complete solar energy system

complete solar energy system

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Correct sizing of solar water heating systems is both a science and an art, developed over the last 40 years by scientists and engineers.

There are three main steps involved in performing such an analysis:

1) Determine the monthly energy requirement for the application. This energy requirement is often referred to as the "load".

2) Determine the contribution of a solar system to satisfy a portion of the load. The contribution is also known as the "solar fraction".

3) Determine the economic value, such as rate of return or payback time.

Solar hot water calculations are performed on a per month basis. The inputs are monthly loads and weather data including solar radiation, and solar collector specifications.

The goal of step number one, determining the load, is to accurately estimate the monthly energy required to heat water. It is calculated from the average gallons of hot water required per month, and the rise in temperature from the cold water mains to the output temperature of the system. The number of gallons per day can be arrived at using ASHRAE hot water consumption data for various types of facilities. Often times, more complex calculations known as "forensic physics" are employed. The best method is to place a flow meter in the hot water line for a few weeks. Non-invasive ultrasonic meters are quick to install and remove but not as accurate as meters placed directly in the line.

The next step of determining the solar contribution involves calculating the amount of solar energy supplied to the load. First, the monthly solar radiation data and the solar collector performance numbers are obtained. Then the monthly load, the radiation, collector parameters, and system size numbers are combined to produce the final result.

Monthly horizontal surface radiation tables come from satellite data provided by NASA and posted on NREL, DOE, and NOAA websites. This data includes monthly radiation on a flat surface, averaged over a 30 year period.

The horizontal surface radiation is translated into the radiation on a tilted collector using the industry standard F-Chart solar equations developed in the 1970s by Duffie and Beckman at the University of Wisconsin. The Canadian government supported the development of a free version called RETScreen, which is used all over the world. All F-Chart programs use the equations from Duffie and Beckman.

The collector parameters that are inputs to the F-Chart program come from tests done at independent laboratories certified by the Solar Rating and Certification Corporation (SRCC). The Florida Solar Energy Center is the foremost of these. Collectors are tested to two standards:ASHREA 93 for performance, and ISO 9806 for durability, thermal shock, etc. Collector testing yields two numbers that define a straight line efficiency curve. The Y-intercept is the absolute maximum efficiency of the collector at ambient temperature, and the slope of the curve represents the drop in efficiency as the temperature increases.

Inserting the load, collector performance numbers, collector area, and storage gallons into the F-Chart program produces a table of energy values. The columns show the monthly values for weather, input loads, solar contribution, solar fraction of total load and energy output per collector.

No assessment of solar hot water would be complete without addressing the financial considerations. The results from the solar calculations are used to determine the economic justification of a solar water heating system. As the size of a solar system increases, the total solar energy goes up, but the energy output per collector goes down. For example, the output of a 32 ft2 collector for different solar fractions is shown here:

Solar Fraction -> Output per Collector

30% -> 8.0 million Btu/yr

80% ->6.5 million Btu/yr

99% -> 4.0 million Bty/yr

The optimum economic value is the acceptable rate of return, or payback time, on the system. The value is a moving target influenced by current and future conventional energy prices, available financial incentives, and the owner's tax status and internal investment rules.

In general, sizing solar water heating systems to supply 30-60% of the total annual load results in the best economic value.

Dr. Ben Gravely is widely recognized as one of the country's foremost authorities on solar hot water systems. With more than 1500 residential and commercial systems to his credit, Ben has a unique knowledge base specific to solar thermal design, installation, and energy output which he shares through http://www.solarhotwater-systems.com. To gain insight and assistance with a solar water heating project, sign up for Ben's blog!

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