In a chilled-water system, heat that other-wise would be lost to the environment (Figure 1) can be captured and used for other purposes. Although this process, known as heat recovery, is not new, the benefits — decreased energy consumption, reduced greenhouse-gas emissions, more points toward Leadership in Energy and Environmental Design (LEED) certification — have never been greater.
This article examines several methods of recovering heat from chilled-water systems, focusing on the capture of sufficient heat for useful application, the minimization of chiller lift and maximization of chiller efficiency, and the control of hot-water temperature without the loss of chiller-plant operation stability.
USES OF RECOVERED HEAT
Heat recovered from a chilled-water system can be used to generate hot water. This hot water can be used to heat the building, to heat service water, or as part of a manufacturing or industrial process.
Understanding hot-water-temperature needs for building-heating purposes is important. Most reheat and building-heat applications do not need 130°F to 140°F water to perform satisfactorily. As discussed later in this article, operating a reclaim chiller at higher leaving condenser-water temperatures (LCWTs) increases lift and reduces chiller-plant efficiency.
In many variable-air-volume reheat applications, hot-water temperatures as low as 105°F can be effective. Lower temperatures maximize chiller-plant efficiency while minimizing system energy consumption. They can be achieved by specifying a two-row reheat coil instead of a one-row coil.
To better understand building-heating-load requirements and the potential of a heat-recovery system, monthly or even hourly heating- and cooling-load data should be considered. Figure 2 shows a typical hot-water boiler-plant load for an office building in Chicago, while Figure 3 shows typical cooling loads for the same building.
A designer's challenge is to determine when both heating and cooling loads exist and how best to capture heat from a chilled-water system. For building heating, some of the best opportunities exist when a facility operates 24 hr a day, with high internal cooling loads and the possible need for heating for perimeter zones.
Nationally recognized and/or code-approved demand flow rates and demand factors must be understood to size service-water-heating systems properly. Hot-water demand flow rates, based on building type and calculated results, should include probable maximum demand, required water-heater output, and any required storage-tank capacity.
Some codes require hot-water supply to satisfy continuous and peak hot-water demand. If ANSI/ASHRAE/IESNA Standard 90.1-2004, Energy Standard for Buildings Except Low-Rise Residential Buildings, criteria are applied, tempering service hot water from “street” temperatures to 85°F is sufficient. Most chiller-based heat-recovery equipment, however, can produce water temperatures of approximately 120°F to 135°F. Regardless of temperature, a heat-recovery system must accommodate continuous and peak hot-water demand while providing a controlled source of service hot water.
Potable water must be of sufficient quality to serve as drinking water, whether or not it is to be used as such. Thus, it must be protected from contamination. Many codes require the separation of potable hot-water sources from non-potable ones by means of a double-wall vented air gap in a heat exchanger. This minimizes the risk of internal leakage and cross-contamination. Other means of separation exist. An engineer must consider code requirements before proceeding with system design.
Process hot water
As with building- and service-water-heating needs, hot-water flow, temperature, and capacity must be understood to support process-heating needs.
MINIMIZING WASTED HEAT
In a traditional chilled-water HVAC system, heat is transferred from the indoors (at the air-handler chilled-water coil) to the outdoors (at the cooling tower) (Figure 4). Not only is this heat “wasted,” energy is consumed at the cooling tower and condenser-water pumps in the process.
Section 126.96.36.199 of ASHRAE Standard 90.1-2004 requires heat recovery on the condenser side of water-cooled systems for the preheating of service hot water in large 24-hr facilities.1 System engineers seeking to meet the intent of the standard can do so with a plate-and-frame heat exchanger located in return condenser water (Figure 5). The ability to obtain 85°F pre-heated service water generally is possible, but is a function of the temperature of condenser water leaving a chiller plant. LCWT is a function of outdoor ambient wet-bulb temperature, condenser flow rate, and chiller load. With service water at a minimum of 85°F required to meet the intent of ASHRAE Standard 90.1-2004 during peak conditions, LCWT must be slightly above 85°F. However, elevating LCWT increases chiller lift, reduces chiller efficiency, and increases chiller-plant energy consumption.
Another way to minimize wasted heat is to divert heat to a device that can convert it to a useful heat source, a device known as a heat-reclaim chiller.
A heat-reclaim chiller (Figure 6) generates within its condenser high-pressure refrigerant that can be used to produce higher-temperature condenser water.
With hot-water requirements varying considerably, several types of heat-reclaim machines are available:
- Single bundle
The single-bundle heat-reclaim chiller is used for non-potable-water applications, such as building heating and process water. An onboard control system maintains the temperature of “hot” water leaving the condenser while simultaneously producing chilled water.
- Double-wall vented desuperheater
The double-wall vented heat-reclaim chiller features a heat exchanger that extracts high-pressure, high-temperature heat from refrigerant, “desuperheating” it to a lower-pressure refrigerant. In the process, “hot” potable water is produced. The amount of heat is less than what could be extracted with a single-bundle heat-reclaim chiller. Because full condensing does not occur in the desuperheater, refrigerant vapor must be piped to a separate refrigerant heat exchanger. This process can take place in a remote water- or air-cooled condenser.
The heat exchanger of a double-wall vented heat-reclaim chiller has an air gap to separate potable water from refrigerant, minimizing the potential for contamination.
- Double bundle with fully condensing heat exchanger
A double-bundle heat-reclaim chiller with fully condensing heat exchanger (Photo A) can produce hot water and provide full refrigerant condensing through a second heat-exchanger bundle.
Some chillers use entering- and leaving-condenser-water temperature to determine the stage of compressor capacity necessary to maintain a hot-water-temperature set point. As lcwt deviates from the heat set point, the chillers adjust compressor capacity to ensure hot-water temperature is maintained.
One exception: During heat mode, chilled-water temperature is allowed to float and no longer is the primary input for capacity control. This could overcool the chilled-water loop during light-cooling-load conditions. However, when leaving-chilled-water temperature falls below the cooling set point, an additional software routine removes a stage of capacity from the chiller. This is known as “low-source protection.” This ensures that the greatest amount of useful heat can be extracted from a heat-reclaim chiller without the stability of the chilled-water system being sacrificed.
Primary/secondary chilled-water system with heat-recovery chiller
A conventional primary/secondary chilled-water system with a small-capacity heat-recovery chiller installed in parallel with the chiller plant (Figure 7) minimizes chiller-plant lift and maximizes energy efficiency while allowing direct control of both hot- and chilled-water temperature. The heat-recovery chiller produces a base load of chilled water while generating hot water controlled to the heat set-point temperature. The heat-reclaim chiller offsets the main chiller-plant load by providing the base load of chilled water before it enters the primary chillers. Under base-load operation, the heat-reclaim chiller provides chilled water to the secondary loop. The low-source-protection feature of the heat-reclaim chiller ensures chilled-water temperature will not fall below cooling-set-point temperature, ensuring stable chiller-plant operation while providing a controlled source of hot water. If the chilled-water temperature cannot be achieved with the heat-reclaim chiller alone, the first chiller in the primary loop can be energized, thus, maintaining secondary-loop water conditions. This configuration provides a stable source of controlled hot water and a base load of chilled water without affecting main-chiller-plant efficiency.
Variable-primary-flow chilled-water system with heat-recovery chiller
Excluding benefits from heat reclamation, a variable-primary-flow system can offer energy savings beyond those of other chiller-plant configurations. One study2 found that “variable-flow, primary-only systems reduced total annual plant energy by 3 to 8 percent, first cost by 4 to 8 percent, and life-cycle cost by 3 to 5 percent relative to conventional constant-primary-flow/variable-secondary-flow systems.”
This system works much the same as the primary/secondary system described earlier, with the heat-reclaim chiller providing a base load of chilled water for the primary chiller plant (Figure 8).
Series-counterflow chilled-water system with heat-recovery chiller
Additional energy savings can be realized with a series-counterflow system (Figure 9). Variable-speed screw chillers offer precise capacity control under varying return-water flow and temperature conditions while providing high full- and part-load efficiency. These chillers are well-suited for such use because the instabilities associated with surge in centrifugal chillers are not present. Although screw chillers with variable-frequency-drive speed control respond well to variations in load and head, other chiller types can benefit from the higher overall chiller-plant efficiency offered by the series-counterflow arrangement. This system takes advantage of the lower lift provided by smaller differences between leaving chilled-water temperature and leaving condenser-water temperature. A series-counterflow plant benefits from significantly lower lift and improved overall efficiency. As with the primary/secondary- and primary-variable-flow configurations, the first stage of cooling is provided by the heat-reclaim chiller.
USING CAPTURED HEAT
Some heat-reclaim chillers can produce water as warm as 135°F. Although that is warm enough to satisfy many hot-water applications, some applications require that a source of water warmer than 135°F always be available. In such cases, a system similar to the one in Figure 10 should be considered. This system captures heat from a heat-reclaim chiller and provides water at the temperature required by an application.
To ensure stable hot-water-temperature control, loop volume should be no less than 6.0 gal. per ton of heating capacity within a water heater and preheat storage tanks and connecting piping.
Note the location of tank piping connections. Thermal stratification or improper thermal mixing within tanks can lead to poor hot-water-temperature control. To ensure proper mixing, the heat-reclaim water pump between a preheat storage tank and a heat-reclaim chiller should remain on at all times.
A backup water heater — a conventional water heater, a hot-water boiler, or a steam converter — must be provided to ensure a controlled source of hot water is available when a heat-reclaim chiller is not operating. This ensures that cold makeup water is preheated by the hot water generated by a heat-reclaim chiller and that the temperature of the hot water supplied to a heat load is controlled.
Understanding how to design a heat-reclaim system to meet the heating requirements of a building or process is important, as is specifying appropriate equipment when a controlled source of hot water is desired. Several chiller-plant configurations are capable of accomplishing these goals. Equipment that can provide a controlled source of recovered heat for hot water must be specified. On-board chiller controls can maintain hot-water-set-point temperatures without sacrificing chilled-water-plant efficiency. The combination of captured heat and optimum chiller-plant efficiency helps to minimize energy consumption and maximize LEED certification points.
ASHRAE. (2004). Standard 90.1-2004 User's Manual. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers.
Bahnfleth, W.P., & Peyer, E. (2004). Variable primary flow chilled water systems: Potential benefits and application issues. Arlington, VA: Air-Conditioning and Refrigeration Technology Institute. Available at http://www.arti-21cr.org/research/completed/finalreports/20070-final2.pdf
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One of Carrier Corp.'s green-building sustainability advocates and a resource for information on high-performance HVAC-system solutions, Brian Key, PE, LEED AP, has more than 20 years of experience in HVAC, including system design, product marketing, business management, software system application, and business development.