Thermodynamics extended the "no free lunch" principle to "some of
the energy is unavailable to do work."
Here we discuss in the context of power utilities:
All electricity utilities have this cycle
Heat from coal, oil, gas or
U-235 creates steam
that drives turbine.
Typical efficiency is 1/3.
3000 MW plant yields
1000 MWe electricity &
2000 MW heat at Tcold.
What happens to this heat?
Must dispose of heat at Tcold: efficiency of turbine generating electricity depends on it! Only two ways:
Neither is without problems, but "dry" or "hybrid" towers seem to raise the least concerns.
Electricity utilities had long used natural bodies of water -- lakes, rivers or coasts -- to cool discharged heat. With nuclear power, the same approach was used, as long as such sources were available.
This external water is recirculated through a heat exchanger, never in contact with water in closed loop of generator. The only question is heat's effect.
Altho returned water is only 5-10 C higher, it was taken from lower non-circulating layer & returned to upper layer in a stratified lake ⇒ 2 adverse effects.
1. Heating the upper layer decreases the amount of dissolved ("entrained") oxygen, essential to chemical processes that clean the water and sustain fish. Changing the water's temperature and oxygen content affects relative stability of fish.
2. The change in the temperature stratification of the lake can seriously affect the lake and its inhabitants.
Most suitable bodies of water have been used. Cooling towers are increasingly used. There are two kinds: "wet" or "dry" (the latter includes "hybrid"). The "wet" are cheaper but slightly affect local climate. The EPA has largely made it impossible to build these anymore.
"Dry" cooling towers use fans to move the air at greater cost. They work better than "wet" in most climates.
My summary. As anticipated growth in electricity plants occurs, "dry" towers seem to pose few problems. ... but
Chemical industry widely uses cooling towers to remove heat from products and processes. Increasingly, such cooling is moving from one-time use of water to recirculating. This decreases the heat added to the source of water but not to the environment. Instead the heat goes into the atmosphere.
In contrast to power plants -- where it seems the cooling cycle does not come into contact with combustion products -- leaks from heat exchanges into the cooling water systems raises concern:
The most common environmental problem is hydrocarbons leaking into the cooling water from "aging" heat exchanges. This problem is characterized in terms of biological oxygen demand (BOD) and chemical oxygen demand (COD) -- the amount of organic and non-organic materials present as determined by a specific test. Examples include:
It does not seem these cooling towers are covered by Clean Air Acts -- discussed in the next lecture.
But in this century, chemical industry sees regulations coming & recognizes standard EPA protocols are ineffective in assessing air emissions.
Differing treatments illustrates the difference between regulated utilities and companies under regulations for which monitoring has not been developed.
Building anything new requires Environmental Impact Statement: so onerous as to undermine their effectiveness. Indeed Congress is making efforts to cut them back, not in a very thoughtful way. Regardless of the politics, there are valid concerns.
These concerns focus most dramatically for proposals to building several power plants at one location for any of several reasons:
Pollution -- ever looming topic not mentioned today -- is next.