The benefits of using water-source heat pumps
Water-source heat pumps prove to be more energy-efficient than alternative systems for commercial buildings.
- Analyze water-source heat pumps and compare them to alternative systems.
- Examine how water-source heat pumps can reduce building energy consumption.
- Implement a water-source heat pump system to meet simultaneous heating and cooling demand.
The first commercial energy code was promulgated in 1975 with the ASHRAE Standard 90-1975: Energy Conservation in New Building Design. Since then, subsequent efforts have led to more stringent energy efficiency standards, as shown in Figure 1.
A complete and uniform adoption of these energy codes would have resulted in approximately a 50% reduction in normalized energy use between 1975 and 2012. In reality, total energy consumption per square foot in commercial buildings has decreased from 114 kBtu/sq ft in 1979 to 79.9 kBtu/sq ft in 2012—a 30% decrease.
While this is a significant achievement, adoption and enforcement of standards by different states has not been uniform, and buildings continue to account for a large percentage of energy consumption in the U.S. According to the U.S. Energy Information Administration, commercial buildings consumed 7 quadrillion Btus of energy. Further, HVAC accounts for 44% of the commercial building energy demand, as shown in Figure 2. This includes space heating, ventilation, and cooling, but excludes refrigeration.
Commercial heat pumps
A heat pump is a refrigeration circuit that can cool spaces during warm weather and heat spaces during cool weather. With a heat pump, you can cool or heat a space by only using electricity. By not burning fuel for heating, as in a traditional central furnace, a flammability risk is eliminated.
Commercially available heat pumps can be categorized into two broad types:
- An air-source or air-cooled heat pump
- A water-source heat pump (WSHP).
An air-source or air-cooled heat pump is a type of heat pump that operates by rejecting heat to outside air during the summer or by absorbing heat from outside air during the winter. A WSHP is a type of heat pump that operates by rejecting heat to a water-pipe system (or water loop) during the summer or by absorbing heat from the same water loop during the winter. If multiple units of WSHPs are installed, they can all be serviced by a common water-loop system (or header).
Advantages of water-source heat pumps
For WSHPs, since the heat is transferred via a heat exchanger into a pipe that is carrying water, the operation is quieter and the system footprint is smaller since water is more efficient at carrying away heat than air. In an air-source system, the limiting heat-transfer coefficient is on the air side and typical forced convection air-side heat-transfer coefficients is in the range of 25 to 250 W/m2 K. In contrast, the forced convection heat-transfer coefficient on the water side is between 50 to 20,000 W/m2 K. This makes WSHP equipment more efficient and smaller in size than air-source heat pumps.
Traditional air source units can require each air handling unit to have a separate condensing unit. For a large, multi-unit system, which is common in a commercial building, multiple condensing units would be needed that are not only noisy but also present a challenge to install since they require a lot of free space. With a multi-unit WSHP installation, heat exchange can be accomplished with a single, central evaporative cooling tower or dry cooler located on the ground or the rooftop. The WSHP units can be placed in dropped ceilings or hidden away from occupied spaces in mechanical rooms or utility closets. Placing the units in ceilings, near to the point of use, also results in less ductwork and less fan-energy consumption. Fan-energy consumption can be among the largest energy components of an HVAC system, and a good overall system design will attempt to minimize it.
WSHPs also offer some of the highest efficiencies in the HVAC industry. ASHRAE sets the minimum efficiency requirements for WSHPs to be higher than traditional air-cooled heat pumps and VRF systems. Tables 1 and 2 show efficiency values for the most directly comparable units and is derived from ASHRAE 90.1-2013: Energy Standard for Buildings Except Low-Rise Residential Buildings. This comparison shows that WSHPs meet the highest minimum energy efficiency ratio (EER) and coefficient of performance (COP) requirements.
WSHPs also are more efficient at heating when compared with packaged furnace air conditioners. In a furnace unit, the maximum efficiency for heating by burning natural gas is about 95% (for a COP of 0.95); electrical heat is 100% (COP = 1.0). With a water-source heat pump in heating mode, not only is the thermal energy from the water loop being extracted and used, but also the heat of compression in the refrigerant circuit is captured and used as a source of heating. Due to this capability of extracting heat from a heat source (i.e., the water loop) and using the heat of compression, the WSHP can easily provide 4 to 6 units of heating for every unit of energy consumed. Clearly, this is a more efficient system.
Simultaneous heating and cooling with WSHPs
Depending on the orientation of the buildings and the demands of different kinds of tenants in a multi-use location, it is common for some of the occupants to demand cooling while others are demanding heat at the same time. Or, there is demand for hot water in a restaurant while the ice cream store next door is demanding cooling. Even within a single office building—for example, in winter—there is usually a need for heating at the perimeter that is exposed to the elements and cooling needed at the core of the building due to heat given off by the occupants and equipment as shown in Figure 3.
This diversity of energy demand exists everywhere. WSHPs can take the heat out of an area that is rejecting it and use it in another area that is demanding it. Energy recovery and transport is an area where WSHPs truly excel. Water is nonflammable, nontoxic, and has a high specific heat value of 4.2 KJ/kg C. It is an ideal medium to transport energy without any of the negative consequences of using a synthetic or flammable refrigerant to move energy, especially in the vicinity of occupied spaces.
The Btus of cooling load and the heat of compression in the refrigeration circuit of a traditional air-source unit is rejected to the atmosphere through the outdoor condensing unit. The quality of heat is low, and it is not economical in an air-source heat pump to recover this energy. This energy is blown into the atmosphere, thus it is wasted. In a WSHP, these Btus are rejected into a common water loop that acts as a reserve of this energy, which can be easily transported to the place that is demanding heat. As water is physically moved through a water pump to different areas of the same building or a group of buildings, thermal energy is also being simultaneously transported. Even though the quality of heat is low, it is efficiently captured and transported to where it is needed due to the high heat-transfer coefficients achievable with water and due to its high specific heat. The energy that is normally wasted in an air-source heat pump is now recovered and used somewhere else, thus offsetting new energy demand. The overall building energy consumption is decreased and no Btu is left behind.
One of the biggest advantages of a WSHP is that thermal energy can be economically, efficiently, and safely transported to wherever water can be pumped. Since moving water is easy and safe, the WSHP can be applied to any situation—from a single condo unit to a large office campus.
The advantages of a WSHP deployment become even more apparent when compared with another popular system, the VRF system. This system has limited ability to transport energy over long distances because it uses refrigerant close to the saturation temperature, or with limited subcooling, in copper pipes as the transport medium. There also are compressor power and pressure drop concerns with pumping so much refrigerant. Most VRF vendors place limits on refrigerant tubing lengths such as how far the indoor units can be from the outdoor units, the total length of system piping allowable, the length from the first separation tube or branch controller to the farthest indoor unit, or the total vertical pipe length.
In addition, there is also a large number of copper pipe connections that have to be made in the field in a VRF system. If there is a refrigerant leak anywhere along the path, it is extremely difficult to detect, isolate, re-braze, re-vacuum, and re-commission the system. In a WSHP, all refrigerant lines are contained within the unit and installed and leak-tested in a controlled factory setting. The energy in a WSHP system is moved by water in normal plumbing systems, with which every HVAC contractor is familiar. This is much safer and easier to maintain, and there are no piping-distance limits.
Another advantage of WSHPs is its ability to achieve highly efficient operation and perform heat recovery while operating with a limited amount of refrigerant charge. High system refrigerant charges are a potential safety hazard, as refrigerant can displace oxygen in closed rooms and cause asphyxiation without warning. ASHRAE Standard 15-2013: Safety Standard for Refrigeration Systems and ASHRAE Standard 34-2013: Designation and Safety Classification of Refrigerants defines the refrigerant concentration limit (RCL) and oxygen deprivation limit (ODL) for different refrigerants, and these can be used to calculate the smallest-sized room where a refrigeration system can be safely installed. For the most commonly used refrigerant, R-410A, the RCL is 26 lbs/1,000 ft3 and the ODL is 140,000 ppm. For institutional buildings, such as hospitals, the threshold is only half of the nominal value (institutional buildings are defined where occupants cannot readily evacuate without the assistance of others).
The small quantity of refrigerant charge needed in a WSHP is contained completely within the factory-tested unit, in comparison with a VRF system that requires higher quantities of refrigerant charge. In a VRF system, refrigerant is not only used within the central units and their internal mini-split units, but it also is present in the extensive pipework and branch controllers between these internal and external units. Some VRF systems require two or three pipe-network designs to work, and all of the pipes carry refrigerant. This complex maze of copper piping presents increased safety and maintenance risks due to the higher refrigerant charge in the system, the higher number of connection points that are potential points of failure, and the hazard it poses if there is a leak. As the industry considers moving to flammable refrigerants to meet the low global warming potential refrigerant requirements, driven by climate change concerns, the safety considerations of using VRF become even more acute. In a VRF system, the RCL/ODL thresholds can be easily exceeded in small offices, hotel rooms, bathrooms, and utility rooms.
WSHP system implementation
A typical WSHP implementation in a large commercial building consists of several WSHPs that are installed close to the areas of demand and fed by a common water loop. The cooling tower, pumps, and boiler are located away from occupied spaces.
In a WSHP implementation, the control schemes generally limit the water-loop temperature to be in the range of 60° to 95°F, depending on the season. Keeping the water-loop temperature in this range is a matter of economics and engineering judgment. The heat pumps can operate with water temperatures outside this range, but their efficiency will be lower. For example, the heat pump has to work harder in heating mode with the water-loop temperature at 55°F than at 60°F. Is it better to let the heat pumps work harder at 55°F (consuming more electricity) or using the gas-fired boiler to raise the temperature of the loop to 60°F? The answer depends on the relative costs of gas and electricity as well as the relative efficiencies of the boiler and heat pumps at these operating points.
When the heating and cooling loads are nearly equal, the WSHPs extract the heat from the areas rejecting it and provide the heat to the WSHPs that are demanding it through the common water loop. Small temporary imbalances in the loads between heating and cooling are taken care of by allowing the water-loop temperature to float within the 60° to 95°F range. In this scenario, there is no need for any net inputs to the system of either heating Btus or cooling Btus and both the cooling tower and boiler can be turned off to improve system efficiency. Due to the large specific heat of water in the loops, a 35°F range in water-loop temperature implies there is a large amount of thermal energy available to take care of demand imbalance. Just 1,000 ft of 6-in. pipe can hold 875 KBtus of thermal energy over a 35°F span of temperature. Over a 1-hour period, this equates to more than 70 tons of refrigeration that can be moved around.
In cases where there is an increase in cooling demand than heating demand—like summer, for example—a central cooling tower is provided for net cooling to keep the water-loop temperature from exceeding 95°F (see Figure 4). Alternatively, the excess Btus can be used to generate hot water using a water-to-water WSHP.
The reverse is true in winter when the heating demands exceed the cooling demands. A central boiler is provided to add Btus to the water loop to make sure that the water-loop temperature does not go below 60°F (see Figure 5). Since the cooling tower and boiler are sized to handle only the maximum net cooling and heating demand, as opposed to sizing for maximum total cooling and heating demand, their sizes can be reduced to result in cost savings.
The WSHP industry continues to make strides in improving efficiency, even at part-load conditions. Technologies like microchannel heat exchangers, variable-speed compressors, building management software integration, wireless thermostats, and occupancy sensors are being tested or have already been implemented to further reduce energy usage and stay at the higher end of the efficiency spectrum.
WSHPs are efficient solutions that have been effectively deployed in commercial buildings. They are especially effective in reducing energy consumption where there is diversity in energy demand, resulting in smaller sizes of cooling towers and boilers. By using water as a heat-transport medium in a common water loop, WSHPs can move thermal energy across large distances safely and more efficiently as compared with alternative solutions on the market. They are smaller and offer design flexibility by allowing the placement of towers, boilers, and pumps in locations remote from occupied spaces. WSHPs eliminate flammability risk associated with furnace heating and they accomplish highly efficient operation using much smaller quantities of refrigerant as compared with VRF systems.
Naveen Halbhavi, PE, is director of marketing for ClimateMaster. He has worked in roles of process engineering, control systems design, product management, business development, and general management.