Closed Loop Water Source Heat Pump
Heat pumps come in many sizes, shapes, and types, some of which are best suited to specific applications; however, the closed loop water source heat pump can be utilized in almost any large facility. The closed loop water source heat pump concept provides an extremely simple, highly flexible and unusually reliable semi-decentralized system approach to year-round space conditioning of multi-room buildings. An inherent characteristic of this system is heat recovery and redistribution of energy gains from within the building due to lights, people, solar radiation and heat producing equipment.
As a combination mechanical and electrical system, it offers an advanced approach to total system capability. This semi-decentralized system provides the benefits of individual choices of "heating", "cooling", or "off" without affecting conditions maintained on other spaces. This choice of function is there any time of the day or year.
If a building has a moderate amount of internal heat gain, needs a heated and cooled environment, and requires a minimum cooling capacity of 35 to 50 tons, then the building should be seriously considered for the closed loop water source heat pump system.
Closed loop heat pumps utilize a system in which there is a heat pump in each zone of a building. The heat extractors of all these units are connected together by a closed loop of circulating water.
The principal components of the system are several small self-contained heat pump units which have the capability of reversing the flow of hot and cold refrigerant gas. These units can be located almost anywhere (vertical, console, horizontal) and come in sizes from 6,000 BTU/hr to over 175,000 BTU/hr, depending upon the style you choose. These units are connected by a water loop of uninsulated piping through which water is continuously circulated.
Obviously, this system requires a very reliable circulating pump to keep water flowing properly throughout the loop and to allow for the proper transfer of heat energy. The two basic requirements of this pump are to guarantee adequate water flow and to maintain the water temperature between 60° - 90°. To keep the temperature within the range, the system must have two additional components-a supplemental heater and a heat rejector or evaporative cooler. As needed, one or the other of these units will operate simultaneously.
How It Works
As we expand our concept to the entire complex, let us now look at energy management principles at work.
Cold Weather Operation
On the coldest day of winter (Figure 1), when most units are on the heating cycle, the supplemental heating system must operate to maintain the minimum (60°) temperature of the water in the loop. Since utility rates vary throughout the country for all forms of energy, a thorough evaluation of all the tariffs or rates is essential when determining the proper energy source for this heater. It could be a single fuel or a combination of several. An example: if the building is on an electric demand rate with a summer/winter ratchet and the building is summer peaking, then an electric element heater may be desirable to level the annual demand. If the winter electric load is similar to the summer, it may be more economical to utilize either a fossil fuel or some form of storage, solar or renewable source. Each facility would have to consult with local experts to determine the best way to go.
Hot Weather Operation
The opposite extreme (Figure 2) to all heating is all cooling, which requires the operation of a heat rejector to maintain the loop temperature below 90°. Since most cooling is done electrically, there is little potential for substitution available; however, making ice during system off peak periods (cooling thermal-energy storage) is gaining in popularity. This ice is then stored and used to control the peak system demand as required. Normally, about 25 percent of the cooling capacity can be stored in a northern climate. In warmer climates, it is difficult to use this technique for controlling demand since there is little chance to make ice during the limited time available.
Moderate Weather Operation
During all other times of the year, the system will be operating at its highest degree of efficiency since neither the supplemental heater of the heat rejector will be required to meet the individual zone requirements.
As the diagram indicates (Figure 3), the unitary heat pumps can be in all modes of operation simultaneously. Those areas with high heat gains are rejecting heat to the water loop which is being pumped to the zones that are extracting heat. Those units which are not in operation are being bypassed; however, heating or cooling capabilities are immediately available when the thermostat requires operation.
Most buildings require year-round cooling in the interior core (Figure 4), thus allowing for the moderate weather operation conditions to exist. No heat energy is being lost through the heat rejector, since the loop is transferring all the heat to the exterior zones that are requesting additional heat.
How to Improve Efficiency
- Adding solenoid valves or water regulating
valves to each heat pump will allow a variable flow pumping system and a significant
decrease in pumping energy by allowing the pump speed to vary with demand. Pumps for WSHP
systems are typically oversized and represent a significant base energy load so improvements
in this area can be very cost effective, especially if the pumps must operate continuously
in order to satisfy a small portion of the building load.
Another method to reduce pumping horsepower requirements is to reduce the system pressure drop. Each piece of equipment in the piping loop (heat pumps, heat rejectors, strainers, and even the piping itself) should be selected for minimum pressure drop.
- Coupling miscellaneous refrigeration and/or computer room units to the WSHP loop can reduce the need for supplemental heat and possibly provide for more efficient operation in the summer (as opposed to air cooled machines, especially those that reject heat to conditioned space).
- Heat rejectors (also known as cooling towers and closed circuit coolers) can have their
efficiency increased in several ways. First, initial unit selection can have a significant
impact on required fan horsepower. In general, for a given heat rejection load, a larger
unit selection results in less fan horsepower. Also there are several different types of
heat rejecters and the fan horsepower requirements can be doubled or triple for some styles
as compared to the most efficient.
Secondly, fans should be selected to allow for part load operation at reduced horsepower ratings. This can either be accomplished by having multiple fans installed and staging their operation or by specifying two speed fan motors.
Lastly, since some water must be circulated through this item at all times to prevent freezing, supplemental heat may be required. The amount of heat lost during the winter can be minimized by adding discharge dampers and insulation to the heat rejector box.
- Adding storage tanks to the piping loop may help the system efficiency during intermediate seasons when afternoon cooling is followed by a need for morning warm up. Large storage tanks will allow excess heat rejection to be stored and possibly eliminate the need for supplemental heat during these conditions. Storage tanks may also allow some "pre-cooling" of the condenser water during summer nights or off-peak heating during the winter.