Solar Water Heating Systems
We are blessed with Solar Energy in abundance at no cost. The solar radiation incident on the surface of the earth can be conveniently utilized for the benefit of human society. One of the popular devices that utilize the solar energy is solar hot water system (SHWS). Solar water heating system is a device that uses solar energy to heat water for domestic, commercial, and industrial needs.
Working of a solar water heater
The system is generally installed on the roof or open ground, with the collectors are glazed on the sun facing side to allow solar radiation to come and connected to a continuous water supply. A black absorbing surface (absorber) inside the flat plate collectors absorbs solar radiation and transfers the energy to water flowing through it. Heated water is collected in the tank which is insulated to prevent heat loss. Circulation of water from the tank through the collectors and back to the tank continues automatically due to density difference between hot and cold water (thermosyphon effect).Water flows through the tubes, absorbs solar heat and becomes hot. The heated water is stored in a tank for further use. The water stored in the tank remains hot overnight as the storage tank is insulated and heat losses are small.
Parts of the Solar Water Heating System
A solar water heating system consists of a solar collector, a storage tank kept at a height behind the collector, and pipelines. The collector usually comprises copper tubes welded to copper sheets (both coated with a highly absorbing black coating) with a toughened glass sheet on top and insulating material at the back. The entire assembly is placed in a flat box.
Storage Tanks
Most solar water heaters require a well-insulated storage tank. Solar storage tanks have an additional outlet and inlet connected to and from the collector. In two-tank systems, the solar water heater preheats water before it enters the conventional water heater. In one-tank systems, the back-up heater is combined with the solar storage in one tank.
Solar Collectors
Three types of solar collectors are used for residential applications:
Flat-plate collector: It is the heart of a solar water heating system. It consists of an absorber plate which is coated on its sun facing surface with an absorbent coating, also called selective coating. The absorber consists of a grid of metallic tubes and sheets. Water flows through the tubes. Sheet absorbs the solar radiation falling on it and transfers it to water. The absorber plate is placed in a top open box to protect it from weather. The space between back and sides of the absorber and the box is filled with insulation to reduce heat losses. The front of the box is covered with a high transmittance glass plate. Flat plate collectors are the most common type used in the domestic solar water heating systems in India as they are relatively cheaper when compared to others.
Glazed flat-plate collectors are insulated, weatherproofed boxes that contain a dark absorber plate under one or more glass or plastic (polymer) covers. Unglazed flat-plate collectors typically used for solar pool heating — have a dark absorber plate, made of metal or polymer, without a cover or enclosure.
Integral collector-storage systems: Also known as ICS or batch systems, they feature one or more black tanks or tubes in an insulated, glazed box. Cold water first passes through the solar collector, which preheats the water. The water then continues on to the conventional backup water heater, providing a reliable source of hot water. They should be installed only in mild-freeze climates because the outdoor pipes could freeze in severe, cold weather.
Evacuated-tube solar collectors: Evacuated Tube Collector is made of parallel rows of transparent borosilicate glass tubes evacuated for providing insulation. Each tube contains a glass outer tube and metal absorber tube attached to a fin. The fin’s coating helps absorption of solar radiation and transfers the heat to the water which flows through the inner tube.
Types of Solar water heating systems
Solar water heating systems are of two categories: They are Indirect/closed loop system and Direct/open loop system.
Indirect/Closed Loop System: Here the heat exchangers are installed to protect the system from hard water obtained from bore wells or from freezing temperatures in the cold regions. After collecting heat in the panels, the heat transfer fluid flows through a heat exchanger, transferring its heat to the potable hot water. When the panels are cooler than the storage tank or when the storage tank has already reached its maximum temperature, the controller in closed-loop systems stops the circulation pumps.
Direct/open loop system: The basic solar thermal models are the direct-gain type, in which the potable water is directly sent into the collector. Many such systems are said to use integrated collector storage (ICS), as direct-gain systems typically have storage integrated within the collector. Heating water directly is inherently more efficient than heating it indirectly via heat exchangers, but such systems offer very limited freeze protection (if any), can easily heat water to temperatures unsafe for domestic use, and ICS systems suffer from severe heat loss on cold nights and cold, cloudy days.
Features of Solar Water Heating System
· Solar Hot Water System turns cold water into hot water with the help of sun’s rays.SWH provides hot water for homes, hostels, hotels, hospitals, restaurants, dairies, industries etc.
· Can be installed on roof-tops, building terrace and open ground where there is no shading, south orientation of collectors and over-head tank above SWH system.
· SWH system generates hot water on clear sunny days (maximum), partially clouded (moderate) but not in rainy or heavy overcast day. Only soft and potable water can be used.
· Stainless Steel is used for small tanks whereas Mild Steel tanks with anticorrosion coating inside are used for large tanks.
· Solar water heaters (SWHs) of 100–300 litres capacity are suited for domestic application. Larger systems can also be used for a variety of industrial applications.
· Around 60 deg. — 80 deg. C temperatures can be attained depending on solar radiation, weather conditions and solar collector system efficiency.
· Fuel Savings: A 100 litres capacity SWH can replace an electric geyser for residential use and saves 1500 units of electricity annually.
· Saves cost on power generation: The use of 1000 SWHs of 100 litres capacity each can contribute to a peak load saving of 1 MW.
· Environmental benefits: A SWH of 100 litres capacity can prevent emission of 1.5 tonnes of carbon dioxide per year.
· Life: SWHs have a life span of 15–20 years.
· Approximate cost: Rs.15000- 20,000 for a 100 litres capacity system and Rs.110–150 per installed litre for higher capacity systems.
· Payback period: 3–4 years when electricity is replaced 4–5 years when furnace oil is replaced 5–6 years when coal is replaced. The payback period depends on the site of installation, utilization pattern and fuel replaced.