Solar Water Heating Explained
The knowledge of using the Sun’s rays to heat water has been known for thousands of years and is easy to understand. A hosepipe lying on a lawn will absorb solar energy and heat any water inside it. This is the basic principle of all modern solar collectors, the major difference being their ability to concentrate the energy reaching the liquid and their ability to massively reduce heat loss.
Hot water consumes a huge percentage of our annual fuel bill, about 40% of the average household energy bill. As natural gas production declines and electricity prices climb the cost of hot water is expected to rise dramatically. If you retrofit your home for solar hot water you could save a lot of money and also help to alleviate the strain on the energy production cycle as a whole.
Before we look in detail at the solar hot water systems available, let’s look at the most common hot water systems in use today. Especially as you may need to integrate your solar hot water system with your conventional system.
CONVENTIONAL HOT WATER SYSTEMS
In most homes, hot water is provided by a conventional gas or electric storage water heater. The storage water heater consists of a free-standing water tank, a reservoir that usually holds from 100 to 500 liters of hot water depending on the size of the household. The tank itself usually consists of an internal storage vessel made from glass, or enamel. It is encased in a steel envelope with a layer of insulation on the outside.
In electric storage water heaters, water is heated by two electric resistors, known as heating elements. The heating elements are positioned at the bottom of the tank and they generate heat when electricity flows through them, very similar to the heating elements of an electric kettle.
Gas powered water heaters use a burner to heat the water as it passes through a copper coil. The burner is connected to a thermostat and ignites when the temperature of the water in the tank drops below a certain point. Because the combustion of natural gas and propane produce carbon dioxide, carbon monoxide, and nitrogen dioxide, which are potentially poisonous to people, gas powered heaters must be vented. Venting is accomplished by a flue pipe that directs hot combustion gases and pollutants through the ceiling. In most gas-fired water heaters, combustion air that feeds the burner comes from room air. There are newer, safer and more efficient power-vented water heaters on the market now. These types include a fan that draws outside air into the combustion chamber and forces combustion gases out through a vent. This is important because bringing outside air in helps prevent leakage through cracks in the building envelope that reduces the energy-efficiency of the house. Secondly it prevents spillage of combustion gases because the combustion chamber is closed. Power-vented water heaters are not only safer, but they are on average about 10% more efficient.
PROS AND CONS OF STORAGE WATER HEATERS
The drawbacks are threefold:
1. They don’t last very long. They need replacing every five to ten years unless you take steps to ensure a longer life span.
2. They use electricity, natural gas and propane, fuels that are finite and will be subject to steep declines and therefore steep price increases in the coming years, starting from now.
3. Perhaps the biggest problem with this type of water heating is that it maintains a large amount of hot every hour of every day of the year. Heat leaks out of tanks between uses, this is called standby losses. This is a big a waste of energy, which some have likened to keeping your car running the garage just in case you might want to use it.
TYPES OF SOLAR HOT WATER SYSTEMS
There are three primary types of solar systems for heating water; batch collectors, flat plat collectors and evacuated tube collectors. We will focus on flat plate collectors and evacuated tube collectors as they are the most efficient systems, they are certified for installation and for receiving a grant from the Junta de Andalucia where applicable.
FLAT PLATE COLLECTORS
Flat plate collectors incorporate a highly absorptive selective metal plate (usually anodized aluminium or copper) backed with copper tubing.
They are usually mounted on a roof, or the ground with maximum exposure to the sun.
The best position is facing south and tilted at an angle of 45 degrees in this latitude. Water is circulated through the tubing by a small electric pump and then to a storage tank for showers, washing machine, taps and all other uses for hot water is. The systems generally require sensors. The simple reason being that you don’t want your pump running if the water in the collector is cooler than the water in your hot water tank.
SPLIT PRESSURISED SYSTEMS
Systems utilizing flat plate collectors can be further sub-divided into Compact systems, where the tank is integral with the panels and usually does not involve a pump. The other type is a Split Pressurised System, this means the tank is separate from the panels and does utilize a pump to circulate the water between the copper coil in the tank and the flat plate collector on the roof. You can see the typical configuration of a pressurized split system in the diagram above. When water flows though the coil it absorbs heat from the water inside the solar collector. One advantage of this system is that almost no water is introduced into the tank (apart from occasional topping up from the automatic feed valv), so corrosion and scale buildup are almost totally eliminated. A well insulated storage tank will keep the water hot even if the weather is cloudy for a couple of days.
COMPACT SYSTEMS
In a Compact system the storage tank is usually horizontally mounted immediately above the solar collectors on the roof. No pumping is required as the hot water naturally rises into the tank.
Typically these systems will function by natural convection to transfer the heat energy from the collector to the tank.
They are usually cheaper to purchase and quicker to install as there are less components in the overall system and less connections to be made.
EVACUATED TUBE COLLECTORS
Evacuated tube collects are the newest of the technologies for collecting hot water. Evacuated tube collectors consist of a series of long transparent glass tubes. Inside each tube is a copper pipe called the absorber tube. The absorber tube is covered with an absorbent material to collect the heat from the sun. Inside the inner tube there is a fluid designed for heat transfer, usually methanol. When the tube is manufactured the air in the outer tube is pumped out creating a vacuum (as in evacuated). Vacuums are very poor conductors of heat. Therefore the heat in the inner tube once captured cannot radiate out again and therefore a higher percentage of the captured heat is retained making them more efficient then conventional flat plat collectors.
The advantage of evacuated tube collectors is that because of their greater efficiency they can work well on cloudy days and in colder climates. Some manufacturers claim they will capture as much as 80% of the available radiant energy. Another advantage of these types of systems is that the vacuum in the tube prevents condensation from collecting, which sometimes happens in flat plate collectors.
The disadvantage is that these systems are more expensive, though they are rapidly becoming cost competitive with more traditional flat plate collection systems and they are not really needed in the warmer climate of southern Spain.
More information on solar hot water can be found at Wikipedia
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