With the increase in electricity and gas prices, the need for renewable sources of energy is growing. Heating water accounts for nearly one-third of a typical home’s energy bill.
Using the sun’s energy to heat water is not a new idea. More than a hundred years ago, water tanks, painted black, were used as simple solar water heaters in Europe. Solar water heating technology has greatly improved during the past century.
Well designed, installed, and maintained solar water heating system can lessen your water heating bills by 50%-80%. In some areas of Southern Ontario, they can supply almost all of home’s hot water requirement. The actual amount of money that can be saved depends on the type and size of the solar heater, the nature of fuel currently being used to heat your water, and on the cost of that fuel.
Although the costs of purchase and installation of solar water heaters are ordinarily higher than those of gas or electric water heaters, ongoing energy costs used to heat the water are considerably lower.
How Does It Work?
The system takes cold water from the bottom of a storage tank and pumps it through a collector, where it’s heated by the sun, and then back to the heat storage tank where it is stored. This continues as long as the sun is up. A controller monitors the temperatures of the solar collector and the storage tank, and it turns the pump on only if the collector is hotter than the tank.
Before you buy and install a solar water heater, you need first to examine your site’s solar supply, as well as the optimal orientation and angle of your solar collector. The efficiency and design of a solar water heating system depend on how much of the sun’s rays reach your site.
You have four options for the placement of your solar panel:
- Flush-mounted on the roof if it faces south and if its ‘tilt angle’ is close to your latitude.
- Rack-mounted on the roof if the tilt angle is lower than your latitude. A rack is used to raise the top of the collectors using special brackets.
- Wall-mounted on a south-facing wall, where a rack is used to tilt the collectors out from the wall to achieve the desired tilt angle.
- Ground-mounted rack on the ground, where a rack accommodates the proper orientation and angle for the solar panels.
Solar panels can also be positioned on a variable-axis sun tracking device. It provides automatic orientation, facing directly at the sun throughout the day and, therefore, increase energy gain by the system.
Although the design of systems can considerably differ, some components are standard to all systems, namely solar collectors and hot water storage tanks.
Solar collectors transform solar radiation into heat and transfer that heat to a medium (water, solar fluid, or air). Based on the collector system, solar water heaters can be of two main types.
1.) Flat Plate Collectors
2.) Evacuated Tube Collectors
Flat plate solar collector:
Flat-plate collectors are the most broadly used kind of collectors in the world for domestic water heating systems and solar space heating.
The panel is an insulated weatherproof metal box holding a dark solar absorber plate under one or more transparent glass covers. The dark absorber absorbs heat from sunlight that transfers through the cover and then gives the heat up to a heat transfer fluid flowing through metal tubes under the absorber plates.
Flat-plate solar collectors have been in service for the last 30 years, without significant changes in their design and operating principles.
Evacuated tube solar collector
Evacuated or Vacuum tube collectors are lately becoming the most popular kind of collector internationally, and present another choice to Canadians. This type of collector consists of two glass tubes each containing a black absorbing surface.
The outer tube is made out of exceptionally durable clear borosilicate glass that can withstand impact from hail up to one inch in diameter. The interior tube is also made of glass, but it is coated with a special selective coating, which features excellent solar heat absorption and minimal heat reflection properties.
The air is removed from the space between the two glass tubes to form a vacuum, that reduces conductive and convective heat loss. The sun’s radiation is absorbed by the coating on the inner glass cover but is blocked from re-radiating out by the silver-coated lining that has been optimized for infrared radiation. This works similarly to a one-way mirror.
Evacuated tube solar collector is very energy efficient. Up to 95% of the sun light’s energy tapping the tube’s surface, is absorbed, while only 5% losses through reflection and re-emission.
The behavior of the vacuum walls blocks any losses by conduction or convection – quite like a thermos bottle. Because of this, the system will work even in very low temperatures, unlike traditional flat plate collectors.
Hot Water Storage Tank
Solar heated water may be stored in a tank that also houses an electric backup heating element, or it may be stored in a separate tank that feeds into the tank of a conventional gas or electric water heater. The alternate heat source can also be an electric or tankless gas water heater.
Some solar systems need a heat exchanger to deliver the solar heat to the water in the hot water tank. This heat exchanger can either be built into the tank, or be a separate external unit.
Regardless of the nature of storage tank, solar thermal energy only preheats the household water throughout most of the year. At night and on cloudy days, the conventional backup heater is needed to boost the water temperature. In many systems, the full hot water load can be met by the solar system alone during most of the summer. The solar storage tank is usually large enough to hold at least a day’s supply of hot water.
Common Types of Solar Water Heating Systems
Systems are classified as either passive or active and as either direct or indirect. Passive systems rely on natural con- vection to circulate the water through the collectors. Active systems use electrically or PV solar driven pumps to circulate the heat absorbing liquid. This allows greater flexibility than their passive counterparts since the hot water storage tank does not have to be above or near the collectors.
Because of the high UV index levels in Canada solar heating technology works very well even on cold days and solar domestic hot water heating systems work very well in Ontario. The need for freeze protection may dominate the choice of what type of system you install. Passive systems are generally the most susceptible to freezing.
The SDHW system most common to regions where the temperature frequently drops below 20°C is the closed-loop antifreeze liquid system. This system circulates a food-grade antifreeze through solar panels and a heat exchanger in solar storage tank.
Closed-loop systems use propylene glycol or similar fluid chemical in the heat transfer fluid to avoid freezing. Glycol is less efficient at transferring heat than water, but it is infused with corrosion inhibitors to prolong the life of copper plumbing.
In other southern locations drain back system or compact solar water heaters can be used when the risk of freeze damage is minimized.[/message]
For a typical single-family residence, there will be one or two solar collector panels. The panels resemble skylights and are about 4 feet wide and 8 to 10 feet long.
There are many reasons why you should try to use solar water heater but, there are also some drawbacks that should be considered as well.
Cost of a System
A solar hot water system will cost more than a conventional water heating system. The overall cost is determined by its size
and complexity. Cost can be estimated by the homeowner or a licensed solar contractor.
Tax incentives and rebates may be available. Check with your local government and utility company to determine what incentives or rebates they may offer, as well. Long-term cash flow analysis should be considered before a decision is made.
A basic system will cost several thousands of dollars per collector to install, and there are some ongoing maintenance costs to consider. It is crucial to have your system properly maintained to optimize its performance and to avoid maintenance problems.
The cost of a typical (2 panel) turnkey solar hot water system in Ontario from a professional installer is currently around $6,000 to $8,000. It is possible for an individual to purchase and install a system for a lower cost, but this is not recommended unless you are a licensed plumbing contractor.
The appropriate collector area and reservoir capacity will depend on your hot water demand. A typical family with two children will require 6 square meters of collector area (i.e., two 4×8’ collectors) and 270 liters (60 gallons) of water storage capacity.
Although a solar hot water system may be unable to provide for all hot water needs, depending on climate and water use, it offers a clean, renewable, and efficient way to cut down on the overall cost of water heating. The energy used to heat the water is sunlight: a clean, renewable, and free source of energy!