Solar Water Heating
How Can Solar Water Heating Save Money?
Solar hot water systems convert the heat in sunlight into usable hot water for your home or business. In most cases, a solar hot water system cannot provide all of the hot water your home requires, but it can certainly meet a large percentage of your needs and significantly reduce energy bills.
Solar energy solutions are very eco-friendly and sustainable alternatives to more conventional natural gas or electrical options, but the cost of solar technology can be a barrier for many customers. Amortising the cost of installing a system over the useful lifetime of the equipment can help put solar technology in a more favourable light. Taking the time to calculate a realistic return on investment can provide an even clearer picture.
Owning and operating a solar water heater with a gas or electric backup is markedly less expensive than relying on a gas or electric water heater alone. A solar water heating system offers long-term performance with very low maintenance costs. A system will typically last for 30 years. Although results vary by region, homeowners can expect to save 60 to 70 percent of water heating costs per year by switching to a solar water heater.
Based solely upon the current cost of utilities, a solar water heating system can reasonably be expected to pay for itself within a 10 year period. That leaves 20 years of additional savings. These figures do not even include periodic rate increases for gas or electric. Unlike conventional energy costs, the price of sunlight remains a constant zero year after year. Unlike gas or electric, there will never be a shortage of sunshine.
How Does Solar Water Heating Work?
Although it is not strictly accurate, it is helpful to think of a solar water heater as two separate pieces. The first piece is the solar collector, which gathers the energy from sunshine to heat the water, and the second piece is the system for delivering hot water to the residence.
There are three different types of solar collectors commonly used for residential applications.
Flat Plate Collectors
A flat plate collector is a large, shallow metal box with a glass or polymer top. These collectors resemble solar panels, but have a deeper design. The sides and bottom of the box are insulated, and box contains a black absorber plate. The absorber plate is usually made of a highly conductive metal to facilitate the transfer of heat. Copper or aluminium is the most common material used for an absorber plate, but some designs use a polymer plate. Copper is much more expensive than aluminium, but it has superior conductivity and better corrosion resistance than the other metal. The absorber plate is often coated with material specifically designed to draw in and retain heat.
Flat plate collectors are usually placed on the roof of a home, and piping is attached to the absorber plate. When sunlight passes through the polymer or glass top layer, which is called glazing, it is absorbed by the black absorber plate. A white plate would reflect most of the energy back up to the glazing, but dark plates absorb most of the energy. As the sunlight heats the absorber plate, heat is transferred to the fluid in the attached piping.
Integrated Collector Storage Systems or Batch Systems
Integrated collector storage systems, often called ICS systems, are similar to flat plate collectors, but the glazed box contains a series of large black tubes instead of an absorber plate. These tubes hold a reservoir of fluid that is heated when sunlight is absorbed by the black walls of the tubes. They function very similarly to flat plate collector systems, except that much larger amounts of liquid are retained in the tubes inside the glazed box.
ICS systems are often called batch systems because the contents of the tanks gradually lose heat overnight and are re-heated each solar day in a batch process.
The fluid from an ICS system is usually delivered to a conventional hot water tank inside the home, and the pre-heating results in greatly reduced operating costs for the conventional hot water heater.
Evacuated Tube Collectors
Evacuated tube collectors are a combination of both previous types of solar collectors. They contain multiple parallel rows of metal tubes and energy-absorbent fins. Each tube and fin is enclosed in an evacuated glass sleeve. The fins act as miniature heat absorber plates and transfer absorbed solar energy to the contents of the tubes.
The evacuated glass sleeves function as a thermos bottle and inhibit heat loss from the metal tubes. The vacuum in the sleeves eliminates the effect of ambient air temperature. Evacuated tube collectors greatly reduce radiated heat loss and outperform flat plate collector systems in cloudy weather, but the evacuated sleeve components significantly increase the cost of these systems.
After solar heat is collected, it must be transferred to the household water supply. This is accomplished by either active or passive methods. Active methods use an electric pump to actively circulate either a heat transfer fluid that has been heated in the collector system, or water that is heated directly in the collector system. Passive methods rely on gravity and thermal gradients to deliver the heated water. There is no active circulation device.
Active Solar Water Heating Systems
Active solar water heating systems can be either open loop systems, which are sometimes called direct systems, or closed loop systems, which are sometimes called indirect systems.
- Open loop systems circulate household water directly through the solar collector. These techniques work well in regions where the temperature never drops below the freezing point of water.
- Closed loop systems circulate a heat transfer fluid, such as a glycol, through a closed loop that comes into contact with household water. The closed loop usually includes a coil of tubing on the inside of a water storage tank. As the heated fluid circulates from the solar collector through the coiled tubing in the storage tank, heat is transferred through the walls of the tubing to the colder water inside the tank. The household water supply is never exposed directly to outdoor conditions that may, even with a well-insulated collector, allow water pipes to burst if the weather should drop below freezing.
Passive Solar Water Heating Systems
Passive systems are less expensive than active systems because there is no cost to circulate water or a heat transfer fluid. They are usually less efficient than actively circulating systems, but they have no electric components and are generally considered to be more reliable and durable than active systems.
Integrated Collector Storage Systems
The ICS heat collector systems described above are passive, open loop systems. Water heated by solar energy is stored directly at the collector and is drawn into an interior hot water tank on demand. The solar-heated inflow of water to the conventional hot water tank reduces, and at times even eliminates, the need for additional heating.
ICS systems work best in regions where the temperature rarely drops to the freezing point. They are also the most beneficial for residences where hot water demand is highest during the daylight hours.
Thermosyphon systems make use of the physical relationship between water density and temperature. Warm water is less dense than cooler water. It will rise to the surface of a container, and cooler water will sink to the bottom. Careful design can create a system where the temperature of the water causes circulation in a limited area. Hot water is stored in a tank directly above the solar collector, and cooler water is continually forced through the circuit to the solar collector for additional heating. Household hot water is drawn from the upper, hotter tank.
Thermosyphon systems are more expensive to install initially, but they have no electric parts and are quite durable. They outperform ICS systems because the upper thermosyphon tank is extremely well-insulated. While batch systems lose heat overnight, thermosyphon systems generally maintain hot water supplies until the following day.
Solar water heating usually requires a conventional hot water system as a backup for cloudy days or for times when demand outstrips the ability of the solar collector to provide hot water. They are often paired with tankless water heaters that eliminate the need to maintain a tank of heated water.