Solar Water Heating Basics
Solar water heating systems are a sustainable and cost-effective alternative to traditional in-home water heaters. The first commercially distributed solar water heating system, the Climax Solar-Water Heater, arrived on the market in 1891. Today, a variety of heating systems are available to accommodate specific usage requirements and climates. A basic understanding of the different types of solar water heating systems can help you decide what best suits your demands (California Solar Center).
Solar Collectors
The most important component of the solar water heating system is the solar collector. There are two different types of collectors responsible for gathering heat from the sun: flat-plate collectors and evacuated tube collectors. A flat-plate collector is a rectangular box made from an aluminum-alloy or galvanized steel and covered with a glass plate. Inside the box are the absorber plate and the flow tubes.
The absorber plate is a thin sheet of metal painted black and generally constructed from either copper or, for a less expensive option, aluminum. As the absorber plate warms up the heat is transferred to fluid passing through the flow tubes. Evacuated tube collectors absorb heat through a coated copper pipe that is placed within two concentric glass tubes.
A vacuum is created between the two glass tubes which greatly reduces heat loss because there is no air for the heat to travel through. Meanwhile, a liquid inside the copper piping is responsible for transferring the heat to the stored water.
Once heat is absorbed by either type of collector it can then be transferred to the water in two different ways depending on whether the system is direct or indirect. In direct solar water heating systems, the water in demand passes through the flow tubes in the collector while being heated by the absorber plate.
The hot water is then available for use. Indirect systems require an intermediary fluid to transfer the heat to the water in storage called a heat transfer fluid or HTF. This non-toxic antifreeze HTF passes through the flow tubes of the collector, absorbs the heat, then transfers the heat to the water in the storage tank through a heat exchanger.
Both systems have their inherent advantages and disadvantages. The direct flow method does not lose any heat in a transfer process. Yet, if the water freezes it can cause damage to the system whereas the HTF in indirect systems protects against freezing (GreenSpec.co.uk).
Circulation
The manner in which the water or HTF is circulated throughout the system is another point to take into consideration when designing a solar water heating system. The options are either passive or active circulation. A passive system utilizes gravity and the tendency of convection to move water when it is heated up.
In this type of system the tank must be placed above the collector. In the case of a batch collector the tank is placed inside a box and acts as both the absorber and storage. These systems are very simple but require attentive planning in order to optimize hot water output (EnergySavers.gov).
An active circulation system uses a pump and temperature sensors to move the liquid through the system. Since the water or HTF can be moved by a pump, the storage tank need not be placed above the collector and can be placed in position that diminishes heat loss. The pump in this system is controlled by temperature sensors and is only activated when the temperature in the storage tank drops below that of the collectors.
Temperature Controls
The temperature controls also protect the system from both freezing and overheating. When freezing temperature are detected hot liquid is circulated through out the system and likewise when overheating is detected cooler fluid is circulated. This ability to monitor temperatures makes the active system much more suitable for situations in which there are long periods of no usage and in climates that vary drastically.
These systems do require more maintenance due to the addition of electrical and mechanical parts. Furthermore, an outside power source is required to keep the system operating. However, photovoltaic cells can be installed to power the pump and electronics, keeping your water flowing and keeping you off the grid and pollution free.
Listed below are links to more information about solar water heating:
- Whole Building Design Guide: A comprehensive overview of solar water heating and it's effects on energy usage.
- National Renewable Energy Laboratory: Introduction to solar water heating and many other renewable energy basics.
- Solar Water Heating Calculator: This calculator helps you analyze your current water usage and look at potential savings by installing a solar water heater.
- Energy Star: Saving and benefits of using Energy Star qualified solar water heaters by the U.S. Department of Energy.
- Florida Solar Energy Center: Resources for implementing solar heating in your house, pool, or commercial building.
Cited Sources:
California Solar Center
GreenSpec.co.uk
EnergySavers.gov