Dedicated Heat Recovery
By Thomas H. Durkin, P.E., Member ASHRAE, and James B. (Burt) Rishel, P.E., Fellow/Life Member ASHRAE
The following article was published in ASHRAE Journal, October 2003. © Copyright 2003 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paper form without the permission of ASHRAE.
The advent of the small scroll or screw chiller, capable of producing condenser water as high as 140°F (60°C), created an opportunity for recovering heat from a dedicated heat recovery chiller’s condenser water circuit for heating or domestic water systems while providing beneficial cooling for the chilled water system. These systems are called “dedicated” heat recovery because 100% of the heat generated by the dedicated heat recovery chiller (DHRC) can be used for hot water heating applications. Also, the DHRC can be piped and controlled to produce the desired evaporator or condenser temperature. Transfer of the recovered heat in this article is limited to clean water applications, such as preheating, heating, reheating, domestic, pool water heating, or snow melting. The ability to recover heat from a chiller’s condenser has been improved due to the development of small tonnage compressors that can operate at high discharge pressures and be controlled from either the condenser or evaporator water temperatures.
Some installations prove that more condenser heat is needed than originally planned. Modular units that can be expanded are a solution to such applications.
The heat recovery chiller introduces an efficient answer to simultaneous heating and cooling loads. Three ways to accomplish this are:
1. Run boilers and chillers,
2. Run boilers and operate air-side economizers, and
3. Run a heat recovery chiller.
Using a particular project’s energy rates, operating boilers and chillers cost $0.87 per 100 MBH (29.31 kW); boiler and economizers cost $0.59 per 100 MBH (29.31 kW), while the heat recovery chiller required only $0.38 per 100 MBH (29.31 kW). This demonstrated that operating the DHRC required only 43.7% of that for running boilers and the chiller, and 64.4% for running boilers and air-side economizer simultaneously for that installation. (Note: To develop the greatest efficiency, air-side economizer operation must be coordinated with the operation of the DHRC. This is very simple. Whenever the chilled water temperature is at 55°F (12.7°C) or lower, the air-side economizers are “locked out” and minimum outside air is provided. Whenever the chilled water supply temperature is above 55°F (12.7°C), the air-side economizers are allowed to operate normally.)
Uses of Recovered Heat
Recovered heat can be used in domestic water systems, airhandling unit preheat or heating coils, or VAV reheat coils. The ability to adjust condenser water temperatures to fit any of these heat recovery applications requires a chiller separate from the main chillers in the chiller plant for greatest potential efficiency. The combination a DHRC, operating at elevated condenser water temperatures, and main, high-efficiency chillers, operating at the most efficient condenser water temperature allowed by ambient conditions, allows beneficial loading of the DHRC to serve heating loads, while the remainder of the cooling load is served by the more efficient main chillers. This combination results in a substantial improvement in the coefficient of performance (COP) for the entire chiller installation, compared to operating the main chillers at an elevated condenser water temperature with only a portion being used for beneficial heating.
When Can Heat Recovery be Used?
Recovered heat can be used when hot water and chilled water are employed simultaneously (i.e., chilled water used in the winter and hot water used in the summer). The same is true for facilities with sufficient internal loads that are served by water-side economizers. Also, installations that use a sizeable amount of domestic water throughout the year may be candidates for condenser water heat reclamation. Hospitals dormitories, computer centers, and hotels are opportunities for economical heat recovery due to their needs for hot water for reheat and domestic use, coupled with air-side economizer operation, or in some cases, winter operation of chillers. This is a requirement of ANSI/ASHRAE/IESNA Standard 90.1-2001, Energy Standard for Buildings Except Low-Rise Residential Buildings, for large facilities that operate continuously with large water heating and cooling loads.
Other applications include:
1. Existing air-cooled chillers. The installation of the DHRC provides a marked improvement in the overall chiller energy consumption whenever a corresponding requirement exists for hot water in the building (especially during winter operation). Further, when properly sized, the DHRC can eliminate the need to run the larger air-cooled chiller during the winter months, thus eliminating the associated operational problems of running air-cooled systems under low loads and low and low ambient temperature conditions (a typical application would be pool dehumidification systems).
2. Installations using condensing type boilers for heating with relatively low hot water temperatures such as 130°F (54°C) supply water and 100°F (37.8°C) return water. These applications increase the transfer of the heat from the condenser to the hot water. Non-condensing boilers must run with at least 140°F (60°C) water temperature and at 80% to 85% efficiency compared to condensing boiler efficiencies above 90%.
3. Installations with water-side economizing. Much of the same equipment can be used for condenser heat recovery.
Dedicated Heat Recovery Sizing
As indicated earlier, the economical application of the DHRC is dependent on a simultaneous need for hot water (at 130°F [54°C] or less) and chilled water. Therefore, size the system to provide for the maximum hours of simultaneous operation to achieve fastest payback.
Most customers who have applied these systems have expressed the desire to shut down their primary chilled water system during winter operation. If this is the case, size the DHRC for the maximum winter chilled water load. In theory, for applications in northern climates, the wintertime heating load will far exceed the winter cooling load. If this is the case, the DHRC will operate throughout the winter season to provide the facility’s chilled water requirements and simultaneously provide hot water to supplement and unload the facility’s main boiler system.
Similarly, some customers want to shut down their hot water systems in the summertime. This is especially the case when a large hot water boiler system is operated to provide a relatively small summer reheat requirement that could otherwise be satisfied by the relatively low hot water temperature (130°F to 140°F [54°C to 60°C]) of the DHRC. In this situation, the DHRC’s heating capacity should be matched to the facility’s maximum summer reheat load. For such applications, the DHRC will operate throughout the summer to provide the facility’s heating and domestic water requirements and simultaneously provide chilled water to supplement, and unload, the main chiller system.
For installations that eliminate boiler operation in the summer and chiller operation in the winter, the payback for the installation of a DHRC system can be less than one year, depending on the local cost of electricity and natural gas. When sizing this system, compare the maximum winter chilled water load and the maximum summer hot water load. Select the DHRC to serve the larger of these two loads
Instrumentation and Heat Balance
The basic instrumentation for the heat recovery chiller is shown in Figure 1. This includes the temperatures in and out of the evaporator and condenser, flows in both of them, and the kW input to the chiller. The heat balance or COP for the heat recovery chiller can be computed by determining the cooling effect plus the heat recovered divided by the kW input in Btu/h. This assumes no heat is rejected to a cooling tower or closed circuit cooler.
For the COP for the chiller and the full article please see PDF enclosed.