What is Solar Energy?
Most of us think of solar energy as sunlight, and that is correct. In addition to the portion of solar radiation that the human eye can see as sunlight, the spectrum includes other forms of energy including ultraviolet light and infrared radiation. We use solar energy either as light energy, which can also be turned into electric current by what are called photovoltaic cells, or as heat, also called thermal energy.
How Much Solar Energy is Available?
This is one of those questions that is best answered as "it depends." If a satellite outside the Earth’s atmosphere were to aim a flat plate at a right angle to the sunlight coming toward it, the amount of solar energy striking that flat surface would be 1,367 Watts per square meter (abbreviated W/m2), if you measure it as a light energy source.
This number is called the Solar Constant and is a measure of the amount of power available from the sun (745.7 W = 1 horsepower). If this energy is considered in terms of heat content, the Solar Constant can be stated as 442 British thermal Units (Btu) per square foot per hour (or 4773.6 Btu/m2/hr since one square meter contains 10.8 square feet).
While a vast amount of solar energy strikes the Earth’s atmosphere, not nearly so much can actually be captured for use at the Earth’s surface. Once solar radiation starts passing through Earth’s atmosphere, much of it is either scattered as it bounces off air molecules, water and dust, or it is absorbed into ozone, water and carbon dioxide molecules. This absorption of solar energy by molecules in the atmosphere is why we feel cooler when a cloud comes overhead and casts a shadow.
Provided below are tables that show how much solar energy is available for use, on average, in Missouri. While data for four different locations are provided, the fact is that very small differences in actual values exist from one location to another. This information can be used to calculate an estimate of how well your energy needs can be met using solar energy. We have included some explanatory information at the end of each table to clarify the use of that data.
The data provided for Columbia (16K), Missouri are called primary data, which means that actual solar insolation data was collected in the immediate Columbia area. Kansas City (15K), St. Louis (15K) and Springfield (16K) data sets are secondary data sets, meaning the values for these three towns were developed by extrapolating the data from Columbia, based on certain weather data values, such as cloud cover, that were different for these three towns versus Columbia’s conditions. The data presented in these tables comes from the Solar Radiation Data Manual for Flat-Plate and Concentrating Collectors, 30-Year Average of Monthly Solar Radiation, 1961-1990.
How Can Solar Energy Be Used in Missouri?
When deciding whether to use any energy source, three components need to be considered: the energy source itself, the reason energy is needed, and the technology that will best match the available energy source to the needs of the user.
As mentioned earlier, solar power can be captured either as light, electricity or as thermal power (heat). Discussions of solar energy systems are often divided into the categories of passive systems and active systems. In the case of solar water heating systems, the additional categories of direct and indirect are also used. However, before we describe solar energy systems further, we need to consider the importance of energy efficiency.
Most people consider using solar energy to improve the quality of their life and the environment, reduce the amount of fossil fuels they use, and possibly save a bit of money. When deciding to use solar, or any other renewable energy source, a key precursor to success is increasing your energy efficiency.
Efficiency saves you money in two ways. First, measures such as installing insulation, reduced-flow shower heads, and compact fluorescent lights save you energy and money even if you don’t change energy sources – you use less energy, so you pay less on your utility bill. Second, energy efficiency saves you money by decreasing the amount of energy you need to live comfortably and conveniently, allowing you to decrease the size and cost of the solar energy system you need to buy.
To read more about energy efficiency in your home, please visit our Consumer Tips page.
The most direct way to use the energy of the sun is to design buildings to allow as much sunlight to come into as much of the building, reducing the need for electric lighting. Windows, skylights, and light tubes can all bring free sunlight into your home or business.
Light tubes act as skylights with a slight difference. A skylight lets light in through a glazed opening (an opening covered with framed glass or other clear material) in the ceiling directly above the room being lit. A light tube uses a similar glazed opening on the roof, but it allows sunlight to be delivered to a room not directly beneath the glazed opening. Light tubes are flexible tubes lined with highly reflective foil that run from the roof glazing to the ceiling of the room. Light tubes are used when people don’t want a skylight opening on the roof on the front of the house or when pipes or other elements in an attic space make construction of a traditional skylight impossible.
Several studies have found that using more natural daylight not only reduces utility bills, but also improves people’s dispositions and reduces absenteeism, definite benefits to both families and employers. When increasing the amount of natural light entering a building, be careful to use quality products and ensure proper installation of the glazing. The location, size and type of window materials are also important considerations and relate to other aspects of passive use of solar energy.
Passive Solar Building Design
Passive solar building design is a combination of energy-efficient design features: proper use of daylighting, landscaping, and thermal mass; and correct orientation of the building so that the long dimension of the structure runs as close to east-to-west as possible. Keeping the long side of the building facing within 15 degrees of due south not only allows maximum heat gain during the winter, it helps avoid heat gain in the summer.
Since this page intends to provide an overview of most solar energy technologies, we won’t go into great detail on passive solar building design at this time. However, remember these things:
- Because of reduced utility costs, a well-designed and well-built passive solar building may cost a little bit more to build, but it costs a lot less to own over several years than a seemingly less-expensive conventional building.
- Passive solar buildings do not have to look "strange." They can be built out of the same materials as those used in conventional buildings. A rule of thumb is that the window area on the south side of a solar building should equal about 10 percent of the floor area. On a solar building, the designer places more of the windows on the south side and fewer windows on the other sides.
- Sometimes existing buildings can be retrofitted to become fairly effective users of passive solar energy. When considering such a project, develop a list of possible passive solar and energy-efficiency changes to the building. Then weigh each change based on criteria including cost, payback, visual appeal, and local building codes before making your decision.
Photo provided by National Renewable Energy Laboratory
The book Passive Solar Design Strategies: Guidelines for Home Building, developed by the Sustainable Buildings Industry Council (SBIC), is an excellent resource for further study of passive solar design techniques. It provides a good introduction to a software package called "BuilderGuide, Energy Analysis Software for Homebuilders," which was developed by the U.S. Department of Energy's National Renewable Energy Laboratory. SBIC also has developed an advanced energy simulation program for commercial buildings called "Energy-10." Information on these programs and training in their use can be obtained through the Division of Energy.
North Carolina State University’s Solar Center web site carries a significant amount of basic information on solar building design. This page contains numerous fact sheets dealing with a wide variety of passive and active solar system information. On the same site, you can find information on how to obtain several different solar home plans, some for free, to get an idea of the variety of good passive solar home designs.
Passive Solar Water Heating
Figures in this page provided courtesy of the Florida Solar Energy Center.
All solar water-heating systems employ a collector area and a storage tank. Usually collectors consist of an insulated box containing a winding array of water pipes attached to metal sheeting or fins that have been painted flat black to absorb solar radiation. Most collectors have a cover of either tempered glass or plastic to better contain the solar energy. The main use of a solar water heater without a cover glass is to heat water for swimming pools. These low-temperature collectors usually do not heat the water above 90 degrees Fahrenheit, which is sufficient for swimming pools.
MID-TEMPERATURE SOLAR COLLECTOR (up to 1600 F)
Passive systems do not use a pump to circulate water from the collector to storage or other locations. They employ three means: gravity, the tendency of hot water to rise above cold water and water pressure.
Passive solar water-heating systems can be categorized as either direct or indirect. A direct passive solar water-heating system is the simplest. The crudest form of direct passive solar water heating is to paint a water storage tank black to absorb heat into the water. Such systems were employed in the past at summer campgrounds where facilities were mostly used during warm, sunny periods of the year. These systems must be drained dry at the end of the season to avoid damage from freezing.
Care must be taken with such a system to avoid scalding injuries since the water in the tank can reach very high temperatures on sunny days. In addition, these systems can hold only as much water as the tank holds. That means that if everyone wants to take a shower at the same time at the end of the day, some folks may end up taking cold showers. Referred to as Integral Collector Storage systems, modern versions of these systems often enclose the storage tank(s) in an insulated box with one or more layers of glazing to let the sun in without letting the heat out as readily.
Next in complexity are passive direct systems that store water in a tank that is separate from the collector area. The term ‘direct’ means the water to be used by building occupants is run directly through the solar collector. One type is called a Thermosiphon System. A box-type collector heats the water, and the storage tank is positioned higher than the collector so that convection draws the heated water up into the storage tank without using a pump.
For more information in solar collectors and heating applications, please visit the U.S. Department of Energy's Solar Heating Basics page.
Active Solar Water Heating
In active solar water-heating systems, pumps circulate water from the storage tank to the collector. Passive systems do not use a pump. The diagram below shows the essential elements of an active system, but there are many variations available. In Missouri, the most important variation is the difference between direct and indirect systems.
In a direct system, the water to be used by the building occupants runs directly through the solar collectors. In an indirect system, either water or another heat-conducting liquid runs through the collectors and then passes through a heat exchanger to heat the water used by building occupants. A heat exchanger requires more piping than shown in the diagram above. Depending on the operating conditions, the cold water delivery pipe would be positioned to allow water to pass either through the heat exchanger loop or directly to the storage tank.
In Missouri, any solar water-heating system designed for year-round use should be an indirect system using a heat-transfer fluid that contains an anti-freeze agent such as non-toxic propylene glycol so that damage from freezing can be avoided. Heat-transfer fluid needs to be changed every three to five years to avoid system failure.
Excessive heat buildup also can harm a system. Controllers are usually solid-state devices that direct the pumps in a solar water-heating system to operate in a manner that optimizes the transfer of heat from the collector to storage to avoid dangerous levels of heat buildup in the collector. Operation of the heating element in backup systems is sometimes also tied into the system's controller.
Heat exchangers in solar water-heating systems resemble a pipe or pipes within a larger pipe. The pipe containing the water to be heated passes through a larger pipe that is flowing the fluid from the collector panel, allowing the heat of the sun to transfer into the water. Most collectors used for water heating are of the mid-temperature type as shown in the box-shaped collector diagram above. Certain industrial and institutional applications of solar water heating use ‘evacuated tube’ collectors. Welding a heat-collecting metal plate to a water pipe and placing it inside a vacuum tube allows the equipment to heat water to a temperature of 3500F. Cost trade-offs must be considered when choosing one type of system over the other.
Pumps used in these systems are small, between 1/100 to 1/12 horsepower, because they move a relatively small amount of liquid through a closed loop. Some manufacturers now provide photovoltaic panels that attach to their solar water-heater collectors to power these pumps. This practice works well because little power is needed to operate the pumps, and the pumps should not need to run when the sun is not shining.
In Missouri, you should connect your solar water heater to a backup water-heating system for use on overly cloudy days, or when your hot water use exceeds your storage capacity. To be most cost-effective, a solar water-heating system should be sized to provide about 60 to 80 percent of your total annual hot water needs.
Whether positioning solar collectors for passive or active solar water-heating systems, the collectors should be angled at no more than 45 degrees east or west of due south. If the angle away from due south is kept to less than 30 degrees, the collector will still capture about 90 percent of the maximum solar energy available. If your system is only going to be used during the summer, it should be tilted at an angle equal to the latitude of your site minus 15 degrees. For example, Jefferson City, Missouri, lies at 38.5 degrees above the equator. To optimize summertime collection of solar heat, a collector should be tilted at 23.5 degrees above horizontal to catch the most energy while the sun is at its high summer angle in the sky.
If your solar water heater is for year-round use, the collector should be tilted at latitude plus 15 degrees. This allows the capture of maximum sunlight during the cold winter months when the sun is low in the sky. If you are going to mount a solar water-heating collector on a pitched roof on your house, the roof angle is usually close enough to an optimal angle to simply mount the collector flush with the surface of the roof. Sometimes additional collector area is needed to capture sufficient solar energy when mounting collectors parallel to the existing roof.
Transpired Air Collectors
Fairly simple in concept and construction, transpired air collectors preheat ventilation air. A dark-colored, corrugated-metal facade covers the south-facing wall(s) of a building. The metal is perforated with thousands of regularly spaced and specifically sized holes. The sun shines on the dark metal (which can be blue, red, black or other dark colors) and heats the metal, which warms the air next to it.
The heated air flows upward between the metal sheeting and the building wall and is captured in a chamber at the roof. A fan pulls this preheated air into the heating system. Since the air has been preheated, less fossil fuel is needed to provide the total heat for the building. These systems even work at night to capture heat that is conducted out through the building wall after the sun goes down.
Well-suited to gymnasiums, warehouses, garages, manufacturing plants and other such buildings, the technology is most cost-effective when there is sufficient south-, southwest- or southeast-facing wall surface; when relatively high ventilation is required; and when the heating season is long. Researchers are studying whether these systems can dry grain effectively.
- For more information on this technology, visit this U.S. Department of Energy Web page.
Photovoltaic (PV) cells are solid-state semiconductor devices (similar to computer chips) that convert light energy directly into electric current. Small PV cells are found on calculators and wrist watches, and larger ones are used to power such devices as lights on buoys in ocean waterways, yard lights, whole-house electric systems, and camel-back refrigerator systems that carry precious vaccines into remote desert communities.
Several different technologies exist for producing PV cells, and continuing research is developing more. Currently, three types - single crystalline, multi-crystalline and thin-film amorphous - all use silicon as the main component in the cell material. Single crystalline cells are round because of the way the crystals are manufactured. Multi-crystalline cells are usually square, while thin-film amorphous cells are made in sheets because, rather than forming solid crystals, the silicon is deposited as a film on backing materials.
Because costs and efficiencies for solar cells are constantly changing, getting current information about them requires you to contact manufacturers and vendors. In general, crystalline and multi-crystalline cells have higher efficiencies and higher prices. Many of these companies offer web sites, including the American Solar Energy Society, the Interstate Renewable Energy Council and Solar Energy Industries Association.
While PV cells used for small appliances like calculators are sometimes less than one square inch in size, solar cells used for larger scale applications are usually about 4 inches wide and either square or circular in shape. Groups of these cells can be wired together, attached to a backing material, and placed into frames with a tempered glass cover. These framed groups of cells are called "modules," and vary in size depending on the amount of power needed. A group of modules can be wired together to form an array. Depending on the voltage and amperage desired for a given application, cells and modules can be wired in a variety of combinations.
PV System Components
PV cells produce direct current (DC) electricity, which is the type of current in automobiles and flashlights. Our homes, offices, and factories use alternating current, or AC, electricity. Using DC electricity requires different wiring, plugs and operation than AC. Also, PV cells obviously produce no electricity unless light is shining on them. To deal with these issues, as well as some safety issues, a complete solar electric system must have, in addition to the PV modules, what are called balance of system (BOS) components:
- Inverters convert DC electricity to AC and ‘condition’ that electricity to make sure it is compatible with the needs of the devices being served.
- Batteries store electricity for use at times when system load exceeds the amount of electricity being provided from the array.
- Battery Charge Regulators or Controllers make sure that batteries are not overcharged (overcharging can lead to explosions in extreme cases) and are not excessively discharged. Charge regulators come in varying degrees of complexity; some models simply display a warning light when discharge is too great, while other models can be programmed to drop certain non-critical loads when necessary.
- Auxiliary Battery Chargers are backup electric generators such as a gas or diesel engine, windmill, or small hydro-powered generator. They are used to keep battery packs fully charged to provide direct electricity to users during periods of low solar delivery or if electric load demands beyond those the system was designed to address. Sometimes electricity directly from the utility distribution system serves these purposes.
- Distribution and Safety Hardware includes wiring, outlets (DC outlets are different than AC outlets), conduits, connectors, switches, fuses, breakers, and surge protectors.
These devices are all needed to complete the installation of a PV system. Even if you are installing the system on a building that is already wired for AC electricity, you will need some amount of these components.
Types of PV Systems
PV systems fall into three categories:
- Stand Alone systems are those in which the PV system is the only source of electricity. They can vary in size, depending on whether they power a wristwatch, a fence charger, a water pump, or something as large as a whole house electrical system.
- Hybrid Systems usually use PV for a majority of the electric generation, but also have a backup generator, such as an internal combustion engine, to supply power in periods of extended low-sun conditions.
- Grid or Utility Intertied Systems use PV for a portion of their electric generation but remain hooked up to the local electric utility lines for backup electricity. In many cases, utility intertied systems do not have any battery backup. Instead, they send any excess electricity generated by their PV system to the utility for use by other customers. Grid intertied systems require close cooperation with the utility to ensure both the safety of utility-line workers and the PV system components, as well as compliance with local building ordinances. Such systems are much more economic and popular in states that allow what is called net metering.
Net metering provides a way to encourage the use of renewable energy systems and makes it more cost effective for small consumers to generate their own power.
In 34 states, consumers can install small, grid-connected renewable energy systems to reduce their electricity bills using net metering. Under net metering, electricity produced by the renewable energy system can flow into the utility grid, spinning the existing meter backwards.
Without net metering, consumers can still use the electricity they produce to offset their electricity demand on an instantaneous basis. However, if the consumer generates more electricity than is needed at the moment, net metering allows the excess to be credited to the consumer and used at other times during the billing period.
Utility interconnection issues are emerging as a major barrier to the commercialization of small grid-connected systems. Standardized technical requirements and contracts for interconnection provide the ability to connect to the electric grid without cost-prohibitive engineering studies and insurance requirements.
Please contact your local electric utility for more information.
Net Metering and Interconnection Resources
- Net Metering and the Easy Connection Act, Fact Sheet--PUB2238 (09/13)
- "Net Metering and Easy Connection Act" (Section 386.890; TAFP HCS SCS SB 54). The full text of the portion of the bill related to Net Metering and the Easy Connection Act can be found on pages 34-41 of this bill.
Other Web Sites of Interest
- Green Power Network -- Net metering policies (includes links to maps and tables for net metering metering provisions nationwide).
Designing a System
Except for small, fully integrated PV systems, you should carefully design your system so that the components are properly sized and matched to arrive at a cost-effective and safe electric generating system. As a general rule, efficiency first; then look at renewable energy generation.
- Define Site Conditions: Will the collectors be located where they are free of all shading from at least 9 a.m. through 3 p.m.? What is the most aesthetic mounting method? Will local ordinances or utility regulations allow installation of a PV system at your location? How energy-efficient is the building or appliance to be served?
- Estimate Loads: Estimating your electric load is fairly straightforward, but it requires thoroughness. List every light, appliance, tool, and gadget that uses electricity and will be using the power from your PV system. Then list the watts of power required to operate each item, and the time of day and length of time you normally expect to use that item. Often this step is the place to identify opportunities to improve your energy efficiency.
An 18-watt compact fluorescent bulb provides the same level of light as a 100-watt incandescent light bulb. If your major appliances are more than 10 years old, they often can be replaced with new appliances that use significantly less energy. Traditionally, we don’t use all our electric appliances at the same time. Avoiding the simultaneous use of major electric appliances can reduce your peak load, thus reducing the total system size required.
- Determine the Type of System: Will your system be a "stand-alone," "utility-intertie," or a "hybrid"? This decision will determine whether you need batteries and what size array you will need.
- Determine the Size of Battery Banks: The type of system you choose is critical to deciding on the size of your battery bank. With a grid-intertie system, which is connected to the electrical grid, some people choose to do without a battery bank. With a stand-alone system, you need sufficient battery storage to see you through extended low-light situations. When deciding on the size of battery packs for hybrid systems, you need to balance cost issues with load demands.
- Determine the Size of Array and Components: As you have figured out by now, deciding on the size of a solar array and its components involves tradeoffs between load, storage, array location and tilt, and system type. Twenty-five or more photovoltaic simulation programs exist for sizing solar arrays and matching them to Balance of System (BOS) components. The best way to ensure proper system design is to work with a reputable solar dealer whose references you have checked. Like any other major purchase, shopping around is a good idea. While Missouri does not have a large number of photovoltaic dealers, you can find out how to contact those that are available through various web sites (American Solar Energy Society, Interstate Renewable Energy Council, Solar Energy Industries Association) and periodical publications.
A very complete discussion of the system design process is provided in:
Stand-Alone Photovoltaic Systems, A Handbook of Recommended Design Practices, Sandia National Laboratories, SAND87-7023, National Technical Information Service, U.S. Department of Commerce, 5285 Port Royal Road, Springfield, VA 22161, 308 pg. plus Appendices, 1995.
Siting refers to the decisions you need to make regarding the location of a solar collector, whether for passive solar building design, water heating, or photovoltaic electric generation. Will the collector or building wall face due south? At what angle should a collector be tilted for your location? How will the placement of trees and other landscaping features or neighboring buildings affect your collector’s efficiency? An excellent description of what things to think about when deciding whether and where to put a solar collector is available on the North Carolina Solar Center’s Web page.
As you consider installing a solar energy system, remember a couple of basic things. Before buying or installing any solar system, please check references of the supplier and installer to make sure you are getting quality products and support. If you prefer to do it yourself, please study carefully all aspects of the type of system you wish to install. Most solar water-heating systems require a few steps beyond simply sweating pipe. In Missouri, you are best advised to use a heat exchanger, or indirect, system for heating water. This need requires use and installation of anti-freeze and electric pumps in addition to pipe work.
Working with direct current (DC) and probably inverters to convert DC to alternating current (AC) for use with your appliances will be required when installing photovoltaic systems. Because many competent electricians do not have much experience with DC, make sure you hire someone who is familiar with its unique attributes. Also, photovoltaic modules are generating electricity whenever light is shining on them, which means installers are working with live current, a situation not normally encountered when working with AC electricity that can be turned off at the breaker box.
Installing and using solar energy systems in your home or business can be a very rewarding and earth-friendly choice. Since solar power is a relatively new industry in Missouri, you will need to make sure the system you buy is a quality system, and that you, or a hired installer, are properly prepared to perform a quality installation.