Solar Water Heater

Obtaining hot water is energy intensive. In developing countries, households often spend a large portion of their energy budget on heating water. Solar thermal water heaters are a sustainable solution for poor households as they allow the barrier of high upfront costs to be overcome.[

The technology of solar thermal water heaters is present worldwide and significant deployments are already occurring in emerging economies and developing countries. Regions that do not experience freezing temperatures can use the simplest and most cost-effective kinds of this technology. In fact, more than 90% of systems worldwide are based on the thermosiphon principle (for a definition see below regarding passive systems).

A SWH with a water tank in Bolivia. (Picture: GIZ Energising Development Bolivia)

The Technology

Solar water heating (SWH) systems are typically composed of:

• Solar thermal collectors (flat plate or evacuated tube)
• A storage tank
• A circulation loop.

State of the art solar water heaters incorporate features such as: selective surface absorbers, anti-reflective glazing, well-designed collector arrays, and efficient storage systems thereby achieving operation efficiencies of the order of 35 to 40%. Even the simplest types allow households to have convenient access to hot water..

Active vs. Passive Systems

SWH can be either active system or pasive systems:

Active system: collector, tank, pipe and controller

• Active systems use either pumps to circulate water or a heat transfer fluid which utilizes electrical components (e.g. a controller). There are two types of active solar water-heating systems:
  

1. Direct-circulation systems use pumps to circulate pressurized potable water directly through the collectors. These systems are appropriate in areas that do not freeze for long periods and do not have hard or acidic water.
   
2. Indirect-circulation systems pump heat-transfer fluids through collectors. Heat exchangers transfer the heat from the fluid to the potable water. Some indirect systems have "overheat protection" which is a method of protecting the collector and the glycol fluid from becoming super-heated when the load is low and the intensity of incoming solar radiation is high.
   

Passive system: collector and tank

• Passive systems transfer and circulate heat naturally. Passive solar water heaters rely on gravity and the tendency for water to naturally circulate as it is heated. Because they contain no electrical components, passive systems are generally more reliable, easier to maintain, and possibly have a longer work-life than active systems. The two common types of passive systems are described below:
  

1. Integral-collector storage systems or batch systems consist of a tank that is directly heated by sunlight. These are the oldest and simplest solar water heater designs and are good for households with significant daytime and evening hot-water needs. However, they do not work well in households with predominantly morning usage because they lose most of the collected energy overnight. These solar collectors are suited for areas where temperatures rarely go below freezing.
   
2. Thermosyphon systems are an economical and reliable choice. These systems rely on the natural convection of warm water rising to circulate water through the collectors and to a storage tank located above the collector. As water in the solar collector heats, it becomes lighter and rises naturally into the tank above. Meanwhile, the cooler water flows down the pipes to the bottom of the collector, thereby enhancing the circulation. Indirect Thermosiphon systems use a glycol fluid in the collector loop as a heating medium.
   

Most solar thermal systems installed in developing countries are thermosiphon systems.

Collectors

Flat Plate Collector

A flat plate is the most common type of solar thermal collector and is typically used as a solar hot water panel to generate hot water. A weatherproofed, insulated box containing a black metal absorber sheet with built-in pipes is placed in the path of sunlight. They can be deployed on the roof of buildings or on the ground. Solar energy heats the water in the pipes causing it to circulate through the system by natural convection. The water is then usually passed to a storage tank located above the collector.

There are many flat-plate collector designs but generally all consist of:

1. a flat-plate absorber which intercepts and absorbs the solar energy,
   
2. a transparent cover that allows solar energy to pass through but reduces heat loss from the absorber,
3. a heat-transport fluid (air, antifreeze or water) flowing through tubes to remove heat from the absorber and
   
4. a heat insulating backing.

One flat plate collector is designed to be evacuated, therefore preventing heat loss. The absorber can be made from a wide range of materials, including: copper, stainless steel, galvanised steel, aluminium and plastics. When choosing an absorber material it is important to ensure that it is compatible, from the point of view of corrosion, with the other components in the system and with the heat transfer fluid used. The absorber must also be able to withstand the highest temperature that it might reach on a sunny day when no fluid is flowing in the collector (known as the stagnation temperature).

The fluid passageways of the absorber may consist of tubes bonded to an absorbing plate or may form an integral part of the absorber. Experience has shown that the simple mechanical clamping of tubes to an absorber plate is likely to result in an absorber with a poor efficiency. A good thermal bond, such as a braze, weld or high temperature solder, is required for tube and plate designs as it ensures efficient heat transfer from the absorbing surface into the fluid.

Matt black paints are commonly used for absorber surfaces because they are relatively cheap, simple to apply and may be easily repaired. Paints, however, have the disadvantage that they are usually strong emitters of thermal radiation (infrared) and at a high temperature this results in significant heat losses from the front of the collector. Heat losses from the collector can be substantially reduced by the use of absorber coatings known as 'selective surfaces'. These surfaces may be applied by electroplating or by dipping a metal absorber in appropriate chemicals to produce a thin semi-conducting film over the surface. The thin film will be transparent to solar radiation while simultaneously appearing opaque to thermal radiation. However, these surfaces cannot be produced or applied easily.

Flat-plate collectors usually have a transparent cover made of glass or plastic. The cover is required to reduce heat losses from the front of the collector and to protect the absorber and the insulation from the weather. Most covers mimic a greenhouse environment. They permit solar radiation to pass into the collector while also absorbing the thermal radiation emitted by the hot absorber.

At night it is possible for the collector to lose heat by radiation resulting in the circulation being reversed and the water cooling. This can be overcome by use of a suitable non-return valve. However, there is a danger with solar collectors when used under clear night conditions (e.g. in arid and semi-arid regions) that they can actually freeze even when the ambient temperature is above freezing point. In such conditions it may be necessary to have a primary circuit through the collector filled with antifreeze and a separate indirect hot water cylinder where the water from the collector passes through a copper coil to heat the main water supply. This problem will only apply in certain desert regions during the cold season or at high altitudes in the tropics and sub-tropics.

Evacuated Tube Collector

Evacuated-tube solar collectors tend to be more efficient but also more expensive. They are used more frequently for commercial applications in the U.S.A. A collector is made up of several pipes in parallel rows that are connected at the top. The tubes consist of a vacuum with a pipe running through the middle containing the working fluid. The water moves up and down and along through the series of pipes to exit the system at a significantly higher temperature. The heat loss is significantly reduced thanks to high negative pressure in the glass tubes. They can be deployed nearly horizontally on flat roofs.

Installation and Maintenance

Basic rules for a good installation include: a preliminary study and a needs assessment to determine the best size for the specific installation and a location that is  well-exposed to the sun with no shading (from nearby buildings, vegetation, etc.). 

The following data is required to design, size and select a solar water heating system:: daily hot water requirement (litres/day), average insolation (kWh/m2 day), water quality and storage requirements.

Find more technical details on SWH: http://www.geni.org/globalenergy/research/solar-water-heaters/solar-water-heater.pdf

Solar thermal systems are relatively maintenance free and only require  an  inspection of the piping for leaks and the cleaning of the collectors on an occasional basis. In some regions, it may also be necessary to inspect the transfer fluid for freeze protection and to remove the build-up of lime scale that chokes the collector and tank recirculating pipes over time.

Advantages and Disadvantages of Thermosiphon SWH

Advantages and disadvantages of thermosiphon Solar Water Heaters

Advantages	Disadvantages
very simple systems	only applicable in regions without frost
very efficient systems	(aesthetic aspects)
very cheap
no electricity, no pump, no controller
self-controlling systems

Costs

Solar water heating systems are usually more expensive than conventional water heating systems when electricity is available. However, a solar water heater can save money in the long run.

How much money can be saved depends on:

• the amount of hot water needed,
• the performance of the system,
• the geographic location and solar resource availability ,
• available public financing and incentives,
• the cost of fuels (e.g. natural gas, oil, electricity, biomass) otherwise used for heating water,
• the cost of the fuel used for the backup water heating system (if existing).

SWH are very efficient, and can reduce either the electricity needs or reduce the costs for heating water by nearly 75% annually.

Initial costs for a solar thermal system vary among countries and depend on the quality of the solar collector, the labour costs of installation, and also on the climate conditions in which the solar thermal systems works. Initial costs depend on the level of governmental support for SWH as well as the development of local industries. Quality of the solar collector: Lower quality products are assumed to have a life expectancy of only half of higher quality products.

See details for different system sizes within the chapter about households, social institutions and productive use.