Plants have also very nicely dealt with the greenhouse problem since the carbon released when they use that stored energy for respiration is the same carbon they extracted in the first place during photosynthesis. That is, they get energy with no net carbon emissions.

While there is already a sizable agricultural industry devoted to growing crops specifically for their energy content, it is almost entirely devoted to converting plant material into alcohol fuels for motor vehicles. On the other hand, biomass
for electricity production is essentially all waste residues from agricultural and forestry industries and, to some extent, municipal solid wastes.

Since it is based on wastes that must be disposed of anyway, biomass feed stocks for electricity production may have low-cost, no-cost, or even negative-cost advantages.

Currently there are about 14 GW of installed generation capacity powered by biomass in the world, with about half of that being in the United States. About two-thirds of the biomass power plants in the United States co generate both electricity and useful heat.

Virtually all biomass power plants operate on a conventional steam Rankine cycle. Since transporting their rather disbursed fuel sources over any great distances could be prohibitively expensive, biomass power plants tend to be small and located near their fuel source, so they aren’t able to take advantage of the economies of scale that go with large steam plants.

To offset the higher cost of smaller plants, lower-grade steel and other materials are often used, which requires lower operating temperatures and pressures and hence lower efficiencies. Moreover, biomass fuels tend to have high water content and are often wet when burned, which means that wasted energy goes up the stack as water vapor. The net result is that existing biomass plants tend to have rather low efficiencies—typically less than 20%. Even though the fuel may be very inexpensive, those low efficiencies translate to reasonably expensive electricity, which is currently around 9¢/kWh.

An alternative approach to building small, inefficient plants dedicated to biomass power production is to burn biomass along with coal in slightly modified, conventional steam-cycle power plants. Called co-firing, this method is an economical way to utilize biomass fuels in relatively efficient plants. And, since biomass burns cleaner than coal, overall emissions are correspondingly reduced in co-fired facilities.

Anaerobic digesters are fairly common in municipal wastewater treatment plants where their main purpose is to transform sewage sludge into innocuous, stabilized end products that can be easily disposed of in landfills or, sometimes, recycled as soil conditioners.

Anaerobic digesters can be used with other biomass feedstocks including food processing wastes, various agricultural wastes, municipal solid wastes, bagasse, and aquatic plants such as kelp and water hyacinth. When the biogas is treated to remove its sulfur, the resulting gas can be burned in reciprocating engines to produce electricity and usable waste heat.

The opening of the transmission and distribution grid to independent power producers who offer cheaper, more efficient,
smaller-scale plants is well underway. Attempts to restructure the regulatory side of utilities to help create competition among generators and allow customers to choose their source of power have been initiated in a number of states, but with
mixed success.

And, partly due to California’s deregulation crisis of 2000–2001, the customer’s side of the meter is being rediscovered and energy efficiency is enjoying a resurgence of attention.

On the customer side of the meter, the power business is beginning to look more like it did in the early part of the twentieth century when more than half of U.S. electricity was self-generated with small, isolated systems for direct use
by industrial firms. Many of those systems were located in the basements of buildings, which were heated by the waste heat from the power plants.

Those old steam-powered, engine generators used for heat and power have modern equivalents in the form of microturbines, fuel cells, internal-combustion engines, and small gas turbines. Using these technologies, customers are rediscovering the economic advantages of on-site cogeneration of heat and power, or tri-generation for heating, electric power, and cooling.

In addition to economic benefits, other motivations helping to drive the transition toward small-scale, decentralized energy systems include increased concern for environmental impacts of generation, most especially those related to climate change, increased concern for the vulnerability of our centralized energy systems to terrorist attacks, and increased demands for electricity reliability in the digital economy.

For comparison, some examples of power demands of typical end uses are also shown. While the power ratings of some of the distributed generation options may look trivially small, it is the potentially large numbers of replicated small units that will make their contribution significant.

For example, the U.S. auto industry builds around 6 million cars each year. If half of those were 60-kW fuel-cell vehicles, the combined generation capacity of 5-year’s worth of automobile production would be greater than the total installed
capacity of all U.S. power plants.

Below chart shows the relative capital expenditures on T&D over time compared with generation by U.S. investor-owned utilities. The most striking feature of the graph is the extraordinary period of power plant construction that lasted
from the early 1970s through the mid-1980s, driven largely by huge spending for nuclear power stations.

Except for that anomalous period, T&D construction has generally cost utilities more than they have spent on generation. In the latter half of the 1990s, T&D expenditures were roughly double that of generation, with most of that being spent on the distribution portion of T&D.

The utility grid system starts with transmission lines that carry large blocks of power, at voltages ranging from 161 kV to 765 kV, over relatively long distances from central generating stations toward major load centers. Lower voltage sub-transmission lines may carry it to distribution substations located closer to the loads. At substations, the voltage is lowered once again, to typically 4.16 to 24.94 kV and sent out over distribution feeders to customers.

Notice the combination of switches, circuit breakers, and fuses that protect key components and which allow different segments of the system to be isolated for maintenance or during emergency faults (short circuits) that may occur in the
system.

Steam power plants tend to be large, coal-fired units that operate best with fairly fixed loads. They tend to have high capital costs, largely driven by required emission controls, and low operating costs since they so often use low-cost boiler fuels such as coal. Once they have been purchased, they are cheap to operate so they usually are run more or less continuously. In contrast, gas turbines tend to be natural-gas-fired smaller units, which adjust quickly and easily to changing loads.

They have low capital costs and relatively high fuel costs, which means they are most cost-effective as peaking power plants that run only intermittently. Historically, both steam and gas-turbine plants have had similar efficiencies, typically
in the low 30% range.

A basic gas turbine driving a generator is shown below. In it, fresh air is drawn into a compressor where spinning rotor blades compress the air, elevating its temperature and pressure. This hot, compressed air is mixed with fuel, usually natural gas, though LPG, kerosene, landfill gas, or oil are sometimes used, and subsequently burned in the combustion chamber.

The hot exhaust gases are expanded in a turbine and released to the atmosphere. The compressor and turbine share a connecting shaft, so that a portion, typically more than half, of the rotational energy created by the spinning turbine is used to power the compressor.

Gas turbines have long been used in industrial applications and as such were designed strictly for stationary power systems. These industrial gas turbines tend to be large machines made with heavy, thick materials whose high thermal
capacitance and moment of inertia reduces their ability to adjust quickly to changing loads. They are available in a range of sizes from hundreds of kilowatts to hundreds of megawatts. For the smallest units they are only about 20% efficient, but for turbines over about 10 MW they tend to have efficiencies of around 30%.

Another style of gas turbine takes advantage of the billions of dollars of development work that went into designing lightweight, compact engines for jet aircraft. The thin, light, super-alloy materials used in these aero derivative turbines enable fast starts and quick acceleration, so they easily adjust to rapid load changes and numerous start-up/shut-down events. Their small size makes it easy to fabricate the complete unit in the factory and ship it to a site, thereby reducing field installation time and cost.
Aero-derivative turbines are available in sizes ranging from a few kilowatts up to about 50 MW. In their larger sizes, they
achieve efficiencies exceeding 40%.

Most peaking plants, which are brought on line as needed to cover the daily rise and fall of demand, are gas turbines based on the Brayton cycle. The newest generation of thermal power plants use both cycles and are called combined-cycle plants.

The basic steam cycle can be used with any source of heat, including combustion of fossil fuels, nuclear fission reactions, or concentrated sunlight onto a boiler. In the steam generator, fuel is burned in a firing chamber surrounded by a boiler
that transfers heat through metal tubing to the working fluid. Water circulating through the boiler is converted to high-pressure, high-temperature steam.

During this conversion of chemical to thermal energy, losses on the order of 10% occur due to incomplete combustion and loss of heat up the stack.

High-pressure steam is allowed to expand through a set of turbine wheels that spin the turbine and generator shaft. For simplicity, the turbine is shown as a single unit, but for increased efficiency it may actually consist of two or sometimes three turbines in which the exhaust steam from a higher pressure turbine is reheated and sent to a lower-pressure turbine, and so forth.

The generator and turbine share the same shaft allowing the generator to convert the rotational energy of the shaft into electrical power that goes out onto transmission lines for distribution. A well-designed turbine may have an efficiency approaching 90%, while the generator may have a conversion efficiency even higher than that.

The spent steam is drawn out of the last turbine stage by the partial vacuum created in the condenser as the cooled steam undergoes a phase change back to the liquid state. The condensed steam is then pumped back to the boiler to be reheated, completing the cycle.

The heat released when the steam condenses is transferred to cooling water, which circulates through the condenser. Usually, cooling water is drawn from a river, lake, or sea, heated in the condenser, and returned to that body of water,
in which case the process is called once-through cooling.

A more expensive approach, which has the dual advantages of requiring less water and avoiding the thermal pollution associated with warming up the receiving body of water, involves use of cooling towers that transfer the heat directly into the atmosphere. In either case, if we think of the power plant as a heat engine, it is the environment that acts as the heat sink so its temperature helps determine the overall efficiency of the power cycle.