April 28, 2004

Green power plants harness the power of wasted energy

Photo courtesy International Sustainable Solutions Combined heat and power plants require some fundamental changes in thinking for traditional power plant designers.

In 2002, only 31 percent of the primary energy consumed for electricity production in the United States was ultimately delivered to end users. The remaining 69 percent was lost during the conversion process or transmission/distribution. That's equivalent to the energy of 4,700 million barrels of crude oil.

This waste dwarfs the 9 to 16 million barrels of oil estimated to be recoverable from the Arctic National Wildlife Refuge, often described as America's best opportunity for new energy reserves. The reality is that our greatest energy reserves lie in changing the basic processes that we use for energy production in our country.

In 1975, with the Arab oil embargo, Denmark faced its own energy crisis, as 93 percent of its energy was dependent on imported oil. The government embarked on an aggressive program of reforms, including promoting higher building efficiencies and the development of combined heat and power (CHP) plants.

Today 53 percent of the electricity in Denmark is produced in tandem with heat production.

The impact of these changes has been remarkable. Since 1990, adjusted gross energy consumption has increased by only 1 percent, despite a 30 percent increase in GDP during the same period. In addition, adjusted CO2 emissions have fallen almost 14 percent. This means that each unit of gross domestic product required 22.6 percent less energy and resulted in 34 percent less emissions of CO2 than in 1990.

Traditional vs. CHP

In a traditional thermal power plant 40 percent to 60 percent of the energy contained in the fuel is dispersed into the atmosphere or cooling water as "waste" heat; in CHP plants this waste heat is captured and used for heating, industrial processes and other production processes. As a result the overall plant efficiency can be increased to 90 percent or more. For example Helsingør, a 57-MW natural gas-fired combined-cycle plant feeding a small town north of Copenhagen, uses 88 percent of the fuel energy to produce electricity and heat.

Larger plants, such as the Avedøre 2 plant serving Copenhagen, operate at an efficiency of up to 94 percent, the highest level in the world. Though Avedøre 2 is capable of burning oil and natural gas, it primarily uses straw and wood pellets, and is equipped with a range of cutting-edge technology to reduce harmful emissions into the environment and meet Kyoto protocol targets to limit climate change.

The efficient utilization of these fuels through cogeneration benefits the environment by considerably reducing emissions of CO2, SO2 and Nox. Avedøre 2 generates 570 MW of electricity, meeting the needs of some 800,000 households, while providing the heat for approximately 110,000 homes.

In 2000 there were more than 678 CHP plants in Denmark, with a combined capacity of 10,231 MW. Of these, 15 were large-scale centralized CHP plants, 576 were small-scale decentralized plants and 87 were industrial plants.

The small-scale plants are on average about 3.5 MW, and are typically designed to meet the electricity and heating needs of a neighborhood or small town. The number of these plants has exploded over the past 20 years, largely due to government policies requiring all district heating systems to convert their gas boilers to cogeneration, in order to increase the overall efficiency of the energy systems and decrease CO2 production.

The plants are highly automated, normally running with a staff of only three to four people during the day shift, and can be easily operated from a centralized utility control center. They are architecturally designed to merge unobtrusively into a mixed-use or even a residential setting, and the sound of these plants is practically inaudible -- far quieter than the traffic noise from adjacent streets. In fact, were it not for the presence of an exhaust stack many of these plants would simply appear to be typical commercial buildings.

New design approach

Although the technologies behind CHP plants are well proven in Scandinavia and elsewhere, they do require some fundamental changes in thinking for traditional power plant designers.

First, for reasons of basic thermodynamic efficiency, the goal of power system engineers has historically been to maximize combustion temperatures, while releasing exhausts at the lowest possible temperature. For a CHP plant to produce useful heat as an intentional, marketable product, the output temperature must be elevated, making the electrical generation process slightly less efficient. However, this reduction in the efficiency of electricity generation is more than offset by the overall increase in plant efficiency gained by capturing and reusing the waste heat.

Secondly, the heating or cooling output of a CHP plant has economic value only if there is a readily accessible market for the output. In the United States this has been primarily restricted to heat intensive industrial processes or space heating of individual buildings, which greatly limits the potential opportunities. It is the widespread adoption of district energy systems, now providing heat for almost 60 percent of the heated floor space in Denmark, that has created a ready-made market for the output of these plants, allowing adoption of these more efficient designs to become much more widespread.

Because CHP plants are designed to deliver both hot water and electricity, even the larger, centralized facilities are located close to urban load centers. This runs counter to the common approach in the United States, where large generating plants are typically sited far from population centers. However, by reducing the losses due to long distance transmission and distribution, this philosophy further increases overall energy system efficiencies.