Burkland, Thomas (Duke University)

Harness the Energy of the Sun: Hybrid PV/T Systems

The United States has cheap, reliable electricity provided to the entire county. The system works now and has worked for years. Why change it? Why is renewable energy needed?

One, fossil fuels are running out.

There is plenty of coal, but coal is the dirtiest fuel source we use. Oil and natural gas are cleaner, only slightly but still better. However, the world's supplies are diminishing, quickly. Estimates suggest that the world will be completely out of oil and gas by 2032 (pdf). Should this crisis arrive before alternatives are implemented, economies and livelihoods will come to a crashing halt.

Two, renewable energy is cleaner.

Right now, about half of the electricity produced in the United States comes from the burning of coal. And most of that coal power is from old plants, built before and not included in the Clean Air Act of 1970. So while new coal burning power plants may be reasonably clean, most of them are not. They provide 59% of total sulfur dioxide pollution, 18% of the nitrous oxides, 50% of particulate pollution, 40% of carbon dioxide, and are the largest source of mercury, sulfur dioxide, and toxic air pollutants.

The pollution put out by these plants is staggering. However, its consequence is not just a dirty sky, but an unhealthy populace. These emissions, especially nitrogen oxide, react in the air with sunlight to form smog. A single coal plant can cause smog for hundreds of miles around it. It is estimated that 48% of the American population, 140,500,000 people, lives in areas harmfully affected by smog. The afflicted populace is at risk of shortness of breath, wheezing, chest pains, asthma, and lung inflammation. The same pollutants are also responsible for acid rain

The high output of carbon dioxide also contributes greatly to global warming. The higher temperatures lead to increased fuel use to cool buildings which in turn creates higher carbon dioxide output. This vicious cycle can only be broken with alternative forms of energy.

Air toxics and particulate pollution cause respiratory inflammation, irregular heartbeats, asthma, heart attacks, and strokes. Mercury causes blindness, mental retardation, and brain damage in unborn children. Each of these consequences alone is unacceptable, but when coal power plants are the source of all of them, how can we justify their use?

Solution: Solar Energy

The Sun is a more abundant source of energy than we could ever need. In three hours, the sunlight hitting the Earth contains enough energy to run the world for a year. It then simply becomes a matter of harnessing this potential for use.

The photovoltaic (PV) panel is a relatively old technology, having been first patented in 1946. However, the first panels efficient enough for consumer use were not developed until the 1970’s. 1980 saw the first 10% efficient solar cell and by 1983 there were only 21.3 Megawatts of PV power worldwide. Photovoltaic power has caught on and efficiency has improved slowly but steadily. By the end of the century, there were 1000 Megawatts of PV power installed worldwide and maximum efficiency had risen over 30%. The 21st century has seen huge increases in installation and efficiency of PV panels. This expansion has been in the form of both grid-source and private installations. For example, Pacific Gas and Electric Company has just added 553 MW of solar power to its system. Also, Google has recently installed 1.6 MW of solar power on its corporate headquarters. Maximum efficiency is now over 40% and even less expensive production panels are at about 18%.

However, electricity is not the only form of energy that the world needs or that solar energy can provide. The sun is the planet's primary heat source, so using it to provide homes with heat in the winter is a logical step. The best method to utilize the sun's heat is to use an absorbent surface to capture it and water to transfer it wherever needed. Today’s solar water heater technology involves a collector put in the sun, a liquid to carry the heat around, a pump, storage of the heat, and a heat transfer when potable water is not used. There are two main types of collector: flat panel, a insulated box that absorbs and traps the sun’s heat, and evacuated tube, three tubes inside one another that allow the sun’s heat in but not back out.

The flat panel collector is simple, reliable, and can be built by the consumer. It consists of an absorber surface with a cover that protects against radiation of heat back out, a fluid flowing through pipes to transfer the heat, and heat insulation.

The evacuated tube collector is the most efficient solar technology, minimizing the amount of infrared energy reflected and, because of its rounded shape, is always perpendicular to the sun, maximizing amount of energy collected. It’s made of a clear outer tube and an absorbent inner tube with a vacuum between for insulation (graphic). Inside the absorbent tube is a copper pipe that carries the heat transfer liquid. Small amounts of water flow through the array of pipes at line pressure and pick up the sun’s heat.

The heat provided by the collectors can be used simply as hot potable water or for more complex applications, such as radiant heating, or even to cool as part of an absorption chilling system.

As mentioned before, the majority of grid-connected power plants are old and burn coal. These plants do not have any heat-recovery process and are only about 33% efficient. The 6% of the grid that is made up of distributed energy facilities which can recycle heat have shown it to be marvelously effective, raising efficiency as high as 90%! The easy way to improve the efficiency of solar energy is to combine technologies with this same principle in mind, called hybrid photovoltaic/thermal (PV/T) systems.

PV panels become quickly less efficient as they heat up. Each degree Celsius above zero brings with it a half percent decreased efficiency (more). However, PV panels need to capture as much sunlight as possible to produce high quantities of electricity. Thus, the same energy that makes a photovoltaic panel useful also decreases its efficiency.

Mounting a PV panel to the front of a hot water collector increases the photovoltaics’ efficiency. The pipes of a flat panel collector carry cool water through the space behind the photovoltaic and pick up the heat generated by the panel and carry it away to be used in the rest of the building. Plus, since about 43% of all the energy that hits a PV panel is converted to heat, it does not detract from the efficiency of the solar collector on which it sits, so there can be substantial collection of heat.

The company leading the hybrid PV/T field, Solarwall, has done a few large-scale projects in the United States since 1987 and received universal acclaim for their product. However, these systems are geared toward air-circulation, not the capture of heat and do not use water. No American company makes PV/T systems that use water (pdf).

The water-cooled PV system, while nearly impossible to find in use, has been shown to work in tests. The tests conclude that:

-A cooled photovoltaic panel is more efficient than an uncooled panel and provides greater power

-Water is a better coolant than air

-After being used as a coolant, the warm water can be applied as preheated water to any other system in a building.

Complete, very detailed account of this test here (pdf).

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