Solar Thermal Energy: More Than Just Hot Air
Monday, 04 October 2010 00:00  |  Written by Aaron Lada, Ph.D. | Article

The Power of the Sun by James RickwoodWhen outside on a hot day, it’s easy to appreciate the amount of energy that reaches our planet from the sun. Solar thermal energy (STE) technology, the most efficient form of solar power, utilizes this resource for a variety of applications. Clean, renewable and suitable for any climate, it’s no wonder that STE has grown into a major player in alternative energy. Here’s what you need to know to tap this free energy source for yourself.

STE vs. Other Solar Power
There are several ways to harness the energy from the sun. Passive solar strategies can be used to heat buildings, and photovoltaic solar cells contain semiconductors that directly convert the energy in sunlight into electricity. STE, on the other hand, captures heat from sunlight for use in heating swimming pools and domestic hot water, providing heating and cooling for buildings and generating electricity in large-scale power-plant applications. According to data in a report presented at the International Energy Agency Solar Heating and Cooling Programme in May 2010(pdf), STE is second only to wind power in its ability to meet global energy demand among renewable sources.

Types of STE devices
There are three main types of STE collectors:

  • Low Temperature - Consist of flat panels (also known as flat plates) that are mainly used to heat swimming pools.
  • Medium Temperature - Also use flat panels as solar collectors, but have a storage tank/heat exchanger.
  • High Temperature - A sophisticated network of mirrors or lenses that focuses sunlight on a central point to produce high temperatures used for electricity generation. This is specifically known as concentrated solar power, or CSP.

Homeowner Applications
STE can be used in any climate, yet should always have a traditional system available for backup. Locations with less sunlight would need more collectors, and colder climates need components designed for use in freezing temperatures.

The most common homeowner applications of STE are medium-temperature systems used to heat hot water and to provide space heating and cooling. Their components include a collector plate, heat-exchange material, storage tank and, depending on the system, a collection of pumps, valves and control units. There are several options for the collector, the simplest being the flat-plate collector consisting of a sealed box containing a dark absorptive material that heats up when the sun hits it. Evacuated tube collectors are more expensive, but more efficient in colder or cloudy conditions.

Inside the collector, a heat-exchange material, either a liquid or air, becomes warmed by the sun and then is delivered to a storage tank. Depending on the system, the tank stores the heated material for use, or the heat-exchange material transfers its heat to air or water for use. Several terms are used to describe the different designs of STE systems:

  • Passive – They rely on convection to move air or water through the system and so no pumps are needed.
  • Active - Pumps are used to circulate liquid or air through the system.
  • Direct - The water or air that will be used is directly heated in the collector.
  • Indirect - Another material, usually a liquid such as antifreeze, is circulated through the system and heated. It then transfers its heat to air or water inside the storage tank.

Hot-Water Heaters
STE hot-water heaters are capable of heating water to 140°F, can store a day’s supply of hot water and most can provide 100% of hot water in the summer, and 40-80% in the winter. Several types are available:

  • Direct-Passive Systems - The simplest of STE models, they have two basic designs. An Integral Collector Storage system combines the storage tank and the collector. A tank is enclosed in a box that will directly heat the water inside for use. Slightly more complex, the Thermosiphon system consists of a collector plate and storage tank that must be located above the collector—often both will be on the roof. As water heats up in the collector, it moves up into the storage tank by convection. Hot water for use is drawn off the top of the tank, and as water in the tank cools, it moves back down into the collector to be reheated. These systems can be damaged by freezing temperatures.
  • Active Systems – They employ a pump allowing the storage tank to be located inside the home and away from the collector. A direct-active system will pump the water through the collector and into the storage tank where it is available for use. Freezing temperatures will damage this system as well.
  • Indirect-Active Systems - This design moves antifreeze through the system. The fluid gets heated in the collector, returns to the storage tank and then transfers the heat to water. This setup is suitable for cold climates.

Space Heating
STE heating systems are capable of supplying 40-80% of heating needs, but are most economical when designed to provide 50% of annual heating demand. A larger system would simply be more expensive with the extra heat, which would be used only on the coldest days and wasted the rest of the year. Several types are available:

  • Direct Systems circulate air to the collectors to be warmed, and then to a storage tank. Indirect systems move antifreeze to the collector to be heated and back to the storage tank where the heat is transferred to air. In both, heated air from the storage tank is used to warm the building and can be supplemented by a traditional heating system if necessary.
  • Radiant Heating - Fluid heated in the collector is moved through a system consisting of a network of pipes installed below the finished flooring. The heat is transferred to the floor that warms the room as the heat rises.

Cooling With STE
As odd as it sounds, heat from the sun can be used for cooling as well. Popular in Asia, STE can be used to supplement, but not replace traditional air conditioners. They can run only when the sun is shining, yet that also coincides with when they are needed the most. Cooling is achieved in one of two ways:

  • Desiccant Evaporation - The heat collected removes moisture from the air, and as with any kind of evaporation, this has a cooling effect.
  • Absorptive Chiller - A refrigerant sealed inside a tube under pressure is heated.  When this pressure is released, it expands and cools the air around it.

Costs and Maintenance
STE systems are more expensive than traditional options, but can pay for themselves in five to ten years since the energy they use is free. The US Department Agriculture offers a handy calculator to help you quantify the potential savings from an STE system based on your location.

Solar water heaters are the most widely used form of STE in the US. Depending on location, total price will vary between $2,000-6,000 for a complete system, but they should save between 50-80% on water heating costs. Actual savings are influenced by location and the energy source being replaced. In addition, there may be state and federal incentives available to reduce your initial investment.

STE systems do require more maintenance than a traditional system. The antifreeze fluid should be replaced every three to five years, an electric component may need replacing every 10 years or so and the collector should be kept clean and free of vegetation and snow. A well-maintained system can be expected to last 30 years or more.

STE Power Plants
Another important use of solar thermal energy occurs in STE power plants. One design uses multiple linear rows of curved mirrors that track the sun’s movement and focus its rays to a central point on an attached fluid-filled tube. As this fluid heats up, steam is created that is used to power an electric generator. Another design uses a large, circular array of reflectors that focus sunlight on a central power tower. The intense heat in the tower is used to generate electricity either through steam-powered generators or turbines driven by the movement of the hot air. STE plants, which are more efficient extractors of solar energy than photovoltaic cell arrays, are currently in use in Spain, and several designs have been tested in the US.

STE systems are a form of renewable energy that can be used in virtually any location to provide heat, cooling and hot water. While the cost is more than fossil-fuel powered systems, they do pay off in energy savings and in the satisfaction of reducing one’s carbon footprint. No single renewable energy source can completely eliminate the demand for oil and coal, but capturing the vast power of the sun is an important piece in the alternative-fuel mosaic that may accomplish just that.

Additional resources:
A Guide to STE Hot Water Systems(pdf)
A Guide to STE Space Heaters(pdf)
US Department of Energy Guide to STE Space Heaters
US Department of Energy Guide to STE Water Heaters
Guide to STE Power Towers(pdf)
Database of Certified Solar Installers

Comments (1)add
Written by Caroline Webb , October 04, 2010
I need to point out that this article makes light of the fact that back-up from other power sources, eg coal, gas or nuclear are needed for this technology to be worth anyone spending money on. It is therefore NOT clean and renewable. It is dependent on power sources that do not fluctuate, the only CO2 clean one of which is nuclear power - of which there should be a great deal more in this world, as fast as possible.

Secondly, Big Solar Thermal power plants are about to rip up pristine desert areas in the Mojave and nearby which are major sinks for CO2 and HOME for hundreds of species, all of which have a right to exist and a right to habitat. Increasingly the 'green agenda' becomes tainted by a willingness either to fudge reality or sacrifice life on Earth. Soon it will be recognized but not soon enough for the living deserts of the Mojave. God help us. Roll on the day that other species can take humans to court to fight for their rights... THAT would be 'green'!
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