Heat recovery chiller insights

• Heat recovery chillers can recover heat being rejected from the building. This process can aid in decarbonization.
• Decoupling ventilation air from the cooling and heating of a building’s rooms can also reduce fossil fuel use.

Water chillers remove heat from chilled water, which is used to cool a building or process. With a water-cooled chiller, the chiller transfers that heat via a refrigeration cycle from the chilled water to a separate condenser water loop, typically that heat is rejected from the condenser water loop to the outdoors via a cooling tower.

The valuable heat being rejected from the building could be recovered by a water cooled, heat recovery chiller (see Figure 1). This can substantially reduce the building’s carbon footprint by decreasing heating energy requirements, whether from a fossil fuel boiler or from an electric heating source. Cooling tower water makeup and chemical treatment is also reduced.

Figure 1: Water cooled heat recovery chiller. Courtesy: Peter Basso Associates Inc.

Operating a chiller in heat recovery mode requires more compressor energy than operating a water-cooled chiller with heat rejection to the outdoors via a condenser water loop and cooling tower.

For example, using the minimum efficiency requirements in the 2019 edition of ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings, a 150-300-ton water cooled chiller uses 0.462 kW/ton when cooling, based on the standard’s required integrated part load value. Operating a chiller with similar capacity in heat recovery mode using the minimum efficiency requirements in ASHRAE 90.1-2019 with a leaving heating water temperature of 120°F increases the chiller’s cooling energy use to 0.650 kW/ton.

However, a heat recovery chiller operating at this minimum efficiency is an extremely efficient heating machine, with an effective heating coefficient of performance of 22.2, when considering only the incremental compressor energy increase between operation in cooling-only mode and operation in heat recovery mode.

Three scenarios for application of a heat recovery chiller are highlighted.

First, during summer, many building heating, ventilation and air conditioning (HVAC) systems reject heat through a cooling tower, via a water-cooled chiller, at the same time fossil fuels are being consumed to generate heat for tempering cool air supplied for ventilation and/or dehumidifcation. The valuable heat being rejected by the cooling tower could instead be recovered via a water cooled, heat recovery chiller, assisting in the decarbonization of the building.

Second, during winter, many buildings have rooms that require cooling at the same time rooms along the exterior of the building require heating. The rooms that require cooling are typically located in the interior of the building and/or are rooms with high heat gains from lights and equipment.

The most common building HVAC systems address the cooling needs of interior or high heat gain rooms in winter by supplying them with cool winter outdoor air, which is then exhausted to the outdoors. While the valuable heat in the warm exhaust air can often be recovered to reduce the heating requirements of the incoming outdoor makeup air, a more effective approach may be to decouple the ventilation air supply from the cooling and heating of the rooms. This allows heat to be removed directly from the rooms which need cooling, via a sensible cooling device such as a chilled beam and then that heat can be transferred to the heating system by a heat recovery chiller and used to heat exterior rooms that need it, thus reducing fossil fuel use.

Third, buildings in cold climates that have high indoor relative humidity requirements will significantly reduce humidification energy use by limiting their outside air intake to the amount required for ventilation, instead of incorporating an airside economizer cycle. This is because an airside economizer cycle will take in a large volume of outside air during moderate winter conditions, often far in excess of the amount required for ventilating most buildings, which must be humidified to the indoor humidity level.

The elimination of the airside economizer cycle typically requires the chilled water system to provide chilled water throughout the winter. A common method to accomplish this without running a chiller is to employ a waterside economizer system in which the cooling tower fan(s) are operated during winter to produce cooling tower water, which is colder than the required temperature of the chilled water. The cold cooling tower water is then used to produce chilled water via an intermediary heat exchanger. Valuable heat being rejected by the cooling tower could instead be recovered via a heat recovery chiller, which transfers the heat from the chilled water to the building’s heating hot water system used to heat the building.

While it may seem somewhat counterintuitive to energize the much larger electrical load of a heat recovery chiller instead of a cooling tower fan for winter chilled water production, because the heat recovery chiller can transfer in the range of 4-7 times its compressor energy into the heating hot water and also capture its own compressor energy as heat, the heat recovery chiller is effective at both reducing carbon footprint and saving energy cost in nearly all cases.

Design considerations for heat recovery chillers

There are several important design considerations when incorporating a heat recovery chiller into a building HVAC system:

Heating hot water system temperature is a significant and limiting factor for the application of water-cooled heat recovery chillers. While there are chillers that can produce hotter water, for practical purposes the heating hot water temperature should be limited to no more than 140°F and lower when possible. This is because the efficiency of the heat recovery chiller is affected greatly by the “lift” at which the machine is operating, with lift being governed by the temperature difference between the cold fluid leaving the chiller’s evaporator and the hot fluid leaving the chiller’s condenser. In a heat recovery chiller, the fluid leaving the chiller’s condenser is heating hot water.

For new construction and renovations in a northern climate, we target the HVAC system’s hydronic heating devices to provide the maximum required capacity at no greater than 120°F heating hot water supply temperature. This typically requires more rows and larger face area in airside heating coils than when hotter water temperatures are used.

Reduced heating hot water supply temperature limits its use for indirectly heating domestic hot water to preheat duty, requiring booster heaters for domestic hot water in most cases.

For existing buildings with heating hot water supply temperatures greater than 140°F, a heat recovery chiller is generally impractical until the heating system is retrofitted to work with a lower water temperature. We plan to discuss design considerations for the application of low temperature heating hot water systems in new and retrofit applications in a future article.

Chilled water supply temperature should also be kept as warm as possible to minimize the required chiller lift, thus improving heat recovery chiller efficiency. This is a parallel consideration to keeping heating hot water supply temperature as low as possible when using a heat recovery water chiller. This means chilled water temperature should automatically be reset by the HVAC control system to be as warm as possible, especially when dehumidification is not required in winter conditions.

The use of chilled beams as sensible-only cooling devices in rooms provides an ideal system for the application of a heat recovery water chiller. With a chilled beam system, chilled water temperatures must be kept warmer than the room dewpoint temperature, thus also reducing the required lift of the heat recovery chiller. Because most or all room heat gain in both summer and winter is removed by the chilled beam into the chilled water, the amount of recovered heat is optimized. The heat recovery chiller moves the heat from the chilled water circulated through the chilled beams into the heating hot water, which can then be supplied to heating devices.

Concurrent heating and cooling demands: For a heat recovery chiller to function, the HVAC system into which the heat recovery chiller is applied must have concurrent chilled water and heating hot water demands. The heat recovery chiller moves heat from its evaporator (chilled water) to its condenser (heating hot water, see Figure 1). The heat recovery chiller’s compressor heat is also rejected through the condenser and into the heating hot water.

The cooling provided by the heat recovery chiller is limited by the concurrent heating load on the heating hot water system. If the heating load is less than the cooling load served by the chiller, plus the chiller’s compressor heat, then the chiller’s operating capacity must be lowered to match the load. Similarly, the heating provided by the heat recovery chiller is limited by the concurrent cooling load on the chilled water system; if the cooling load is less than the heating load served by the chiller, minus the chiller’s compressor heat, then the chiller’s operating capacity must be lowered to match the load.

To maximize the fossil fuel reduction achieved by the heat recovery chiller by means of its heating output, the chiller can be piped to load it before loading other cooling-only chillers. A side stream chilled water piping arrangement for this is indicated in Figure 2. With this piping arrangement, when the cooling load exceeds the cooling capacity of the heat recovery chiller, the cooling-only chillers must maintain the system leaving chilled water temperature.

Figure 2: Heat recovery chiller side-stream piping arrangement. Courtesy: Peter Basso Associates Inc.

There are other possible piping arrangements for incorporating a heat recovery chiller with cooling-only chillers — this side stream arrangement minimizes the lift on the heat recovery chiller and allows it to be fully loaded when the cooling-only chillers are not.

If there is potential for inadequate heating demand as needed to balance the heat recovery chiller’s chilled water production, it can sometimes be beneficial to add heat rejection capability to the heating hot water system that can be activated when needed (making sure that other heat sources on the heating hot water system, such as boilers, have already been turned off).

Among the ways to accomplish this is by adding a heat exchanger that transfers heat out of the heating hot water loop and into the cooling tower loop, where it can be rejected by the cooling tower (or by the geothermal ground loop, if one exists).

It is important to consider that a heat recovery chiller typically consumes more power, due to its greater lift, than a cooling-only chiller that is rejecting heat to a cooling tower. Because of this, the amount of heat being rejected to the outdoors from the heating hot water system should not be allowed to exceed the tipping point where the beneficial heat being recovered into the heating hot water system is less than the increase in compressor energy between the heat recovery chiller and the cooling-only chiller.

Careful consideration should be given to concurrent heating and cooling load profiles to avoid oversizing heat recovery chillers.

Another approach for systems with inconsistent heating demands would be to provide smaller increments of heat recovery and cooling-only chiller capacity, allowing them to be staged as heat recovery and cooling-only demands vary. An example applying specialized modular heat recovery chillers that are available on the on the market, is described in the case study: Retrofit requires modular chiller with heat recovery.

Sizing of cooling-only chillers and supplemental heating sources

When heat recovery chillers are incorporated into an HVAC system, their cooling and heating capacities can be factored in when determining the required capacity of other cooling-only chillers and heating boilers, but important considerations must be taken, including:

• If heating hot water temperature is seasonally adjusted, the heat recovery chiller’s capacity must be derated to the capacity it can provide at the heating hot water temperature for which the boiler capacity was selected.

• There could be an imbalance between heating and cooling loads at certain times, as described above, which could limit the heat recovery chiller’s cooling and/or heating capacity at the time the cooling-only chillers and/or boilers are required to provide their maximum capacity.

Chillers should no longer be considered only as machines that remove heat from the building for it to be rejected to the outdoors. Heat recovery chillers can be employed to capture that heat, reducing the building’s carbon footprint. Designing the building heating system to work with low heating water supply temperatures, designing the building cooling system to effectively capture internal heat gains and careful evaluation of concurrent cooling and heating load profiles are among the complexities that need to be considered when employing heat recovery chillers.

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