Solar energy is a gift that is vastly underutilized by humans. As oil and natural gas become increasingly rare and therefore expensive, more and more people will turn to the sun to meet their energy needs. We will use the suns energy to heat our homes and to produce electricity. Many will use the sun’s massive output of energy to heat water for use in our homes. Click here to see a video on the history and future of Solar Hot Water.
Hot water consumes a huge percentage of our annual fuel bill, about 20% of the average household energy bill according to the US Department of Energy. As natural gas production declines and 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 the solar hot water systems available, let’s look at the most common hot water systems in use today. Especially as you will need to integrate your solar hot water system with your conventional system.
CONVENTIONAL HOT WATER SYSTEMS
In most homes, hot water is provided by conventional gas or electric storage water heater. The storage water heater consists of a free-standing water tank, a reservoir that holds 40 to 80 gallons (roughly 150 to 303 liters) of hot water. The tank itself consists of an internal storage vessel made from glass. It is encased in a steel envelope with a layer of insulation in between.
In electric storage water heaters, water is heated by two electric resistors, known as heating elements. The heating elements are positioned at the bottom and the top 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 at the bottom of the tank. 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 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 seven to ten years unless you take steps to ensure a longer life span.
2. They use natural gas and propane, two fuels that are finite and will be subject to steep declines in the coming years, starting now. There are many studies to suggest that declines in natural gas supplies will be very steep. http://canada.theoildrum.com/node/3095
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 too 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. A much better alternative is the tankless water heater.
TANKLESS WATER HEATER
Tankless water heaters heat water instantly when it’s needed and because they don’t expend energy to heat water when it’s not needed they use about 20% less energy. For example when someone turns on a hot water tap a water flow sensor in the tankless water heater sends a signal to open the gas valve and ignite the gas in the combustion chamber which heats the water. As soon as the tap is turned off the water flow through the heater ceases and the flame goes out.
Pros and Cons of Tankless Water Heaters
1. Slightly higher purchase cost than storage water heaters.
2. Difficulty of installation is slightly more than for a conventional storage water heater.
1. They only heat water as it is needed,therefore they are about 20% more efficient than storage water heaters.
2. Due to the fact that there is no standby heat loss there is less waste heat to deal with, which can be an issue in summer.
3. They last longer than storage water heaters, typically they are designed to last for twenty years.
4. They are also easy to repair. If a part fails it can be replaced fairly easily without replacing the whole unit as is often the case with storage water heaters.
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. Each has specific advantages and disadvantages depending upon the climate you live in and the demands you have for hot water. These will be explained below.
Batch collector systems are very popular in tropical or warm climates. They usually consist of one or more water tanks that sits on your roof instead of your basement or garage and they use the heat of the sun directly to warm the water. Most batch systems use one or two hot water tanks of between 20 and 40 gallons. These sit in glass enclosed containers which capture the heat of the sun. The plumbing for these types of systems is pretty simple. Cold water enters the tank on the roof. It is heated by the sun and then is drained off as hot water is needed. They are sometimes referred to as direct or open-loop systems.
Batch type solar water heaters tend to be used only in warmer climates because they are susceptible to freezing. One option is to drain your tanks during the winter but getting on your roof in the middle of winter to drain your tanks is not high on most people’s favorite things to do.
If you live in a very warm climate such as California or Arizona, you can actually find that batch type collectors can heat the water to too high a temperature (over 160 degrees Fahrenheit). In these situations you might also need a mixing or tempering valve which allows the overly heated water to be mixed with cool water in order to obtain an optimal temperature. Most commercial batch hot water systems come be purchased with sensor driven valves of this type.
Flat Plate Collectors
A more common type of solar hot water system are those that use flat black metal plates to collect heat. The heat can then be transferred to heat the hot water. early solar hot water systems used an electric pump to circulate hot water over the plate to collect the heat. The problem with this approach is that the water in the tubes can freeze in winter. To solve this problem some newer systems circulate a freeze proof liquid, think antifreeze, over the solar plates and then use a heat exchanger to transfer that heat to your hot water tank. Such systems are a bit more expensive but have a long life cycle.
Some solar hot water systems are actually hybrid systems in that the use the sun directly to heat the water, but also indirectly by adding photovoltaic panels to run the pumps that pump the fluid over the collector plates. These can add even greater efficiency to the system.
Hot water 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.
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 great thing about 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. Being a fairly new technology reliability data is rather limited but this is definitely a technology to keep your eye on.
Solar Hot Water Government Grants and Rebates
Australia – Department of the Environment – Solar Hot Water Rebate Program
Canada – Natural Resources Canada – Retrofit Your Home and Qualify for a Grant!
United Kingdom – Search for grants and offers from the Energy Saving Trust