only for winter heating. Sometimes these heating-only systems incorporate a groundwater cooled coil that can provide cooling in moderate climates. Geothermal heat pumps can also differ in the way they are designed. Self contained units combine the blower, compressor, heat exchanger and coil in a single cabinet. Split systems allow the coil to be added to a forced-air boiler and utilize the existing blower.
- How efficient is a ground source heat pump system? Modern systems are very energy efficient. For each kilowatt of electricity used to run the heat pump, three to four kilowatts of heat are delivered to the building.
- How large are these units? A typical heat pump unit for a domestic dwelling is about the same size as a large fridge.
- Can it supply hot water for the house? Yes. Some domestic systems are able to heat domestic hot water via a modern high efficiency indirect water cylinder. An immersion heater can then boost the temperature which can be done at night using off peak rates.
- Can the systems provide cooling? Yes. There are reverse-cycle heat pumps that can deliver both heating and cooling.
- I have an older style property. Can I still fit a GSHP system? Yes, you can, but your building must be well insulated for you to gain most benefit. The cost of a system is directly related to its size and with heat losses being fairly high from older buildings, this can add substantially to the capital cost of installation. Money spent on upgrading insulation levels can save a considerable amount on the capital cost. Regrettably, many older buildings can never be made sufficiently energy efficient to use a modern heating distribution system such as low temperature underfloor heating, or low temperature radiators.
- Can I install trenches on a downward sloping site? Yes, provided you can physically dig the trenches, a moderate downward slope is not a problem. Consideration needs to be given to purging air from a system with ground loops higher than the heat pump.
- I have some very wet land. Can I use this? Yes, wet land is better at conducting heat so, as long as you can physically dig a trench, its ideal.
- Are GSHP systems really environmentally friendly? Yes. In the UK, there is now a strong move towards alternative technologies that are sustainable and environmentally much more acceptable. It has been calculated that 40% of CO 2 emissions are derived from the heating of buildings. By using renewable sources of energy to heat your property you can help to reduce these emissions, particularly when compared to burning fossil fuels such as oil. Most electricity suppliers are now offering 'clean green' electricity from a renewable energy source and, if you use this to power your heat pump, your property will be totally heated from renewable energy with zero carbon emissions.
- Are Ground Source Heat Pumps dangerous? What about servicing and maintenance? There are no hazardous gas emissions, no flammable oil, LPG or gas pipes, no flue or chimney and no unsightly fuel tanks. GSHP systems have absolutely NO site emissions. There is no need for regular servicing or annual safety checks and maintenance is very low.
- How do running costs compare with conventional alternatives? It depends what you are comparing. In a modern, well insulated house, a Ground Source Heat Pump system can offer very high efficiency and moderate running costs. An oil-fired boiler would cost considerably more to run, and electric heating would be at least three times as expensive. It is true that the very best of the modern condensing gas boilers may only be a little more expensive to run but that is on current gas prices, which are set to rise. Also, all fossil fuel boilers need regular servicing and maintenance.
- Are these systems expensive? The initial purchase costs of a ground source heat pump system will be quite a lot more than a conventional oil or gas fired boiler. The initial one-off expense is offset by the lower running costs, lower maintenance and low servicing requirement. There is also the security of knowledge that the majority of your heating and cooling energy comes out of your ground, is under your control and will not increase in price.
- How are the pipe sections of the loop joined? The only acceptable method to connect pipe sections is by thermal fusion. Pipe connections are heated and fused together to form a joint stronger than the original pipe. Mechanical joining of pipe for an earth loop is never an accepted practice. The use of barbed fittings, clamps, and glue joints is certain to result in loop failure due to leaks.
- Will an earth loop affect my lawn or landscape? No. Research has proven that loops have no adverse effect on grass, trees, or shrubs. Most horizontal loop installations use trenches about six inches wide. This, of course, will leave temporary bare areas that can be restored with grass seed or sod. Vertical loops require little space and result in minimal lawn damage.
- Can I reclaim heat from my septic system disposal field? No. An earth loop will reach temperatures below freezing during extreme conditions and may freeze your septic system. Such usage is banned in many areas.
- Can I install an earth loop myself? It's not recommended. In addition to thermal fusion of the pipe, good earth-to-coil contact is very important for successful loop operation. Non-professional installations may result in less than optimum system performance.
Note: All FAQs and Answers are taken directly from Ground Source Heat Pumps FAQs22
- Geothermal International. http://www.geothermalint.co.uk/index.html
- Geothermal Heat Pumps (Virginia Tech). http://www.geo4va.vt.edu/A3/A3.htm
- Geothermal Heat Pump Consortium. http://geoexchange.org/
- Geothermal heat pump. http://en.wikipedia.org/wiki/Geothermal_heat_pump#Closed_loop_fields
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