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Ground-Coupled (Closed-Loop) Systems

Ground source closed-loop heat pumps use the same concept as the ground source open-loop units - the temperature of the earth near the surface is typically around 55 ° F. The difference is no water is taken from the ground or disposed of. The water is circulated to the individual heat pumps and the returned to a ground closed-loop to be cooled or warmed.

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When more units are heating than cooling the circulating water temperature drops and is warmed back up by the earth in the ground heat exchanger. Conversely, when more units are cooling than heating, the circulating water is cooled down by the ground heat exchanger buried in the earth.

The closed-loop ground-coupled system uses a buried or submerged geothermal heat exchanger. This heat exchanger can reject or draw heat from a source such as the earth, a lake, or a pond, by circulating water through a closed-loop. This heat exchange loop may be pipe coils installed horizontally, vertically (down-hole), in a coil, or in other configurations. The down-hole system is used when surface area is limited since horizontal or even spiral coils can take up a lot of room and run up excavation costs.

Advantages:

  • Geothermal heat pump systems have lower operating costs, lower maintenance costs, lower life cycle costs, increased reliability, and greater comfort than alternative cooling and heating systems.
  • There is no outside equipment exposed to weather and vandalism, and no exposed equipment where children can get hurt.
  • In both commercial and residential installations, geothermal heat pump systems typically have lower maintenance costs than conventional systems as all equipment is installed inside the building or underground. All refrigerant systems are sealed, similar to household refrigerators.
  • Geothermal systems are very flexible. They can be easily and inexpensively subdivided or expanded to fit building remodeling or additions. They are particularly well-suited to "tenant finish" installations.
  • In commercial installations, systems can save money by recovering excess heat from building interior zones and moving it to the perimeter of the building. They can also save money by allowing management to isolate and shut down unoccupied areas of the building.
  • Since units can be installed in a portion of an equipment room or small closet, it gives owners more usable space.
  • In retrofit situations, they can replace rooftop equipment or a central chiller and boiler.
  • Industry standards, set by the Air-Conditioning and Refrigeration Institute, provide consumers with easy to find and consistent support in choosing equipment. They establish standards for testing and rating products, and certify the performance ratings that participants publish.
    Geothermal heat pump systems have lower operating costs, lower maintenance costs, lower life cycle costs, increased reliability, and greater comfort than alternative cooling and heating systems.
  • There is no outside equipment exposed to weather and vandalism, and no exposed equipment where children can get hurt.
  • In both commercial and residential installations, geothermal heat pump systems typically have lower maintenance costs than conventional systems as all equipment is installed inside the building or underground. All refrigerant systems are sealed, similar to household refrigerators.
  • Geothermal systems are very flexible. They can be easily and inexpensively subdivided or expanded to fit building remodeling or additions. They are particularly well-suited to "tenant finish" installations.
  • In commercial installations, systems can save money by recovering excess heat from building interior zones and moving it to the perimeter of the building. They can also save money by allowing management to isolate and shut down unoccupied areas of the building.
  • Since units can be installed in a portion of an equipment room or small closet, it gives owners more usable space.
  • In retrofit situations, they can replace rooftop equipment or a central chiller and boiler.
  • Industry standards, set by the Air-Conditioning and Refrigeration Institute, provide consumers with easy to find and consistent support in choosing equipment. They establish standards for testing and rating products, and certify the performance ratings that participants publish.

Disadvantages

  • Geothermal heat pump systems may require a higher initial investment to cover the incremental cost of the ground loop.
  • Space is required for installing the ground loop,
  • They require a knowledgeable contractors to assure correct installation, check-out, start-up and operation.
  • Since accessibility to terminal units is important in geothermal systems, architects and mechanical and structural designers must carefully coordinate their work.
  • Each unit requires both electrical and plumbing service.
  • Duct systems must be installed to bring outside air to each space.
  • Secondary or backup heat sources, and use of a loop anti-freeze solution, are required in cooler climates.

Applications

Closed-loop heat pumps can be applied wherever there is space to install the horizontal, vertical or other configuration ground loop. Current installations range from small residences to schools and large commercial or institutional buildings.

Commercial Application Benefits In addition to the general benefits of a geothermal system, there are other advantages for the commercial user to install a geothermal heat pump system.

Efficiency

In the heating mode the geothermal heat pump system extracts two-thirds or more of its energy from the earth and moves it to the heat pump unit. Only one-third of the energy needed is purchased power. This is used to run the compressor, circulating pumps and fan.

In the cooling mode, the unwanted heat is transferred back into the earth for reuse in heating.

No ground level outdoor equipment

All of the equipment needed can be installed in one or more mechanical equipment rooms, above dropped ceilings, or on the roof. Except where roof-top units are installed, there are no visible outdoor units to suffer from deterioration due to weather or vandalism, and no outdoor noise. The rest of the equipment consists of an array of piping buried beneath the ground. These pipes can be installed in a vertical well system, horizontally buried under the surface, or installed in a pond. In some cases, well or pond water can be used directly without buried pipe.

Environmental

Geothermal heat pump systems produce no on-site pollutants, improve indoor air quality, and reduce the amount of pollutants at the generating station when compared to other forms of electric heat. They also result in lower carbon dioxide emissions which reduces the threat of global warming.

Service hot water

In cooling mode, the unwanted heat can be used to preheat hot water at a very low operating cost. In winter, surplus heat can be used in the same way.

Warmer air distributed during heating

No more seemingly cold drafts from the air outlets often encountered with conventional heat pumps.

No supplemental heat

Most geothermal systems can operate year-round without supplemental direct heat.

Little or no out-of-pocket incremental cost

Any added initial cost can often be included in the project financing, and the operating cost savings used to amortize the added investment, if any.

Technology types (resource)

Manufacturers of products certified by the Air-conditioning and Refrigeration Institute (ARI) Standard 330 for ground source closed-loop heat pump equipment, publish a capacity rating for each model cataloged at the standard entering water rating conditions of both 77 ° F for cooling and 32 ° F for heating. The cooling EER, heating COP and fluid flow for both is also published. In calculating the cooling Standard Energy Efficiency Rating (EER) and the heating Coefficient of Performance (COP), a penalty for the water pump effect of 0.8 watts per gallon per minute per foot of head is added to the measured power input. This approximates a 25% total pump efficiency. Further more, a 17 foot pressure drop shall be added for the loop to determine the total pressure drop of the system for the pumping penalty.

You should not confuse the ARI Standard Energy Efficiency Rating with the expression SEER. The latter stands for Seasonal Energy Efficiency Ratio. SEER is a measure of seasonal cooling efficiency under a range of weather conditions, assumed to be typical for a location, and of performance losses due to cycling under part load conditions. Most manufacturers publish performance ratings on their various models. These ratings are usually specified by a model number at a fixed air flow in cubic feet per minute (cfm) with the following variables:

  • Several fluid flow rates in gallons per minute,
  • Entering fluid temperatures from 25 to 100 ° F,
  • Entering air temperatures at 80 ° F dry bulb and 67 ° F wet bulb for cooling, and
  • Entering air at 70 ° F dry bulb for heating.

The following rating values are given for cooling for each of the above variables: Total capacity in:

  • Btu per hour,
  • Sensible capacity in Btu per hour,
  • Heat rejection to the loop in Btu per hour,
  • Power input in watts, and
  • Energy efficiency rating (EER), which is the total capacity divided by the power input, or Btu per watt.

Also, for each of these variables, the following rating values are given for heating:

  • Total capacity in Btu per hour,
  • Heat absorption from the loop in Btu per hour,
  • Power input in watts,
  • Coefficient of Performance (COP), which is the total capacity divided by the power input converted to Btu per hour, and multiplied by 3.412 Btu per hour per watt, and
  • Performance includes provision for an antifreeze solution used at 35 ° F and below entering water temperatures, but not the pumping penalty for the antifreeze use.
It is important not to extrapolate from the data tables provided by manufacturers. Extrapolation removes the data from the context and boundaries of the table, and is not a good engineering practice. This is why most manufacturers say that interpolation between ratings within a table is permissible, but extrapolation is not.

Other published data typically includes:

  • Correction factors given for various other entering air conditions, dry and wet bulb, and other air flow ratings,
  • The unit water pressure drop is also published for each flow rate, in either feet of water or pounds per square inch (psi),
  • Blower performance including fan motor brake horsepower (bhp), and external static pressure capability,
  • Electrical data on voltages and current draw,
  • Physical data including operating weight and refrigerant charge, and Hot water generating capacity.

Efficiency

In general, efficiency is defined as the Useful Work or Energy Delivered divided by the energy supplied to do that work. In heating and cooling with heat pumps, the definition is changed somewhat. For cooling the efficiency is expressed as the Energy Efficiency Ratio - EER.

If the unit cooling capacity and EER is known, the kW power input can be calculated. For example, a unit having a total cooling capacity of 26,000 BTUH at an EER of 11.4, the cooling unit power input is:

26,000 BTUH / 11.4 x 1000 watts/kW = 2.28 kW

For heating, the efficiency is expressed as the Coefficient of Performance or COP. If the unit heating capacity and COP is known, the kW power input can be calculated. For example, a unit having a heating capacity of 33,500 BTUH at a COP of 3.9, the heating unit power input is:

33,500 BTUH / 3.9 COP x 3,413 BTUH/kW = 2.51 kW

Keep in mind the unit EER and COP are dependent on the entering ground water temperature, water flow rate through the unit, airflow rate and the entering air temperature. High efficiency equipment comes with a higher price tag. It is difficult to state a general rule about selecting equipment on the basis of efficiency because of the economic considerations and the merits of each system and the equipment used with it.

Air flow is typically selected to be between 300 and 525 cfm per ton of cooling capacity. 400 cfm per ton is typical. The final selection will be governed by the sensible to total load ratio.

Water flow through each unit should be designed to simplify water balancing. To do this, the system should be designed to keep unit water pressure drops as close as possible to each other. Though the target water temperature rise is usually 12 ° F. or 2.0 gpm per ton, they can range from 8 to 15 ° F, equal to 3 to 1.6 gpm per nominal ton of cooling capacity.

The gpm per ton can be derived from the following formula: 12,000 divided by the result of the temperature rise, times 500. In this formula, 500 equals water at 8.33 pounds per gallon times 60 minutes per hour.