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Active solar thermal systems use solar collectors to collect the sun’s energy to heat water, another fluid, or air. The heart of a solar collector is an absorber that converts the sun’s energy into heat. The heat is then transferred by circulating water, antifreeze, or sometimes air to another location for immediate use or storage for later use. Applications for active solar thermal energy include providing hot water, heating swimming pools, space heating, and preheating air in both residential and commercial buildings.

Definition

Active solar thermal systems use solar collectors to collect the sun’s energy to heat water, another fluid, or air. The heart of a solar collector is an absorber that converts the sun’s energy into heat. The heat is then transferred by circulating water, antifreeze, or sometimes air to another location for immediate use or storage for later use. Applications for active solar thermal energy include providing hot water, heating swimming pools, space heating, and preheating air in both residential and commercial buildings.

Solar Collectors

A standard solar hot water system is typically composed of solar collectors, a heat storage vessel, piping, circulators, and controls. Solar radiation is absorbed by the collector, and the heat collected is commonly used to heat or preheat water or air. Systems are typically integrated to work alongside a conventional heating system that provides heat when solar resources are not sufficient. The solar collectors are usually placed on the roof of the building, oriented south, and tilted around the site’s latitude, so as to maximize the amount of radiation collected on a yearly basis.

Although there are several options for using active solar thermal systems for space heating, the most common method involves using glazed collectors to heat a liquid held in a storage tank (similar to an active solar hot water system). Heat from the tank is transferred to radiators, a radiant floor heating system, or through a heating coil in a forced-air system.

Solar-heated water can also be used to preheat outdoor air drawn into a building. Other techniques directly preheat fresh air with solar air collectors. This is used more in commercial buildings and industrial applications. Whereas some solar air collectors are quite sophisticated, one simple system of this type uses a darkly colored, perforated aluminum sheet mounted on a south-facing wall. As the sun heats the sheet, a fan draws outdoor air through the perforations and warms the air. This preheating can significantly reduce energy use by conventional heaters to bring fresh air up to room temperature.

Use / Application

To determine the financial feasibility of an active solar system, conduct a life-cycle cost analysis of at least a 10-year projected life of the system. The life-cycle analysis can determine what the upfront and operational costs and expected energy savings would be compared with conventional systems.

Determine whether the climate and building usage is appropriate for an active solar system. The energy savings for active solar thermal systems depend on the amount of available solar radiation, projected uses of the system, and proper system design.

The results of this analysis can determine the financial feasibility of investment in an active solar system.

An active solar collection system can provide the following benefits:

·        Offsetting the heating energy from conventional systems over the life of the building and/or the life of the system

·        Hot water energy savings given that the demand for hot water is fairly constant throughout the year

·        A cost-effective way to extend the yearly use of your pool

·        Overall reduction of fossil fuel use and emission of ozone-depleting gases

Solar Collector Diagram (click to enlarge)

Solar Service Water

Solar service water systems are often used in residential applications to heat water for basic household needs, such as laundry, bathing, dishwashing, and cooking. In larger buildings, such systems can be especially cost effective where there are large, year-round hot water loads (e.g., laundromats, restaurants, hospitals, car washes, etc.). Hot water systems typically use solar energy to preheat incoming water from wells or water mains. If solar energy is not sufficient to heat the water to the desired temperatures, a conventional water heater is used to make up the difference. The warmer the water from the solar heater, the less conventional fuel will be needed.

Pool Heating

Pool Heating

Simple solar pool heaters work by circulating pool water directly through collectors and then rerouting the heated water to the pool. System controllers sense when collectors are warmer than the pool water and open valves to divert water from the pool circulator through the collectors and then back into the pool. Collectors for these simple pool heating systems are generally unglazed and quite inexpensive. The systems are not freeze-proof, so they are used in mild climates or during summer months when there is little chance for freezing. Heating indoor pools in colder climates is also an excellent use of solar energy, but these systems require more expensive collectors and rigorous freeze protection.

Transpired Collector Components (NREL)

Ventilation Preheat

In a typical ventilation solar preheat system, fresh air is drawn across a heat absorbing south-facing wall or other form of solar collector. The preheated air is drawn into a building’s primary heating system where it is further heated, then distributed throughout the building. A solar air heating system augments a conventional heating system rather than replaces it because the system is primarily used to preheat ventilation air for commercial and industrial facilities, such as factories, warehouses, and hangars. Because the air going into the building’s primary heating system is already warmer than the outside air, less energy is needed to heat it further.

Space Heating

The chosen solar heating system needs to be compatible and interactive with conventional HVAC systems in the building. Heat-delivery methods that require lower fluid temperatures, such as radiant floor heating or some forced-air heating systems, can often take more advantage of solar heat than high-temperature devices (such as many radiators or convectors). Designers should also be very sensitive to the control of solar heating systems. Solar energy should be used whenever available, but backup heating should be integrated seamlessly to optimize efficiency and comfort.

The ever-increasing worldwide demand for energy and the increasing understanding of the link between fossil fuels and the global environment will continue to drive active solar thermal system technology as a viable and important tool in whole-building system design.

Nevada Solar One

Increased use of active solar thermal systems is occurring in the United States, the European Union, Japan, and China. In the United States, many states now offer sales tax exemptions, income tax credits or deductions, and property tax exemptions or deductions for active solar thermal systems.

The use of active solar thermal systems to convert heat to electricity is also growing. One such project taking place in the United States is using solar receivers to convert energy from the sun into electricity by using concentrated solar radiation from parabolic mirrors. The temperature of an oil fluid flowing through receivers is thereby heated to over 750° F. This heated fluid is then used to turn water into steam, which drives a turbine and generates electricity. Once this project is completed, it is estimated that it will result in a reduction of greenhouse gases equivalent to removing approximately 1 million cars from the nation’s highways.

Use an Integrated Approach

A new way of thinking must be adopted in order to meet the goal of reducing carbon emissions associated with buildings.  Your solutions can begin by integrating four possible methods.  None works alone, and they are not all relevant in considering every strategy.  However, considering the following tactics is necessary:

·         Reduce the overall energy use in your building.

§         By reducing the overall energy use, various building systems (i.e., HVAC) can be reduced in size and cost to make way for further energy-saving materials, designs, and technologies.

·         Specify energy-efficient equipment and technologies.

§         Use an integrated system approach to spec out the most cost-effective, energy-efficient equipment and technologies that can include active solar thermal systems.

·         Use renewable strategies and purchase green power.

§         Use present solar income as a part of a whole-house design strategy.

§         Use active solar thermal systems to decrease energy loads and offset emissions of conventional, built energy systems.

·         Educate building owners, operators, and occupants.

§         Provide information on function and operations of installed technology.

Resources

Associated Strategies

All 50to50 strategies relate to each other in some way. However, we recommend that you consider investigating these selected 50to50 strategies to assist you in gaining a deeper understanding.

• Photovoltaics
• Building Orientation
• Life Cycle Assessment
• Daylighting
• Renewable Energy Resources
• Sun Shading