How do solar water heaters work?

Submitted by: Tholakele Nene, Friday, July 5, 2013

<p>Water heating is typically one of the higher consumers of electricity for households. SWHs use the energy of the sun and result in monthly savings on electricity. Image credit: <a href='http://www.123rf.com/photo_15590243_water-heating-solar-panels-on-the-roof.html'>sspopov / 123RF Stock Photo</a></p>

Water heating is typically one of the higher consumers of electricity for households. SWHs use the energy of the sun and result in monthly savings on electricity. Image credit: sspopov / 123RF Stock Photo

Solar water heaters (SWH) are a renewable energy technology that heats cold water using the sun’s energy. The solar energy is absorbed by the panels or collectors, usually installed on top of the roof, and heats cold water. Solar water heaters are a form of thermal heating, which relies on the direct energy emitted by the sun. This is in contrast to other Solar Photovoltaic (PV) systems which convert the sun’s energy into electricity.

According to solar water heater supplier, KZN Solar’s managing member Phil Van Achterbergh, SWHs are a good investment, especially for households with a monthly electricity bill exceeding R1000.  A household that installs a solar water heater will significantly reduce its monthly electricity bill as it will no longer need to use Municipal-supplied electricity to heat water but will instead rely on the solar water heater system to provide  hot water. 

Components of a SWH and movement of water

A typical solar water heating system has three major components: A solar collector, a transfer medium and a storage container. The solar collector absorbs solar radiation and transfers the energy in a form of heat, to the fluid within the collector. The collector is often made up of a simple glass-topped insulated box with a flat solar absorber made of sheet metal attached to copper pipes and painted black, or a set of glass tubes surrounded by an evacuated (near-vacuum) glass cylinder.

Once the water has been heated by the collector it needs to be moved into a storage tank. The storage tank should be well-insulated to ensure the water remains hot for as long as possible. The volume of the tank needs to be large enough to allow for bad weather days when the collector is unable to heat water. When installing a new solar water heater system, households with existing geysers in good condition can save money by using the existing geysers as the storage tank, says Van Achterbergh.

Water moves around a solar water heater system either by natural convection or through the use of electrical pumps. In a system using natural convection the water heats up in the collector and rises naturally into the storage container above it, while the cooler water in the storage container flows down, to the bottom of the collector creating an unaided circulation. For a natural convection system the tank used for storing the water needs to be located above the solar collector. In cases where the water is stored below the collector, an electrical or solar PV pump is used to circulate water.

Direct and indirect systems

Residential SWHs fall into two groups, direct and indirect systems. Direct SWH systems, generally used in coastal applications, provide heat generated in the panel directly to the geyser, where it is then utilized by the home owner. The indirect system, designed to overcome problems associated with frost in colder areas as well as poor quality water issues, uses a food grade antifreeze (Glycol), that is circulated through the panel into either an outer jacket or element in the geyser where the heat is then transferred into the water that is in the tank. With indirect systems there is an additional step to the process of heat exchange.

According to Van Achterbergh most of the solar water heater installations around KwaZulu-Natal done by KZN Solar, are direct SWHs, which means that the hot water is directly heated by the sun. Direct systems are commonly found in coastal parts of South Africa where frost and cold weather are not an issue and there is a good supply of sunlight. However, the colder areas situated inland use indirect systems where the water in the panels is replaced by the anti-freeze solution.

Direct systems can be installed inland if evacuated tube technology is used with silicone inserts.

Electrical Backup

Most solar water heaters include a backup electrical element in the storage tank.  The electric element is used to heat water in the tank when it falls below a minimum temperature setting. The backup system is normally required in cases where there is constant cloud cover and not enough sunlight to heat the panels. This ensures that the users will always have hot water regardless of the weather conditions. The volume of water held in reserve will determine how long a system can last without electrical support, leading to the conclusion by Van Achterbergh that it is essential to do a site inspection in which the extent of water held in reserve can be determined.

Maintenance

Maintaining Solar Water Heater collectors is cheap and easy to do.  The solar collectors need to be dusted off and cleaned from time to time to allow for maximum solar output. To do this you can use soapy water and a sponge once every quarter or hose and mop depending on your installation. However, owners are advised against using detergents.

The benefits of installing a solar water heater

There are multiple benefits of installing solar water heaters, these include

  • Saving money on your monthly electricity bill
  • Reduced demand of electricity and pressure on Eskom and the Municipality
  • Reduced greenhouse gas emissions
  • Reduced dependency on finite fossil fuels
  • SWHs are a sustainable renewable energy method with easy installation and maintenance

However, like most technologies solar water heater installations have had their share of challenges and successes. Speaking about some of the challenges that he has experienced to date with installations, Van Achterberg says that some of the clients have “uninformed expectations of what solar can do and in some cases do not take advice about which installations are suitable to them”. Therefore, creating an in- balance between what the client wants and what is practical or what system would suit them best.

The big challenge is to get the reality of savings across, moving away from “going solar” to understanding the financial impact of a reduced bill, using the right technology, he says. “Solar is based on volume” and a solar water heater must be designed to accommodate the maximum volume of hot water that will be required by a household.  “This is why we generally try to use at least one of the existing geysers in a “pumped” or “retrofit” application and then back it up with another system either in or on the roof. This gives us volume but essentially also provides us with only half the portion of the entire volume to heat should we need to use an element. The advantage being that all the hottest water remains in the house feeding geyser and all the cold water mixes with the pre-feeding geyser”.

Van Achterbergh further encourages residents to take advantage of the Eskom rebate programme which provides a discount for residents that purchase a heat pump or solar water heater from one of Eskom’s accredited suppliers. 

“Do it now while the Eskom rebate is making it easy. Right now the program is one of assistance, in the future it will be one of insistence, driven by Government legislation. If your bill exceeds R1450.00 the savings derived from a solar system may well offset the finance costs of purchasing the system, “says Van Achterbergh.

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Tholakele Nene