August 30, 2013

Heat Pumps - 1

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.

Therefore I am putting some basics for this useful device which can be utilized very effectively.

1. Introduction 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 replaces 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.

2. Why can a heat pump save money? Heat pumps use waste heat that would otherwise be rejected to the environment; they increase temperature to a more effective level. Heat pumps can deliver heat for less money than the cost of fuel. Therefore, the cost of fuel of different types is very important in the selection of heat pumps.

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. See figure below.

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.

3. 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:

a. Receipt of heat from the waste-heat source.

b. Increase of the waste-heat temperature.

c. Delivery of the useful heat at the elevated temperature.

One of the more common heat pump types, the mechanical heat pump, will be used to show how these functions work. Below is given a picture of typical system for energy saving.

Waste heat is delivered to the heat-pump evaporator in which the heat-pump working fluid is vaporized. The compressor increases the pressure of the working fluid, which in turn increases the condensing temperature. The working fluid condenses in the condenser, delivering high-temperature heat to the process stream that is being heated.

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.

For example, if natural gas costs $3.00/ (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 consumed.

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:

a. The heat output is at a temperature where it can replace purchased energy such as boiler steam or gas firing.

b. The cost of energy to operate the heat pump is less than the value of the energy saved.

c. The net operating cost savings (reduction in purchased energy minus operating cost) is sufficient to pay back the capital investment in an acceptable time period.

In the next post we will discuss about different type of heat pumps with specific variations.

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Global Energy Systems said...

Your blog seems very informative, Thanks for sharing this kind of information. Your blog was very helpful for me. Keep further posting.

Heat Pumps

gk said...

Very helpful information for me..thanks a lot

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