Heat from the sun can be used to heat water for your showers or swimming pool or to provide heat for your house. To be clear, solar thermal energy is an altogether different technology than solar electric power (discussed in the next chapter), in which light from the sun is converted into electricity. Using the sun is the easiest way to gain and use free heat and can be very cost-effective. Simply leaving a garden hose out in the sun can provide useful hot water; from there, it’s not such a big step to moving and storing that water with a simple controller and a small pump. This chapter provides an overview of solar thermal systems for water heating.
Types of Systems
S o l a r thermal hot water systems can have many different configurations, depending primarily on the climate, how much hot water is needed, and when it’s needed. All systems have one or more collectors (the “panels” where the water is heated) and a storage vessel that holds the heated water. The stored water can be used directly, or it can be delivered to a secondary water heater to boost.
Passive Solar
Passive solar thermal can be described as simply allowing the sun to heat something. It could be heating the air and floor in your living room, or water held in a black barrel outside. A passive hot water heating system often relies on a naturally induced thermosiphon to transport heated fluid between a solar collector and a water storage tank. A thermosiphon requires a temperature difference to work: Hotter fluid (being less dense and therefore more buoyant) will rise to the top of the system, while cooler fluid is moved toward the bottom of the system. This means that the storage tank must be at a higher elevation than the collectors since the collectors will be hotter than the storage tank when the sun is out and you want the hotter fluid to move upward into the tank. In practice, thermosiphon systems are somewhat uncommon, because typically the water storage tank is in the basement and the collectors are on the roof. In this situation, hot water stored in a tank in the basement will naturally want to thermosiphon up to the collectors on the roof when the collectors are cool. Unwanted thermosiphon must be stopped with a check valve in the plumbing, ensuring that water flows in only one direction.
Active Systems
Active systems use the sun’s heat to warm a fluid in the collector, and the fluid is moved by a pump so that the heat in the fluid can be used or stored. Pumps are turned on and off by a controller that responds to the temperatures at the collectors and the storage tank. Active systems can be directly or indirectly heated. In a direct system, the water that’s heated in the collectors is the same water you use at the faucet, and it’s pumped between the storage tank and collectors. Also called “open-loop” systems, these are often used in climates where freezing temperatures are not a concern. Hard water can be problematic for directly heated systems because minerals in hard water build up on hot surfaces, restricting or even stopping water flow. In such cases, a water softener is used to reduce mineral content in the water supply.
Types of Systems
S o l a r thermal hot water systems can have many different configurations, depending primarily on the climate, how much hot water is needed, and when it’s needed. All systems have one or more collectors (the “panels” where the water is heated) and a storage vessel that holds the heated water. The stored water can be used directly, or it can be delivered to a secondary water heater to boost.
Passive Solar
Passive solar thermal can be described as simply allowing the sun to heat something. It could be heating the air and floor in your living room, or water held in a black barrel outside. A passive hot water heating system often relies on a naturally induced thermosiphon to transport heated fluid between a solar collector and a water storage tank. A thermosiphon requires a temperature difference to work: Hotter fluid (being less dense and therefore more buoyant) will rise to the top of the system, while cooler fluid is moved toward the bottom of the system. This means that the storage tank must be at a higher elevation than the collectors since the collectors will be hotter than the storage tank when the sun is out and you want the hotter fluid to move upward into the tank. In practice, thermosiphon systems are somewhat uncommon, because typically the water storage tank is in the basement and the collectors are on the roof. In this situation, hot water stored in a tank in the basement will naturally want to thermosiphon up to the collectors on the roof when the collectors are cool. Unwanted thermosiphon must be stopped with a check valve in the plumbing, ensuring that water flows in only one direction.
Active Systems
Active systems use the sun’s heat to warm a fluid in the collector, and the fluid is moved by a pump so that the heat in the fluid can be used or stored. Pumps are turned on and off by a controller that responds to the temperatures at the collectors and the storage tank. Active systems can be directly or indirectly heated. In a direct system, the water that’s heated in the collectors is the same water you use at the faucet, and it’s pumped between the storage tank and collectors. Also called “open-loop” systems, these are often used in climates where freezing temperatures are not a concern. Hard water can be problematic for directly heated systems because minerals in hard water build up on hot surfaces, restricting or even stopping water flow. In such cases, a water softener is used to reduce mineral content in the water supply.
