Get the most out of waste heat: Exergy analysis reveals the amount of useful work you can get from a warm exhaust stream.(Engineering Practice)

The process industries are extensive users of energy. But it is now clearly established that a large amount of this energy is wasted, essentially in the form of heat rejected to the environment through gaseous or liquid effluents. The temperature level of this heat makes it of no use by the process; and at the same time, in many cases, it cannot be economically upgraded. Examples of industries rejecting considerable thermal energy include: thermal power plants, metallurgical processing, the cement industry, pulp and paper, and petrochemicals. Table 1 gives typical exhaust temperatures for some such industries.

On the basis of the second law of thermodynamics, the quality of an energy source is determined by its ability to perform work. In the specialized literature, this work potential is referred to as availability, or more precisely "available energy" or exergy. From this standpoint, heat (thermal energy) is considered to be the most degraded form of energy.

A simplified approach to evaluate the exergy content of a hot waste stream is presented here. The objective is to provide engineers a means to easily estimate the amount of useful energy they may recover from industrial heat exhausts. Such energy can be used, for example, to drive a turbine for onsite production of electricity. Some of the new and emerging technologies for converting waste heat into work are also briefly described. Many of these technologies are already on the market or on the verge of commercialization.

The concept of exergy

Exergy is a thermodynamic property that was first introduced in the mid-1950s as a measure of the capability of a hot stream to produce useful work. Exergy is related to the well-known thermodynamic properties enthalpy and entropy through the equation:

Ex =(H-[H.sub.0]) - [T.sub.0] x (S-[S.sub.0]) (1)

where H is the specific enthalpy of the stream, S the specific entropy of the stream, and T is the temperature in Kelvins. The subscript 0 indicates a reference condition, which usually corresponds to the environment containing or surrounding the system. *

Several properties of exergy can be noted with the definition of Equation (1). The exergy content of the stream is a fraction of the enthalpy content of the stream. This enthalpy content alone does not give any indication of the potential to extract some work from the heat resource. Equation (1) also shows that a heat resource, regardless of its amount, carries no work potential (zero exergy) when it is at the same (thermal, mechanical and chemical) conditions as its surrounding. In other words, if a stream is in equilibrium with its surroundings, it can produce no work. In fact, this represents the main difference between exergy and Gibbs free energy which is a property of the stream, regardless of the surrounding environment.

Exergy variation between an initial state (subscript i) and a final state …