8/10/2019 WSHP - Water Source Heat Pump Sequence of Operations

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Closed Loop Water Source Heat PumpHeat 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 a lmost any large facility. The closed loop water source heat pu mp concept provides an extremely simple,

highly flexible an d unusually reliable semi-decentralized system approach to year-round space conditionin g of multi-room buildings. An

inherent characteristic of this system is heat recovery and redistribution of energy gains from within the building due to li ghts, people, solar

radiation and heat producing equipment.

As a co mbinatio n mecha nical and electrical system, it offers an advance d ap proach to total system capabi lity. This semi-decen tralized system

provides the benefits of in dividual 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.

System Components

The principal components of the system are several small self-contained heat p ump 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 reliabl e circulating pump to keep water flowing properly throughout the lo op and to allow for the proper

transfer of heat ene rgy. The two basic requirements of this pump are to guarantee ad equate 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 hea ter and a

heat rejector or evaporative cooler. As needed, one or the other of these uni ts will operate simultaneously.

How It Works

As we expand our concept to the e ntire co mplex, l et us now look at ene rgy mana gement princi ples 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 wa ter in the loo p. 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 a n electric demand rate with a summer/winter ratchet and the building is summer

peaking, then an electr ic element heater may be de sirable 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 renewabl e 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 li ttle potential for substitution available; ho wever, making ice during system off

peak periods (cooling thermal-energy storage) is gaini ng 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.

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Moderate Weather Operation

During all other times of the year, the system will be operating at its highest degree of efficiency since neither the suppl emental heater of the

heat rejector will be required to meet the individual zone requirements.

As the d iagra m ind icates (Fig ure 3 ), the unitary heat pumps ca n be in all modes of ope ration simultane ously. Those areas with h igh 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 o peration conditions to exist.

No heat ene rgy 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

A. Adding solen oid valves o r water regula ting valves to each heat pump will allo w a variabl e flo w pu mping system and a sig nificant 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 red uce p umping horsepo wer re quire ments is to redu ce the system pressure drop. Ea ch pi ece o f equ ipment in the pi ping

loop (heat pu mps, heat rejectors, strainers, and even the piping i tself) should b e selected for minimum pressure drop.

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B. 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).

C. Heat rejectors (also known as cooling towers and closed circuit coolers) can have their efficiency increased in several ways. First, initial uni t

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 ho rsepower. Also there are several different types of heat rejecters and the fan horsepower requirements can be dou bled 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.

D. 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 win ter.