Solar water heating basics

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The collector plat

Modern solar heating systems can keep swimming pools warm, heat your home’s water and heat your home’s interior space. Their popularity is increasing for several reasons. Solar heating systems are cost saving, reliable, adaptable, and pollution-free because they use renewable energy from the sun. Modern systems include sleek, attractive, low-relief collectors that blends in with the look of the modern house. Did you know that modern solar heating systems work well even in winter?
A solar heating system is a rewarding investment. It can cover 100% of your monthly heating bill, ensure hot water during power failures and increase your property value. When you purchase a solar heating system you are making a conscious, responsible decision to help reduce harmful emissions from fossil fuels, while maintaining your quality of life.
What is solar heating?
A solar heating system collects the sun’s energy to heat water or another heat transfer fluid. The water or other fluid then transfers solar heat directly or indirectly to your home, water, pool or industrial process. Solar water heaters are a very good investment. Although solar water heaters cost more initially than conventional water heaters, the fuel they use—sunshine—is free. To take advantage of solar energy you need to have an unshaded area, such as a roof, that faces north, northeast or northwest.
What are the basic components of a solar thermal system?
Solar water heaters and solar space heaters are made up of solar collectors, and all systems except pool heaters have some kind of storage. In pool systems, the swimming pool itself is the storage, and the pool’s filtration pump circulates the pool water through the collectors. Active systems also have circulating pumps and controls, passive systems work without this added equipment. Two types of solar collectors are used for residential applications: flat-plate and evacuated-tube collectors.
Flat-plate collectors are traditionally the most common type because they are easy to manufacture. Glazed flat-plate collectors essentially are weatherproofed boxes that contain a dark absorber plate under one or more glass or plastic (polymer) covers.
Evacuated-tube solar collectors can be seen as the new generation of solar collectors. They are made of a copper “manifold” where the water flows through and a number of evacuated tube heat pipes connected to it. Figure 1 shows a 24 heat-pipe Evacuated-tube solar collector.

Figure 1. An installed ITS-58/1800-24 Evacuated tube solar collector.
Evacuated tube solar collectors convert direct and diffused solar radiation into heat. Infra red rays, which can pass through clouds, are also absorbed and converted into usable heat. The collectors efficiently collect and transfer this energy through a special collector plate and a rapid heat transfer channel ‘the heat-pipe’, situated in an evacuated glass tube, to a highly insulated manifold heat exchanger –see Fig 2 and Fig 3.

e has a special wavelength ‘selective’ coating using a semi-conductor layer. This special absorber plate converts the maximum amount of solar radiation into heat whilst having very low radiation losses. The heat-pipe has a very low heat capacity but an exceptionally rapid conductivity and therefore is a very efficient and speedy heat conductor.
It also provides the system with a diode function i.e. heat transfer is always in one direction – from the absorber to the water and never the reverse. The special fluid inside the ‘heat-pipe’ evaporates when heated, transferring heat energy to its top. The vapor condenses inside a special heat exchanger (which is located within a water manifold) and the fluid then returns to its original position, at the bottom of the ‘heat-pipe’, due to gravity and the cycle is repeated continuously. The vacuum in the glass tube, being the best possible insulation for a solar collector, suppresses heat losses and also protects the absorber plate and the ‘heat-pipe’ from external adverse conditions. This results in exceptional performance far superior to any other type of solar collector. Due to the huge growth in the solar heating market over the last number of years, this cutting edge technology has come down drastically in price. Table 1 gives a comparison between flat-plate collectors and evacuated-tube collectors.
How does a ETHP work – Click Here!
Evacuated Heat Pipe Tubes
Flat-plate Solar Panels
The collector is hermetically sealed inside an evacuated glass tube, eliminating convection and conduction heat losses and isolating the collector from adverse ambient conditions. Therefore, no heat losses due to convection and conduction and no change of performance during the service life of the collector due to corrosion. The collector is put in a casing with a glass shield to reduce heat losses. The air gap between absorber and cover pane allows heat losses to occur, especially during cold and windy days. Build up of condensation will in due course influence the collector greatly due to corrosion, reducing performance and durability.
Uses a heat-pipe for super efficient heat conduction. No water enters into the collector. Circulates water inside insulated areas. Prone to leakage, corrosion and restriction of flow due to possible air lock.
Thermal diode operation principle. The heat pipe’s thermal flows one way only; form the collector to the water and never in the reverse. Flat-plates can actually rob the water of built up heat if the collector becomes colder than the water temperature.
Corrosion and freeze free; there is nothing within the evacuated tube to freeze and the hermetic sealing of each tube eliminates corrosion. Flat-plate collectors contain water and unless well protected can burst upon freezing. Corrosion can become a major problem reducing performance!
Easy installation and no maintenance. Lightweight individual collector tubes are assembled into the system at the point of installation. Each tube is an independently sealed unit requiring no maintenance. Installation is difficult. Entire panels have to be hoisted onto the roof and installed. If one has a leak, the entire collector has to be shut down and removed.
Relatively insensitive to placement angle, allowing architectural and aesthetic freedom. Requires accurate northern exposure and elevation placement.
Table 1. Comparison between Evacuated Tube and Flat-Plate solar collectors.
Solar Storage tanks are basically geysers with an additional outlet and inlet that gets connected to and from the solar collector. Most types also contain a backup heater should the need arise. Another difference between solar storage tanks and a standard geyser is that they are usually much better isolated. This ensures that the water temperature is kept even through the coldest night. A normal geyser can be converted to a solar storage tank by connecting a special valve to it that provides you with the extra inlet and outlet ports.
The collectors can be connected to the storage tank in various ways. The main systems on the market are Closed-Coupled systems and Split systems.
Close-Coupled systems consist of roof-mounted solar collectors, combined with a storage tank located immediately above the collectors. The main disadvantage of this system is out of a esthetic point of view…you now have a big tank mounted on top op your roof.
Split Systems consist of the solar collector on the roof and the storage tank inside the roof. Water circulation between the storage tank and collector can be done in two ways:
  • Thermosyphon circulation relies on the natural rising of warm water. This however requires that the base of the storage tank be at least 300mm above the collector’s top which is not always possible in modern roofs.
  • Forced circulation uses an electronically controlled electric pump to circulate the hot water from the collectors to the storage tank. The pump used very little power and can even be run from a photovoltaic panel. Forced systems allow for maximum efficiency and flexibility as the storage tank can now be placed wherever. It also provides freezing and overheating protection.
All of the above systems can be configured to be direct or indirect circulation systems. Direct circulation systems directly heat the water in the system. Indirect systems are used in areas where the water is hard and corrosive or in extreme cold areas. Indirect systems usually use non-freezing heat transfer fluid that is circulated through the collectors and a heat exchanger inside the storage tank. Thus the water in the storage tanks is indirectly heated.
How much money will my solar thermal system cost, and how much will it save?
As a practical example lets consider a house that already has a 200 liter geyser. An Evacuated-tube collector suited for this size geyser (18 pipe for Cape Town) using thermosyphon circulation will cost you around R9500 (For a forced circulation systems with electronic control and digital display you can add about R2000).
Evacuated-tube solar heating systems are designed to provide you with more than 25 years of service. Using solar energy information obtained from NASA and measurements we have done on the system we can calculate the average daily kWh collected from the sun by this system. Using projected Eskom tariffs it can be calculated that the system will save you more than R33 000 over 10 years in electricity fees.
We are also very likely to see tax rebates for households with solar water heating systems in the near future.
How much will your solar heating system help the environment?
On average for every 1kWh of energy produced by a coal power station, 0.966kg of carbon dioxide is produced. Now the Solar water heater that we used in the cost saving calculation will provide you with a yearly average of over 2400 kWh. Therefore, you will personally be responsible for saving our environment of another 2320 kg of carbon dioxide per year. Carbon dioxide traps heat in our atmosphere, contributing to the greenhouse effect, which alters our planet’s climate and ecological systems. Using solar energy in place of nonrenewable fuels may also reduce nitrous oxides and sulfur dioxides, which are components of smog.
What are the maintenance issues and repair costs for an Evacuated tube solar thermal system?
Since the collector’s manifold and piping to it is in essence just a copper pipe loop to and from your geyser the same maintenance will apply as to your normal household copper pipe plumbing. The Evacuated-tubes should periodically be wiped for optimal performance. The tubes are very durable and can handle hailstones as large as 25mm. If a tube should get damaged for some or other reason it can easily be replaced by even a person with minimal skills and the tubes are very cheap. The water does not need to be disconnected since it only flows through the manifold section.
John Ruskin wrote the following lines:
It’s unwise to pay too much, but it’s unwise to pay too little. When you pay too much, you loose a little money that’s all. 
When you pay too little, you sometimes lose everything, because the thing you bought was incapable of doing the thing you bought it to do.
The common law of business balance prohibits paying a little and getting a lot… it can’t be done. If you deal with something cheaper, it’s well to add something for the risk you run. And if you do that, you will have enough to pay for something better!
www.its-solar.com

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