Heat Recovery System Performance

As with heat pumps, the energy available to a heat recovery system consists of the refrigeration (or cooling) effect, plus the driving energy input. In English units, when driver energy for a vapor compression system is measured in hp, the heat available from a system can be expressed as:

ha = [12,000 Btu/tan-hcoolmgeffect +

hp/ton x 2,545 Btu/hp-hcompraswn] x tons capacity

When driver energy is measured in kW, the equation becomes:

ha = [l2,000 Btu/ton-hcoolingeffect +

(kW/ton x 3,413 Btu/kWh)compresiwn] x tons capacity

In SI units, this can be expressed as:

(kWhm/kWhr)COmpremJ x kWr capacity

In this equation, subscript h refers to the heat energy rejected and subscript m refers to mechanical energy input. It is understood that the mechanical energy input is converted to heat as a result of the compression process.

Total system performance includes the cooling benefit of the refrigeration effect and the heating benefit of the refrigeration effect plus the driving energy input. Combined, the total useful energy output usually reflects a very efficient use of the energy input. However, a principle disadvantage is the chiller capacity and efficiency penalty associated with the higher pressure differential against which the compressor must work. This particularly affects summer energy consumption, because the compressor speed is optimized for the high-head duty through selection of gear ratio and/or impeller duty. Annual performance is increasingly degraded by increasing elevated leaving condenser hot water temperature. This penalty increases as the hot water temperatures produced increase and may exceed 25% of the total chiller energy input. As a result, the energy saved by utilizing heat recovery is partially offset by the additional energy required per unit of cooling output.

Table 36-2 provides a representative comparison of two heat recovery system design set points. In winter, leaving condenser temperature is 100°F (38°C) in one unit and 105°F (41°C) in the other. In both cases, through high gear ratio, leaving condenser water temperature is 95°F (35°C) in summer.

This example is based on a constant leaving chilled water temperature of 44°F (7°C). In addition to minimizing leaving condenser water temperature, chilled water reset can be used to minimize performance degradation. This is accomplished by allowing the leaving chilled water temperature to be raised to the maximum temperature that will still satisfy the cooling load. This reduces peak compressor pressure requirements during winter operation (or cooler ambient temperature periods) when lower humidity levels and cooling capacity requirements allow for elevated (or reset) chilled water temperatures.

The economic performance of heat recovery systems is a function of the achievable net energy cost savings and the cost of the heat recovery equipment and any design modifications necessary for the distribution and use of relatively low-temperature hot water. As with any low-temperature heat recovery system, capital costs are negatively impacted by the need to increase distribution system size. If, for example, the hot water is used for space heating, the capital cost penalty includes larger size pipes, pumps, and coils. Pump energy use is also increased. Selection of distribution temperature must balance these factors with the desire to minimize leaving condenser water temperatures to minimize the system performance penalty.

A key operating factor to system economic performance is the concurrence of heating and cooling loads. When there is insufficient cooling load to support the heating requirement, a false cooling load must be applied to the chiller in order to generate the required heat rejection, rendering the system highly inefficient. An alternative to this operating mode is the provision of heat from a supplemental source. When operating in cooling only mode, with a lower pressure differential, the chillers can

Operating Mode

Leaving CWT (°F)

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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