Solar hot water systems collect the Sun’s energy to heat water. The systems can provide supplemental hot water to your home domestic hot water system, radiant heating system, or pool heating system. They are more efficient than solar photovoltaic and solar hot air systems.
Solar hot water systems are composed of three main components: a solar collector, a distribution system, and an optional heat storage unit. Other accessory elements may include pumps, sensors, filters, valves, and system controls.
transfers heat from the sun to water.
pipes circulate water from the collector to a storage tank, radiant heating system, or pool using a pump.
Heat Storage (optional)
a tank that stores water or thermal energy for distribution at a later time.
There are three main types of solar hot water collectors: flat plate collectors, evacuated tube collectors, and integral collector storage systems.
Flat plate collectors
consist of glass or plastic sheets, a dark absorber plate, and water pipes with insulation. Sun light heats up the dark plate, which in turn heats the water. These are the most common type of solar collector, and they operate more efficiently during the summer months than in the winter months due to heat loss to the outside air.
Evacuated tube collectors
consist of rows of glass tubes nested with metal pipes and aluminum or copper fins, heat exchanging fluids, and a copper manifold heat exchanger. The external glass tubes and the metal tubes are separated by a vacuum air space to limit heat loss. Located above all of the pipes there is a copper manifold that serves as a heat exchanger to a home’s hot water system. These systems are the most efficient solar collector type for extremely cold environments, but they are also the most expensive collectors. In Ohio, these collectors work more efficiently than flat plate collectors during the winter months, but less efficiently than them during the summer months. Check out “Solar Collector Efficiency Calculator” by BuildItSolar.com to determine the efficiency of these collectors during different operating temperatures.
Integral collector storage systems (ICS)
also known as “batch” or “bread box” solar water heating units, these systems consist of a big tank or many large diameter tubes that are insulated within a box located outside of a home. They effectively combine the solar collector and storage into one unit. These systems are the cheapest solar water heating system available, but have the lowest efficiency. Since the hot water storage is outside of the conditioned environment, these systems are only viable in places where temperatures do not drop below freezing. For this reason, they’re not widely used in Ohio.17
These systems can either be active systems, which use pumps, valves, and controls to move water through the system, or they can be passive systems, which do not include any such mechanical elements.5
Direct active circulation
utilizes a pump to move water through the solar collector and directly into the house’s radiant system or a storage tank in the house. There are no heat exchangers associated with these systems.
Indirect active circulation
uses a secondary fluid in the solar collector and exchanges heat to the water used in your home using a heat exchanger. Typically, the storage tank and heat exchanger are both located in a hot water storage tank. This is the most common distribution method for solar hot water heating.
utilizes natural convective heat transfer to deliver hot water to a storage system. These systems are less efficient than the active systems, but they do have a lower upfront cost. Due to freezing conditions in Ohio, these systems cannot work in the winter time, so the system will need to be flushed to avoid damage to the solar collector.
Energy harnessed from the solar collectors can be stored in a storage tank, masonry mass, or phase changing materials.
stores hot water. These systems are most common and can be supplied actively or passively. When purchasing a domestic hot water storage tank, consider cost, size, insulation, durability, tank placement, and auxiliary heating fuel type and efficiency. These systems can be used to supply domestic hot water or radiant heating.
materials such as concrete or floor tiles have the ability to absorb large amounts of heat, which act as thermal storages. These materials are used in radiant heating systems and can add additional heat storage to the system.
Phase Changing Material (PCM)
PCMs can use a variety of substances that change phases at specific temperatures. It can be used for thermal energy storage in homes if the critical temperatures are close to the set point temperatures for hot water tanks. However, PCMs are not common in residential solar hot water systems.16
Selecting a Domestic Solar Hot Water System
Before purchasing a domestic solar hot water system, you’ll first want to understand solar availability of your site, the system’s energy efficiency, heat output, cost and payback, and local codes and regulations associated with your property. Solar hot water for radiant heating design is much more complicated, so it is recommended that you directly contact a contractor for your system design.
To know if solar is right for your home, you first need to determine if your site is viable. To do so, you will have to determine the available sunlight for the site, and place your system accordingly. We do recommend you contact a professional in your area to do this, which can be found at Green Energy Ohio's "Ohio Renewable Energy Installers List" webpage.
For a do-it-yourself site-assessment, follow these steps:
1. Determine True South
True solar south is not the same as magnetic south. For Ohio, when pointing a compass south, you are actually pointing towards 5° - 8° E of true solar south. To correct this, if you are looking at magnetic north, true north can be found at roughly 8° E (8° W of South). You can determine true south here.
2. Determine the altitude (incline) of your solar collectors.
Can affect the amount of power generated by your system. For fixed-oriented systems, most homeowners orient the incline of the solar panels to the latitude angle.
3. Determine shading patterns
Lastly, determine if there are any intrusions that may cause unwanted shading at any time throughout the year. This can be done using a sun pathfinder.
For solar water heating efficiency, most panels are rated with a solar energy factor (SEF) and solar fraction (SF). SEF is the ratio of the energy delivered by the solar hot water system to the electrical or gas energy put into the system. Typically, systems have an SEF of around 2 or 3. The SF is the proportion of water heating supplied by the solar system to the overall hot water demand. This means that the SF ranges from 0-1, with 1 indicating that all of the hot water is supplied by the solar hot water system . Learn more at “Estimating the Cost and Energy Efficiency of a Solar Water Heater” by the DOE.
The size of your solar collectors will be determined by the hot water usage of your home and the storage volume of your hot water tank. Typically, this is done by your solar hot water contractor. Check out “Sizing a New Water Heater” by the DOE for more information.
Cost and Payback
According to homeadvisor.com, a solar hot water heater ranges from $800 to $6,000 and has a payback of 5 to 15 years (corrected for no federal tax incentive). If you are building a new home, the price of a solar water heating system for a 30-year mortgage is typically around $13-$20 per month, and federal income tax deductions reduce the payment by $3-$5 per month. So If you save more than $15 a month on water heating, the system will be profitable. Learn how much your system will cost by visiting “Estimating the Cost and Energy Efficiency of a Solar Water Heater” by the DOE.
Local Codes and Regulations
Every site has a different set of local zoning codes and regulations. Learn more about these restrictions at “Building Codes and Regulations for Solar Water Heating Systems” by the DOE.
Solar hot water systems will likely require a contractor since they do use pumps, plumbing, and heat exchanging elements. You can find installation contractors at Green Energy Ohio’s “Renewable Energy Installers” webpage.
- Solar Systems Design Basics by James M Pleasants Company - PowerPoint for understanding solar variables (complements of ASHRAE presentation)
- "The Zen of Passive Heating Panel Design" by Morris R. Dovey - Passive design strategy presented by a home owner that implemented an active solar collector approach with natural convective air movement. This website presents a step-by-step assembly of the system.
- "Heat your Water with the Sun" by the Department of Energy - Consumer's guide to implementing solar heating technologies
- "Solar Heating and Cooling" by Solar Energy Industries Association
- "Active Solar Heating" by the Department of Energy
- "Solar Water Heaters" by the Department of Energy
- "Siting your Solar Water Heating System" by the Department of Energy
- "Building Codes and Regulations for Solar Water Heating Systems" by the Department of Energy
- "Heat Exchangers for Solar Water Heating Systems" by the Department of Energy
- "Estimating Cost and Energy Efficiency of a Solar Water Heaters" by the Department of Energy
- "Heat Transfer Fluids for Solar Water Heating Systems" by the Department of Energy
- "Solar Water Heating System Maintenance and Repair" by the Department of Energy
- "Solar Swimming Pool Heaters" by the Department of Energy
- "Solar Hot Water" by Alternative Energy Tutorials - Great solar source
- “Solar Collector Efficiency Calculator” by BuildItSolar.com - Calculate the average efficiency and heat output of a solar thermal hot water system using ambient air temperature, collector temperature, and solar intensity.
- “Using Phase Change Materials (PCMs) For Space Heating and Cooling in Buildings” by University of South Australia - Research conducted by Dr. Frank Bruno at the Sustainable Energy Centre concerning PCMs and their use in various applications.
- "Integral Collector Storage" by Alternative Energy Tutorials
- "Break-even Cost for Residential Solar Water Heating in the United States: Key Drivers and Sensitivities" by Hannah Cassard, Paul Denholm, and Sean Ong
- "Solar Hot Air Collectors" by Green Building Advisor
- “DIY Solar Air Heating Collectors: Pop Can vs Screen Absorbers” by Build It Solar - Introduction into solar hot air heating followed by simple solar hot air collector instructions, costs, and conclusions.