Sizing The Solar Thermal Heat Exchangers
Heat exchangers are a key component in closed-loop solar heating systems. And, properly sizing your heat exchanger is critical to the performance, efficiency and cost-effectivness of your solar thermal system.
Many in the solar thermal industry believe that closed-loop systems are far less efficient (and therefore inferior) to open-loop systems. However, over the years, we have determined that much of this is due to improperly sized heat exchangers. Most likely, these heat exchangers were sized for traditional applications instead of solar.
The difference in temperature between the hot and cold fluid in the separated loops of a hot water system is known as the approach. In traditional boiler systems, the approach is typically 80F (180F – 100F). In a solar system, however, we want the collectors and the storage tank temperatures to be as close as possible (this will significantly increase collector efficiency). Typically, we try for an approach of 10-20F in solar systems.
To accomplish, we need to skew the output numbers of heat exchangers that have been rated for traditional boiler systems. The guidelines we use are based on these “low approach” solar temperatures (the maximum heat load is determined from the typical operating temperature under full sun).
Using these principles, closed loop systems with a brazed plate heat exchanger will operate within 5% of an open-loop system.
Sizing Brazed Plate External Heat Exchangers
Brazed plate heat exchangers have high efficiencies and compact sizes, which allows them to be sized for low approach temperatures (down to 10F) and still remain cost-effective. Using these sizing methods, these systems can parallel open-loop system efficiency.
Pressure drops at design flow rates will be around 1.5psi
Sizing Internal Heat Exchangers
Typically, the efficiency of these heat exchangers is lower than brazed plate external heat exchangers. This is mostly due to the fact that they rely on natural convection for heat transfer away from the exchanger. For this reason, they can be economically sized for approaches down to about 20F.
The actual heat transfer from your heat exchanger will depend on the configuration of your heat exchanger, there is a simple rule of thumb that will help your heat exchanger with little difficulty. Use 1 square foot of tube for every 14 square feet of collector area. This should give you an approach around 20F and a pressure drop at the design flow rate of about 1psi.
Space Heating
Many large buildings need ventilated air to maintain indoor air quality. In cold climates, heating this air can use large amounts of energy. But a solar ventilation system can preheat the air, saving both energy and money. This type of system typically uses a transpired collector, which consists of a thin, black metal panel mounted on a south-facing wall to absorb the sun’s heat. Air passes through the many small holes in the panel. A space behind the perforated wall allows the air streams from the holes to mix together. The heated air is then sucked out from the top of the space into the ventilation system.
Solar water-heating systems are designed to provide large quantities of hot water for nonresidential buildings. A typical system includes solar collectors that work along with a pump, heat exchanger, and/or one or more large storage tanks. The two main types of solar collectors used for nonresidential buildings—an evacuated-tube collector and a linear concentrator—can operate at high temperatures with high efficiency. An evacuated-tube collector is a set of many double-walled, glass tubes and reflectors to heat the fluid inside the tubes. A vacuum between the two walls insulates the inner tube, retaining the heat. Linear concentrators use long, rectangular, curved (U-shaped) mirrors tilted to FOCUS sunlight on tubes that run along the length of the mirrors. The concentrated sunlight heats the fluid within the tubes.
Space Cooling
Space cooling can be accomplished using thermally activated cooling systems (TACS) driven by solar energy. Because of a high initial cost, TACS are not widespread. The two systems currently in operation are solar absorption systems and solar desiccant systems. Solar absorption systems use thermal energy to evaporate a refrigerant fluid to cool the air. In contrast, solar desiccant systems use thermal energy to regenerate desiccants that dry the air, thereby cooling the air. These systems also work well with evaporative coolers (also called swamp coolers) in more humid climates.
For more information about solar process heat, visit the following resources:
Active Solar Heating U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy
How a Solar Thermal System Works
Collectors absorb the sun’s heat energy and transfer it to a heat transfer fluid in the system. A pump moves the hot fluid to a domestic water tank where the heat is transferred to the water through a heat exchanger. The now cool heat transfer fluid circulates back to the collector to gather more heat.
Stiebel Eltron has been designing and manufacturing solar thermal components for 40 years. Like all of Stiebel Eltron’s products, our solar components are carefully engineered. They are designed to work both individually and system-wide to bring you the best in performance and reliability.
Explore a Live Solar Thermal Installation
You can interact with a live Stiebel Eltron solar thermal installation in real-time to learn more about how our system functions.
Systems to Meet Any Needs
Solar thermal systems can be tailored to meet just about any need or existing mechanical situation. The diagrams show three common solar thermal installations. Many components of a solar thermal system are universal to all systems.
Solar Thermal Collectors Absorbs energy from the sun, converting it into heat.
Heat Transfer Fluid A propylene glycol food- and pharmaceutical-grade fluid that holds and transfers heat from the collectors to the tank. The heat transfer fluid is freeze-proof for cold nights.
Pump Station Moves the heat transfer fluid around the system.
DHW Storage Tank The internal heat exchanger transfers the heat from the heat transfer fluid to the domestic hot water the tank holds. A backup heating element keeps the tank hot when the sun isn’t shining.
Controller An electronic device that controls the operation of the pump and the safety of the system.
Backup Options A backup system is used to add additional heat to the household hot water if necessary. This situation can happen, for instance, on a very cloudy day if the solar system can not make enough hot water to satisfy the demand or make it hot enough. The Stiebel Eltron SB E tanks in SOLkits have integral electric backup, but other possible solutions include Stiebel Eltron Tempra ® tankless electric water heaters or existing site-specific backup systems.
SOLkit Water Heating Packages
Stiebel Eltron SOLkits highlight our 40 years of solar thermal experience by combining the best solar components into complete packages. SOLkits come in 1, 2, or 3 panel sizes in up-roof configuration, and 2 or 3 panel sizes across the roof. Selection of the correct kit depends on family size, domestic hot water needs, and space heating needs if required. Our expert service representatives are available by phone or email for assistance and recommendations. Kits come complete with recommended pump station, controller, and tank. A rack kit, and the line set for a particular installation, completes the package, supplying every component needed. Our components are designed for maximum compatibility, ease of installation, and reliability.
Where to Get a Solar Water Heater
Solar hot water heater components are readily available in many national chain stores, such as Home Depot. Units are also available for purchase directly from producers, with Duda Diesel and Sunbank Solar offering several great residential options. Local installers may also offer quality units.
For solar pool heating and small-scale use, check out the heaters below:
- Duda Solar 30 Tube Water Heater Collector: This system is the perfect choice for heating pools, hot tubs and closed-loop systems. Thirty highly efficient unpressurized tubes provide excellent sunlight absorption and are rated at up to 45,000 BTUs a day.
- Sunbank Solar 40 Gallon Solar Water Heater: This is designed for households with one to three members. This thermosyphon system offers exceptional absorption efficiency (92-96%) and keeps hot water hot all day in an ultra-insulated built-in tank. Weighing in at just 180 pounds, it can be installed on most roofs.
- Duda Solar 200 Liter Water Heater Active Split System: This full kit comes with a stainless-steel water tank, controller and submersible water pump. It’s a dual-coil system, which allows you to heat the water in the tank both with solar power and a secondary electricity or heat source.
Because so many factors influence which solar water heater you should buy, it’s advisable to work with a professional when choosing and installing a larger solar water heating system.
Solar Hot Water Heaters Vs. Home Solar System
Solar water heaters are less common than they used to be. This is largely due to the drastic decline in the cost of solar panels, causing many people who would otherwise install solar water heaters to forgo them and heat their water with electricity generated from their own solar panels.
Solar water heaters take up precious real estate, and for a homeowner interested in producing their own solar-generated electricity, it may make more sense to maximize the space available and nix solar water heating altogether, buying solar panels instead.
However, if you don’t have the space for solar panels, solar water heaters may still be a great fit, as they take up far less room than solar panels do. Solar water heaters can also be a great option for those living in remote locations or as an environmentally friendly add-on for existing solar electricity generation. Modern electric water heaters are incredibly efficient and, when powered with solar electricity and paired with a solar water heater, will yield significant savings for your book and cut down your greenhouse gas emissions.
For many homeowners, the decision comes down to price. Solar hot water heaters can cost upwards of 13,000. To see how much a full home solar system would cost for your home, you can get a free, no-obligation quote from a top solar company in your area by filling out the form below.
FAQ: Solar Hot Water Heater
Is a solar water heater worth it?
Whether a solar water heater is worth it all depends on where you live, your needs and preferences, and whether you plan on installing solar panels. They have been losing ground due largely to the surge of home solar: The folks that would install them also want solar for electricity generation and often choose to eliminate solar water heaters that compete for valuable rooftop space.
If you have the space, a solar water heater will likely lower your water heating bills. Used in tandem with other renewable energies, it is still a great choice for nearly any application.
What is the price of a solar water heater?
A typical solar water heater system will cost around 9,000, with higher-end models reaching upwards of 13,000. Small-scale use heaters will be much cheaper, running between 450,000 and 3,000.
What are the disadvantages of solar hot water heaters?
The biggest disadvantage of a solar water heater is that it won’t work on foggy, rainy or cloudy days, nor at night. While this can be overcome with a conventional auxiliary heater, it is still a disadvantage all solar technologies share. Maintenance can be another turn-off. While generally requiring little maintenance, some need regular draining, cleaning and protection against corrosion.
How does a solar water heater work?
Solar water heaters circulate liquid through a solar collector — most commonly a flat-plate collector or tube collector — heating the liquid and sending it either to a tank for use or an exchanger, where the liquid is used to heat water for home use.
Christian Yonkers is a writer, photographer, filmmaker, and outdoor junkie obsessed with the intersectionality between people and planet. He partners with brands and organizations with social and environmental impact at their core, assisting them in telling stories that change the world.
Christian Yonkers is a writer, photographer, filmmaker, and outdoor junkie obsessed with the intersectionality between people and planet. He partners with brands and organizations with social and environmental impact at their core, assisting them in telling stories and spreading verifiable information that change the world for the better. Christian aims to have a sizable impact on the world helping foster a sustainable relationship between humans and the planet.
Irma joined the EcoWatch team in August 2015. She holds a Master’s degree and Bachelor’s degree from the E.W. Scripps School of Journalism at Ohio University in Athens, Ohio. She is a member of Kappa Tau Alpha – a national honor society of journalism.
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