An open-loop, ground-water heat pump, uses a surface or underground water source (such as a lake, river, or well) as the heat source and sink. Well water designs are the most common and seem to be the most cost effective. The well supplies both domestic water and water for the heat pump. Approximately three gallons per minute of well water are needed per ton of cooling capacity.
Ground water source open-loop heat pumps use the same concept as the ground coupled units - for example, in the Midwest the temperature of the earth near the surface and the water in it (aquifer) is typically around 55°F. Water is taken from the ground or surface water (pond, lake, etc.), circulated to the individual heat pumps and the returned to the ground via a disposal well, returned to the lake or pond, or where permitted discharged into a stream or river.
When more units are heating than cooling the circulating water temperature drops prior to disposal. Conversely, when more units are cooling than heating, the circulating water is warmed prior to disposal.
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. This means that there is no outside equipment exposed to weather and vandalism. All refrigerant systems are sealed, similar to household refrigerators.
- Open loop systems have less loss in heat transfer
- Open loop systems have lower heat pump energy costs
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.
- Problems associated with disposal of the water after once-through the heat pump, disposal wells can be costly,
- Water which has suitable qualities could change with time to poor quality that causes problems of corrosion later in time,
- Costs associated with the drilling of the water well can be high and unknown at the outset,
- Water tables and well output can change over time and cause future problems,
- Permitting and its costs are usually required,
- Open-loop systems have more potential problems than either conventional systems or closed-loop geothermal systems because they bring outside water into the unit. This can lead to clogging, mineral deposits, and corrosion in the system.
- Open-loop systems require a large supply of clean water in order to be cost effective. This often limits their use to coastal areas, and areas adjacent to lakes, rivers, streams, etc. In addition, there must be an acceptable method of returning the used water to the environment. This may be limited not only by environmental factors (such as no place to dump that much water), but also by local and state regulations.
- 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 are required in cooler climates.
Open loop heat pumps can be applies wherever there is an ample source of good quality water is available along with an adequate and permitted disposal method.
Best Applications Where water supply and quality is known and water use is not now, or expected to be, regulated; and where a closed loop system may not be applicable.
Technology Types (Resource)
Manufacturers of products certified by the Air-conditioning and Refrigeration Institute (ARI) Standard 325 for ground water-source heat pump equipment publish a capacity rating for each model catalogued at the standard entering water rating conditions of both 70 and 50°F for cooling and heating. The cooling EER, heating COP and water flow for both is also published. In calculating the cooling Standard Energy Efficiency Rating (EER) and the heating Coefficient of Performance (COP), a water pump penalty effect is added to the measured power input.
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 water temperatures from 35 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 water 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 water 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.
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.
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.
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