Fact one: The Sun bestows enough energy upon the Earth every second to meet the needs of the entire world’s population for an entire year.
Fact two: Buildings are energy hogs. They use 40% of the world’s energy; consume 65% of total U.S. electricity; and account for 36% of total U.S. primary energy use and 30% of total U.S. greenhouse gas emissions.
Fact three: It’s time to seriously consider the benefits of solar energy.
Let's Look at Solar EnergySolar energy is all around us in the form of energy wavelengths and particles, heat and light. It is a renewable, environmentally friendly resource available everywhere on Earth, at least part of the time. This energy is provided to us free of charge, we just need to harvest and make use of it. Currently, solar energy use is much more widespread in Europe than North America.
Solar energy can be used in many ways to provide heat, lighting, mechanical power, and electricity. But often, there is confusion about the various methods used to harness the sun’s abundant and clean energy. For our purposes, the Sun’s energy can be categorized in two ways:
Light Energy can be converted directly into electrical current through photovoltaic devices. Photovoltaics (PV) is a technology often confused with solar thermal and is, in fact, what many people mean when they refer to solar energy. Photovoltaics (photo=light, voltaics=electricity) is a semiconductor-based technology (similar to the microchip), which converts light energy directly into an electric current that can either be used immediately or stored, such as in a battery or capacitor, for later use.
PV panels/modules are very versatile and can be mounted in a variety of sizes and applications; e.g., on the roof or awning of a building, on roadside emergency phones or as very large arrays consisting of multiple panels/modules. Currently they are being integrated into building materials, such as PV roofing material, which replaces conventional roofing shingles.
Solar Thermal technologies use the sun’s heat energy to heat substances (such as transfer fluids or panels) for applications such as space heating, pool heating, and domestic water heating. There are a variety of products on the market that utilize thermal energy. Often the products used for this application are called solar thermal collectors and can be mounted on the roof of a building or in some other sunny location.
The sun’s heat can also be used to produce electricity on a large-utility scale by converting the sun’s heat energy into mechanical energy. These central utility solar plants are sprouting up around the world.
Great strides are being made in PV technology, but solar thermal systems are generally more economically feasible at this time. Solar thermal collectors are about five times as efficient as currently available photovoltaic panels, yet they cost about one-tenth as much.
Not So SimpleA simple explanation of a solar thermal system would be to say that the sun’s energy heats up water, which becomes “hot water.” Simple, right? Yes and no. Life is never that simple once engineers get involved (see the system schematic in Figure 1). We have an internal drive that forces us to find ways to improve efficiencies and to complicate matters.
We know that the higher the input temperature is above the ambient temperature, the higher the potential heat loss will be. This will result in a lower output temperature, and, therefore, lower efficiency. Because heat naturally migrates from more to less, this result is not surprising. The system will experience conductive and convective heat losses.
The level of efficiency can depend on which type of collector is used: evacuated tube or flat plate. There is no collector type that is always most efficient, because efficiency is a factor of input times efficiency (in formulaic terms, Output = Input x Efficiency).
Evacuated tube collectors (see Figure 2), which can cost significantly more than flat plate collectors, are best if high temps are needed or in low solar energy conditions, like winter. Ironically, because of their higher thermal efficiency, these collectors are slower to melt ice, snow, and frost - inhibiting their function. Hot compressed air is the preferred method of defrosting or deicing vacuum tubes. Debris such as leaves, etc., can also present maintenance problems.
Flat plate collectors are more efficient if there is lots of sun and a hot water temperature of only 140°F is needed. They can be at least as efficient if 140°F water is desired and ambient temperatures are below 32°F. As an added bonus, these types of collectors have a lower initial cost and requires less maintenance.
However, collector type is only one system component that needs to be considered for each project.
Figure 1. LEGEND
- 1. SOLAR COLLECTORS
2. CIRCULATING PUMP
3. TEMPERATURE / PRESSURE RELIEF VALVE
4. PUMP CONTROLLER
5. SENSOR: COLLECTOR TEMPERATURE
6. SENSOR: TANK TEMPERATURE
7. CHECK VALVE
9. HEAT EXCHANGER
10. STORAGE TANK
11. AUXILLARY HEAT SOURCE
12. WATER SUPPLY
13. TO BUILDING
14. FLOW METER
15. DRAINBACK TANK
16. EXPANSION TANK
17. THERMOSTATIC MIXING VALVE
Solar-Assisted Service Water Heating SystemsSolar-assisted service water heating systems generally provide maximum savings when the systems are designed to deliver 45% to 70% of the load (f = 45% to 70%). Oddly enough, in cold climates like Canada, we might get 25% of our hot water load during the summer, but 65% of our load during the winter. To determine the amount of energy required to heat the service water, the following information is needed:
To meet the requirements set forth in the Federal Housing Administration’s Minimum Property Standards, the hot water supply temperature must be 140°F as a minimum. This temperature also helps reduce water-born pathogens such as Legionella. Depending on the season and geographical location, the cold water supply temperature may vary from 40°F to 70°F, although it is possible to have lower or higher temperatures. This should always be considered when calculating water heating or cooling applications.
Although many system types, applications and design techniques may be considered for various service water-heating requirements, a few basic guidelines should be followed in all cases:
Types of Solar Water Heating SystemsDirect Systems (Active)
This system uses a pump to circulate potable water from the water storage tank through one or more collectors and back into the tank. The pump is regulated by an electronic controller, an appliance timer, or a photovoltaic panel.
Indirect Systems (Active or Passive)
In these systems, a heat exchanger heats a fluid that circulates (with or without a pump) in tubes through the water storage tank, transferring the heat from the fluid to the potable water.
A thermosiphon solar water heating system has a tank mounted above the collector. As the collector heats the water, hot water rises up to the storage tank, while heavier cold water sinks down to the collector. No pump is required.
In cold climates, this type of system prevents water from freezing in the collector by using electric valves that automatically drain the water from the collector when the temperature drops to freezing. Drainback systems, a variation of this approach, automatically drain the collector whenever the circulating pump stops.
Swimming Pool Systems
In solar-heated swimming pools, the pool’s filter pump pumps water through a solar collector, and the pool itself stores the hot water.
System Sizing: Some Rules of ThumbApproximately 440 BTU/hr/sq. ft. of energy generated by the sun could potentially reach the earth. Of that potential energy, 30% to 60% is lost in the journey through the atmosphere, and 170 to 315 BTU/hr/sq. ft. eventually reaches the surface. Here in Illinois, we receive 1260 to 1575 BTU/sq. ft./day of energy from the sun. So for optimum output, panels should be installed at a 45° angle and face south. Optimum output from a panel is roughly 220 BTU/hr/sq. ft.
For residential applications, collector size is calculated at about 20 square feet of collector area for each of the first two family members and 8 sq. ft. for each additional family member if you live in the South [see Figure 3]. Up North, collector size is calculated at about the same 20 square feet for the first two family members but increased to 14 square feet for each additional family member.
Solar storage tank size is calculated at a ratio of about two gallons storage capacity per one square foot of collector area. This provides a buffer to reduce overheating in warmer climates like the southern parts of the United States. As we go north, we actually provide less storage capacity per square foot of collector!
In the central band of the country, we would want to calculate at a ratio of at least 1.75 gallons of storage capacity to one square foot of collector area. This still helps to prevent the system from overheating when the demand for hot water is low. In more northern climates, we calculate at a ratio of 1.5 gallons storage capacity to one square foot of collector area.
Collectors come in different shapes, as do tanks. Using the calculations above, we can see that a family of five, in a place similar to Chicago, for instance, would require about 73 square feet of collector area, and about 127 gallons of storage capacity.
A circulation pump is required for the solar thermal heat loop, unless it is a passive system. This can easily be accomplished for a typical home-sized system, with a PV-powered DC pump at about 20-30 watts.
In this case, we would want to consider a drainback system, which will eliminate the need for glycol to protect against freezing. Glycol becomes acidic and aggressive, especially when it is heated repeatedly. We would also recommend flat plate collectors for this project because of the snow that Chicago is likely to experience.
For additional sizing information, including all of the formulae behind every aspect of solar design, refer to the solar chapter in the American Society of Plumbing Engineers Design Handbook Volume 3, or other similar publications.
StandardsThe Solar Rating and Certification Corporation (SRCC) is a not-for-profit organization that provides a valuable service by serving as a third-party testing, rating, and certification agency for solar thermal systems and equipment. The current standards are OG-100 and OG-300.
OG-100 addresses testing and quality-assurance issues of solar thermal collectors. Units are tested for durability as well as performance characteristics and reaction to adverse conditions.
OG-300 focuses on testing and quality-assurance of the solar thermal systems. Systems are tested for durability as well as performance characteristics. Visit www.solar-rating.org for more information.
The SRCC will release a new standard for rating installers soon. To date, 72 installers have been certified by the National American Board of Certified Energy Practitioners. NABCEP also deals with PV and wind systems. Visit www.nabcep.org for more information.
These standards and certification programs should lead to inclusion into building codes. I guess that’s a good thing, but that’s sure to bring more challenges with it. The standards are also related to qualification for tax credits, etc. There are myriads of tax credits available, and they can be researched at this site: http://dsireusa.org.
SpecificationsSpecifications are an important part of any system design. The new CSI format has rearranged the sections and changed the numbering system. Plumbing is now found in the 220,000 sections.
Please note that your system may require sections other than those listed. These listed here are only an example. Some of the sections that you may need to include in a solar water heater system specification are:
But regardless of the format used, the basic objectives and requirements are the same.
As always, be the engineer! No system or equipment can be the best solution for every application.
Additional ResourcesAmerican Solar Energy Society (www.ases.org)
The Solar Hygrogen Civilization, by Roy McAlister
Solar Hot Water Systems, by Tom Lane
Solar Energy Industries Association (www.seia.org)
Energy Star (www.energystar.gov/index.cfm?c=products.pr_tax_credits#8)
Tax Incentives Assistance Program (www.energytaxincentives.org)
Solar Rating Certification Corporation (www.solar-rating.org)