Space Cooling Systems
For most facilities, cooling interior spaces is one of the larger energy expenses. Often combined with space heating systems, electric-powered cooling systems offer the advantages of lower equipment and maintenance costs as well as safety and cleanliness.
Unitary Heat Pumps
In addition to year-round temperature control, unitary heat pumps add the energy-saving advantage of individual room or zone control. Factory-packaged and refrigerant based, these units are available in a number of application categories which include:
- Packaged terminal heat pumps (PTHP)
- Closed water loop heat pump systems
- Ground-Coupled (Closed-Loop) Systems
- Ground water-source heat pumps
- Large unitary air- and water-source heat pumps
In each category there are several possible configurations, including:
- Single package, with all components in a single enclosure
- Roof-top packages – a variation on the single package
- Split units, with remote outdoor coil, fan, and/or compressor
- Air and water-source heat pumps
- Heating only heat pumps
Package units and the indoor sections of split units are available in several configurations:
- Vertical for closet installations,
- Console for installation under windows, and
- Horizontal for ceiling or outdoor locations.
Some models have decorative casings. Others can be built-in. With the exception of large unitary heat pumps, the units are designed for free-air delivery or with short duct connections between the unit and the conditioned space. Most heat pump manufacturers participate in the Air-Conditioning and Refrigeration Institute (ARI) Certification Program. Product performance (below 135,000 BTUH sizes) is listed in the ARI Directory of Certified Products. This allows a performance comparison of various models from various manufacturers. Sizes range from l/2-ton package terminal heat pumps up to rooftop units of 30-tons or more. Many are provided with supplemental electric heaters to satisfy the load when the outdoor temperatures drop below the set point. Gas fired supplemental heat is also available in a limited range of equipment. Most heat pumps are sold and installed by local air conditioning contractors, who also provide routine service and repair. Depending on the nature of the application, this availability of fast service can be very important in equipment selection.
Advantages vs. Disadvantages
Unitary Heat Pumps:
- They provide individual temperature control in small occupied zones during nights and weekends without the need for a large central plant chiller or boiler and their associated pumps,
- The heat pump’s consumption of electricity can be separately metered.
Water Loop Heat Pumps:
- They don’t require wall openings to reject heat from air-cooled condensers,
- They aren’t exposed to the weather and therefore tend to have a longer service life,
- If a unit fails, the entire system doesn’t shut down, however, failure of a loop pump, heat rejection device or secondary heater can affect the entire system.
- Imprecise temperature and humidity control,
- In-room or in-space maintenance including frequent filter replacement,
- Water loop systems require regular loop maintenance, and
- These systems require space for pumps heat exchangers and boilers (if a boiler is required).
Unitary heat pumps can provide year-round electric air conditioning at a competitive cost with the feature of individual room or zone control. They can be used in almost any type room or building. They can be used in a variety of engineered system arrangements (i.e. closed water loop, etc.) to provide economic operating costs and desired features (night setback, etc.) not available in other systems.
They are best applied:
- Where there aren’t competitive alternative heating energy sources,
- In climatic areas with moderate heating loads, and
- Where individual room or zone control is required for customer satisfaction.
- They offer the most advantages when combined in any of the many available engineered system arrangements.
Unitary heat pumps can be applied in almost any building as they are extremely versatile. The basic customer decision is whether the requirements of a particular application can be best served by unitary system or by a central chilled water system. The required features, floor space and weight, ceiling height, cost factors, maintenance factors, operational factors all need to be considered and evaluated.
Unitary air and water source heat pumps are available as larger capacity commercial self-contained units which serve large zones using ducts for air distribution. Air source units must be located along outside walls or on the roof. They tend to have higher operating costs than central plants or water-source heat pumps.
- Water-source units can be located anywhere but require ventilation air ducts. They are usually connected to a cooling tower circuit for heat rejection when they are in the cooling mode, and to a central hot-water heater or strip heater when heating is required. Advantages include low first cost and the availability of optional accessories (variable air volume control, economizer cycle, night setback and morning warm-up).
- Large system heat pump applications are often applied in, buildings using two- or four-pipe water distribution systems, or in industrial applications. Many large buildings require cooling the year-round due to large internal loads from lighting, electronic and other business equipment. Only the perimeter zones of these structures ever need heating. The warm condenser water from the water chillers serving this cooling load can be used as a heat source. The water-to-water heat pump is piped in a cascade system, using this waste heat to preheat domestic hot water or provide hot water to satisfy building space or reheat loads. Units are available to heat water from 105°F to 120°F, or even higher if needed. The lower the hot water temperature required, the lower the energy consumption.
In some cases, the units are combined into a single heat recovery chiller with a double-bundle condenser. The house water condenser serves the hot water loop for the building. When the waste heat exceeds the heat requirement, the excess heat is rejected in the second tower water condenser. Thermal storage can also be integrated into this system. Other options include integration with closed loop water-to-air heat pumps, or secondary heat recovery from water loop heat pump systems.
- Air-to-water heat pumps perform in a similar manner but typically use warm exhaust air as the heat source. They are often referred to as heat pump water heaters and are used in hotels, restaurants, laundries and other applications needing a lot of hot water.
Many industrial processes have low-level waste heat that must be rejected. Factory-packaged, closed-cycle refrigerant-based heat pumps are available to heat water to 120°F or warmer. Using waste heat for this purpose off-loads cooling towers or evaporative condensers while reducing boiler fuel consumption and the corresponding products of combustion.
Most heat pump manufacturers rate their equipment in accordance with ARI standards and participate in the ARI certification programs. ARI Standard 210/240 covers air-source units, ARI Standard 330 covers ground source closed loop heat pumps, ARI Standard 325 covers ground water-source heat pumps, ARI Standard 310/380 covers packaged terminal heat pump air-source units, and ARI Standard 320 covers water-source heat pumps.
Packaged terminal heat pumps typically have cooling energy efficiency ratios (EER) in the range of 9.0 to 11.0 Btu per watt of electrical energy input and a heating coefficient of performance (COP) in the range of 2.7 to 3.3 .
Air-source heat pumps typically have cooling seasonal energy efficiency ratios (SEER) in the range of 10.0 to 14.0 Btu per watt of electrical energy input and a heating seasonal performance factor (HSPF) in the range of 6.0 to 9.0 Btu per watt of electrical energy input.
Water-source heat pumps typically have cooling energy efficiency ratios (EER) in the range of 10.0 to 15.0 Btu per watt of electrical energy input and a heating coefficient of performance (COP) in the range of 3.7 to 5.2.
Refer to the segment on ground coupled and ground source heat pumps for a discussion on the efficiency of these heat pumps.