|Industrial Heat Pumps for Steam and Fuel Savings|
|Written by USDOE Office of Energy Efficiency and Renewable Energy|
|Tuesday, 22 June 2010 12:32|
(This is an excerpt of Industrial Heat Pumps for Steam and Fuel Savings: A BestPractices Steam Technical Brief, available from the DOE Information Bridge)
Industrial Heat Pumps for Steam and Fuel Savings
U.S. Department of Energy, Energy Efficiency and Renewable Energy
Industrial heat pumps are a class of active heat-recovery equipment that allows the temperature of a waste-heat stream to be increased to a higher, more useful temperature. Consequently, heat pumps can facilitate energy savings when conventional passive-heat recovery is not possible.
The purpose of this Steam Technical Brief is to introduce heat-pump technology and its application in industrial processes. The focus is on the most common applications, with guidelines for initial identification and evaluation of the opportunities being provided.
1.0 INTRODUCTION TO HEAT PUMPS
A heat pump is a device that can increase the temperature of a waste-heat source to a temperature where the waste heat becomes useful. The waste heat can then replace purchased energy and reduce energy costs.
However, the increase in temperature is not achieved without cost. A heat pump requires an external mechanical- or thermal-energy source. The goal is to design a system in which the benefits of using the heat-pumped waste heat exceed the cost of driving the heat pump.
Several heat-pump types exist; some require external mechanical work and some require external thermal energy. For the purpose of discussing basic heat-pump characteristics, this brief will first introduce the mechanical variety, and then address the thermal types.
1.1 Why can a heat pump save money?
Heat pumps use waste heat that would otherwise be rejected to the environment; they increase air temperature to a more effective level. Heat pumps can deliver heat for less money than the cost of fuel.
Heat pumps operate on a thermodynamic principle known as the Carnot Cycle. To aid understanding of this cycle, it is helpful to contrast the Carnot Cycle with the more familiar thermodynamic cycle that underlies the operation of steam turbines, the Rankine Cycle.
Degrading high-grade thermal energy into lower-grade thermal energy creates shaft work, or power, in the Rankine Cycle. In a steam turbine, this is accomplished by supplying high-pressure steam and exhausting lower-pressure steam.
In contrast, mechanical heat pumps operate in the opposite manner. They convert lower-temperature waste heat into useful, higher-temperature heat, while consuming shaft work (Figure 1.1).
The work required to drive a heat pump depends on how much the temperature of the waste heat is increased; in contrast, a steam turbine produces increasing amounts of work as the pressure range over which it operates increases.
Heat pumps consume energy to increase the temperature of waste heat and ultimately reduce the use of purchased steam or fuel. Consequently, the economic value of purchasing a heat pump depends on the relative costs of the energy types that are consumed and saved.
1.2 How does a heat pump work, and how much energy can it save?
Several types of heat pumps exist, but all heat pumps perform the same three basic functions:
One of the more common heat pump types, the mechanical heat pump, will be used to show how these functions work (Figure 1.2).
A key parameter influencing the savings that a heat pump achieves is the temperature lift realized in the heat pump. Temperature lift is the difference between the evaporator and condenser temperatures.
Figure 1.3 illustrates how the cost of heat delivered by an electric-motor-driven mechanical heat pump depends on the cost of electric power and on the temperature lift that the heat pump achieves.
For example, if natural gas costs $3.00/million British thermal units (MMBtu), the cost of delivering heat from fuel at 80% efficiency will be $3.75/MMBtu. Figure 1.3 shows that the effective cost of heat supplied by the heat pump is lower than the cost of purchased fuel that otherwise would be con- sumed. However, this advantage erodes as the temperature lift increases, because more work is required to obtain the higher lifts. Also, because electricity is the work source for this heat pump, lower power costs result in greater benefits.
Under the right circumstances, a heat pump can reduce energy costs and provide an attractive cost-reduction project, particularly when:
For industrial applications, simple paybacks of 2 to 5 years are typical. Different types of heat pumps accomplish the three basic heat-pump functions in different ways, but in all cases the goal is the same: recover waste heat, increase its temperature, and deliver it at a higher, more useful, temperature for a reduced cost compared to the alternative. The common variants are described below.
1.3 Common types of industrial heat pumps
A brief description of the most common types of heat pumps and their key operating principles is provided below.
Closed-Cycle Mechanical Heat Pumps use mechanical compression of a working fluid to achieve temperature lift. The working fluid is typically a common refrigerant. Most common mechanical drives are suitable for heat-pump use; examples include electric motors, steam turbines, combustion engines, and combustion turbines.
Open-Cycle Mechanical Vapor Compression (MVC) Heat Pumps use a mechanical compressor to increase the pressure of waste vapor. Typically used in evaporators, the working fluid is water vapor. MVC heat pumps are considered to be open cycle because the working fluid is a process stream. Most common mechanical drives are suitable for heat-pump use; examples include electric motors, steam turbines, combustion engines, and combustion turbines.
Open-Cycle Thermocompression Heat Pumps use energy in high-pressure motive steam to increase the pressure of waste vapor using a jet-ejector device. Typically used in evaporators, the working fluid is steam. As with the MVC Heat Pump, thermocompression heat pumps are open cycle.
Closed-Cycle Absorption Heat Pumps use a two-component working fluid and the principles of boiling-point elevation and heat of absorption to achieve temperature lift and to deliver heat at higher temperatures. The operating principle is the same as that used in steam-heated absorption chillers that use a Lithium Bromide/water mixture as their working fluid.
Key features of absorption systems are that they can deliver a much higher temperature lift than the other systems, their energy performance does not decline steeply at higher temperature lift, and they can be customized for combined heating and cooling applications.
Four heat exchangers—an evaporator, condenser, generator, and absorber—are found in a typical absorption heat pump (Figure 1.4). High-temperature prime energy (steam or fuel) is supplied to the desorber, where vapor is boiled out of the working fluid at high pressure. The high-pressure vapor is condensed in the condenser, where the heat is recovered into a process stream. High-pressure condensate from the condenser is throttled to a lower pressure in the evaporator, where the waste heat is recovered to vaporize the low-pressure condensate. In the absorber, concentrated working fluid from the desorber contacts low-pressure vapor from the evaporator, creating heat that is recovered into a process stream. The working fluid returns to the desorber to complete the cycle.
In a typical absorption heat-pumping application, waste heat at low temperature is delivered to the evaporator, and prime heat at high temperature is delivered to the generator. An amount of heat equivalent to the sum of the high- and low-temperature heat inputs can be recovered at an intermediate temperature via the condenser and absorber. This is analogous to the thermocompression heat pump, in which high-pressure steam is used to increase or lift low-pressure waste vapor to a higher pressure and temperature. However, in the case of the high-lift absorption heat pump, the temperature lift can be 200 to 300° F, rather than the 20 to 50° F of the thermocompression system.
An important variation of the Type-1 Absorption Heat Pump is obtained by selecting operating parameters so that the device effects chilling at the ‘cold-end’ of the cycle while delivering hot water. The ability to provide simultaneous cooling and heating provides additional benefits over a ‘heating- only’ heat pump and improves the economics of an installation.
An alternate configuration for an absorption heat pump allows a medium-temperature waste-heat stream to split into one higher-temperature stream and one lower-temperature stream. Adjusting the operating pressures and working-fluid concentrations accomplishes this reconfiguration.
Figure 1.5 illustrates the energy balances for Type-1 and Type-2 absorption systems.
For the rest of Industrial Heat Pumps for Steam and Fuel Savings: A BestPractices Steam Technical Brief, visit the DOE Information Bridge