Heat Pump Water Heaters: Efficient Water Heating and Space Cooling Combined
By Staff -- Foodservice Equipment and Supplies, 8/1/2007
 Professional foodservice operations that feature a relatively constant demand for hot water and continuous need for cooling or dehumidification are natural candidates for heat pump water heaters. |
When to choose an HPWH. Your building may be a good candidate for an HPWH if:
- You need to replace an electric water heater
- You’re looking to add air conditioning to spaces where it’s normally cost-prohibitive
- Natural gas is not available in your area
- You require a large, steady load of hot water throughout the day
Because HPWHs produce cool, dry air as a by-product of heating water, the best applications take advantage of both outputs simultaneously. Consequently, HPWHs work well in commercial-sector applications that feature a relatively constant demand for hot water and continuous need for cooling or dehumidification, including commercial kitchens.
Although geography does not limit commercial HPWH applications, air-source HPWHs do work best in mild climates where the ambient temperatures remain hot enough in nearly every season to provide hot water.
Pick the right size. Commercial-sized heat pump water heaters (HPWHs) are add-on systems in which the heat pump apparatus stands alone. Heat is transferred from the heat pump condenser to the water in the tank of an electric water heater via a heat exchanger and a small pump, using the tank's water as a heat exchange medium. Add-on systems come in a variety of sizes for commercial and industrial applications, producing up to 3,000 gallons per hour of hot water and more than 30 tons of air conditioning.
Choosing the right size HPWH system requires estimating daily hot water needs in gallons, just as you would size any other water heating system. However, for HPWH systems, an allowance must be made for high peaks in hot water demands. HPWH systems are quite efficient, but they are slow and steady. A key factor to consider is the rate of hot water production, listed in product literature as the “recovery rate” and measured in gallons per hour. Recovery rates are typically half those of traditional electric water heaters, but the instantaneous power consumption is typically 40 percent to 70 percent less. Accordingly, electric demand savings with HPWH systems can be substantial, but only if the use of backup electric resistance heat is quite low. If you’ll be using HPWH systems in applications that require considerable hot water over a short time, choose either a larger tank than a traditional hot water system has or an HPWH system with a high recovery rate. Either choice will help smooth over peak hot water loads.
Perform a cost/benefit estimate. Utility rates and water use heavily affect the cost-effectiveness of an HPWH. Commercial HPWHs initially cost much more than an electric or gas-fired boiler, but the large annual savings mean paybacks often range between two and three years when an operation consumes large amounts of hot water. Some of the parameters a commercial designer should take into account include water inlet and setpoint temperatures, HPWH location, air-conditioning and dehumidification loads, and water consumption rates. This makes estimating commercial HPWH economics a trickier process, so contact a vendor or system designer to see if an HPWH is appropriate for your application.
Integrate plant systems. For buildings that use rooftop cooling towers or large refrigerators, it may be worthwhile to harvest waste heat from these units, using the HPWH system both to produce hot water and to help meet the air-conditioning load.
Pick a good location. Install all HPWH systems with careful attention to the flow of air across their evaporators. First, because airflow is a necessity (several hundred cubic feet per minute, even for smaller systems), do not place systems in isolated, tight areas. Second, because they produce dry, cool air, put them where their output air will be useful, such as damp areas or spaces that need cooling most of the year. It can also be advantageous to place the HPWH evaporator in a location where it can take advantage of the heat from the dishwasher. Of course, operators can use ducts and dampers to achieve the needs of source and output air, thus allowing flexibility in choosing a location. Finally, as with refrigerators, the compressor motor on an HPWH system produces some noise, so it may be wise to pick a location where the noise won’t be a nuisance.
Perform regular maintenance. Heat-exchange surfaces perform better when clean, and HPWH systems are no exception to the rule. To maintain good energy performance, keep the filter that protects the evaporator’s heat-exchange surfaces clean. This is particularly important in kitchens and other areas that contain airborne pollutants.
