Cogeneration—Efficient Use of Energy
Cogeneration Plants are power plants that generate electricity and also collect the low-grade thermal energy (material temperatures less than roughly 80 to 90°C) that is produced during the electrical generation process. If the thermal energy is made available for use in domestic, commercial, or industrial heating and drying applications, the effective efficiency of the generating facility is greatly increased. Cogeneration plants are only useful if they can be deployed near the energy markets they serve, because low-grade thermal energy is difficult to transport over long distances.
In general, the ability to utilize the low-grade thermal energy produced by a power plant means that the technology used for generating the electricity must be scalable, that is, plant efficiency must be independent of system size. If the power-generating technology is scalable, power plants can be sized for the energy markets they serve and located close to those markets, whether the market is small, like an individual farm or business, or larger, like a residential neighborhood or an industrial park, or even a portion of a large city. When power plants are close to the markets they serve, the thermal energy produced can be effectively utilized.
Solar-cell-based power plants are scalable; their efficiency is pretty nearly independent of system size. The interconnected solar cells that power a hand calculator have roughly the same efficiency as an industrial-scale array of solar panels. Coal, gas, and nuclear power plants are not scalable. If they were built as units with generating capacities of a only few megawatts, as opposed to hundreds of megawatts or gigawatts, capital cost per unit of power generated would rise significantly and overall plant efficiency would fall. Scalability is a big advantage that solar-cell-based systems have over other power generating technologies because scalability enables cogeneration.
It’s worth looking at the different ways we generate and use energy, especially electrical energy, and it’s worth going back in time to see how we got to where we are today. When we do that, we can see how to make the best use of solar energy and and we can see why it’s so important to make solar an essential part of our energy infrastructure. But first, let’s make sure we’re on the same page by explaining a few terms.
Primary Energy Resources are supplies of energy that exist in nature. They can be renewable resources, like wind, solar, or hydropower; or they can be non-renewable, like coal, petroleum, natural gas, or nuclear. For the most part, we don’t use primary energy resources directly. We usually change the form of the energy provided by primary resources before we actually use it.
Energy Conversion Systems are the machines and processes that act on primary energy resources to produce the forms of energy that we use every day in our homes, in our businesses and industries, and in our vehicles. Examples of energy conversion systems would include petroleum refineries that process crude oil, electrical power stations that burn coal or natural gas, or wind turbine systems that capture the power of the wind.
The forms of energy that energy conversion systems produce by altering or modifying energy derived from primary energy resources are sometimes called Secondary Energy Resources, although we usually refer to them as Energy Carriers. In our modern societies, the most important energy carriers are electricity and the various types of fuels produced from crude oil and natural gas, although thermal energy could also be on the list because we burn large quantities of high-quality fossil fuels to provide heat and hot water for our homes and businesses and to provide process heat for a variety of industrial and agricultural tasks.
The energy flow chart below shows US Primary Energy Resources on the left and Energy End Use Categories on the right. The secondary resource streams flowing to the end use boxes are the energy carriers that drive the US economy. The box labeled “Electrical Generation” is representative of the energy conversion systems (power plants) that act on different primary energy resources to produce electricity, our most important energy carrier. Not represented on the chart are the petroleum refineries and natural gas processing plants that turn crude oil and raw natural gas into high-quality fuels and chemical feed stocks.
When taken together, the petroleum resource—used mainly to produce fuels for the transportation sector — and the energy resources consumed in generating electricity, account for nearly 75% of all primary energy resource usage in the US. The striking thing about the transportation and electricity generation sectors is that, in these sectors, roughly 75% of the primary energy is rejected. The reason for this is that nearly all of the mechanical energy that powers our transportation vehicles and electrical generators is produced by Heat Engines—and the efficiencies of heat engines are limited by the fundamental laws of thermodynamics. Automotive engines, for example, typically have efficiencies in the range of only 20 to 30%. Steam-driven electrical generating systems have slightly higher efficiencies, usually in the range of 30 to 35 per cent. Some Brayton/Rankine combined cycle electrical generating systems claim efficiencies in the range of 50 to 55 per cent.
For any energy conversion system, including heat engines, only a portion of the input energy is converted to useful output; the rest is rejected, usually as low-grade thermal energy. For the heat engines discussed above, anywhere from 50 to 80% of the input energy is converted to low-grade thermal energy. This seems like an unfortunate but necessary loss until we look inside the end use boxes on the right side of the flow chart.
The energy charts below give a detailed breakdown of residential end-use applications in the United States for 2016. The charts show that a large portion of the energy we use is for low-grade thermal applications. (Similar but somewhat smaller thermal energy percentages exist for the commercial and industrial sectors.) The low-grade thermal energy we currently use is now obtained either directly by burning high-grade fossil fuels or indirectly by using electricity that is generated from fossil fuels. This energy could easily be provided by the low-grade “waste heat” that is normally discarded from electrical generating plants—if those plants were scalable and located in close proximity to the energy markets served. In particular, the low-grade thermal energy collected and stored by SEA’s scalable cogeneration systems could be used to meet these low-grade end use applications. Doing so would significantly reduce the quantity of fossil fuels that we consume and it would greatly decrease the amount of electricity that we generate. (In our homes and businesses, more than 40% of the electricity we use and more than 70% of the total energy we use, is for low-grade thermal applications and low-grade heat transfer operations. That’s an unnecessary and unacceptable waste of high-end forms of energy.)
How did our energy infrastructure evolve to allow this type of waste and, more importantly, what can be done to change the present situation? The answer to these questions can be found back at the very beginning of the electric power industry.
The first central electrical generating plant in the US was a coal-fired facility built in lower Manhattan in 1882 by the Edison Illuminating Company. This plant, known as the Pearl Street Station, provided Direct Current (DC) electrical power for “electric lamps” in a one-square-mile area immediately surrounding the plant. The steam-driven “dynamos” generating the electricity were fairly reliable in meeting customer needs, but the plant’s relatively low-voltage DC output limited the distances over which power could be economically transmitted. The development of practical Alternating Current (AC) transformers by William Stanley in 1885 set a new direction for the electric power industry. The Pearl Street Station was obsolete even before it was shut down in 1895.
However, Edison’s plant did one very important thing that was lost as the electrical industry grew and developed. The Pearl Street Station was the world’s first cogeneration plant. Since the plant was located right in the heart of the energy market it served, waste heat from the engines that drove the dynamos could be circulated in the form of steam to nearby buildings for space heating and manufacturing operations.
The new AC technology, as developed by Stanley and his boss George Westinghouse, enabled the generation of higher voltages, which in turn made it possible to efficiently transmit electrical energy over longer distances. Larger, more efficient coal-fired power plants were built to realize economies of scale and consequently, the customers served were far from the plants generating their power. That made cogeneration impossible because the thermal energy rejected by the plants’ heat engines could not be efficiently transmitted over long distances. As a result, for more than a century, we’ve used only about 30 to 35% of the input energy that we supply to the heat engines that power our electrical generating plants. The rest of the energy is lost, rejected as low-grade thermal energy.
But now that’s all changing. SEA has created a practical cogeneration system that converts more than 90% of incident direct sunlight into electricity and valuable low-grade thermal energy. Since the SEA energy conversion technology is completely scalable, our systems can be sized for, and located in close proximity to, the energy markets they serve. This means that, instead of discarding low-grade thermal energy as waste heat, it can be used in a wide variety of end-use applications that currently rely on high-end fossil fuels or electricity derived from fossil fuels. Nothing is wasted when SEA cogeneration systems are used to convert sunlight into the types of energy we need.
And we’ve gone one step further. We’ve developed technology for season-to-season storage of both the electricity and the thermal energy that our SEA systems produce, so it’s now possible to cost-effectively bridge the longest temporal variations of the solar resource. Our systems will produce a complete decentralization of our energy infrastructure, creating millions of jobs and other important socio-economic benefits at the local level.