Solar Hot Water
Solar hot water is perhaps the most widely used renewable energy source. It has many uses from heating the water for your pool to heating your house through radiant flooring.
It is one of the most cost-effect solar technologies on the market and can provide year round heating of your water. There are many state and federal incentives to subsidize the cost of your solar hot water system.
The basics
The principles are simple. The sun shines on a collector that heats up. This heat is transferred to a liquid that is is then stored in tanks for use in your house.
Open or Closed?
All solar hot-water heaters fall into one of two categories: open loop or closed loop. The difference is simple. Open-loop systems heat the water that you actually use, while closed-loop systems heat an antifreeze-water solution (water and glycol) that transfers its heat to the domestic hot water.
If you live in a region where the temperature stays above freezing, you can get by with the simpler open-loop type. If you live in an area that experiences freezing temperatures for much of the year, you’ll need to go with a closed-loop system.
The most basic open-loop type is called a batch collector, or integral collector storage system. Large-diameter pipes or one or more tanks are mounted in an insulated box with double or triple glazing. Batch collectors are plumbed into the household water system and feed hot water to the domestic water heater, or bypass it when there’s ample sunshine. The disadvantage of a batch collector is that it is also a storage tank. That means that if you don’t use the hot water quickly, you’ll lose the heat it contains.
A more efficient open-loop system is a flat-plate collector type that transfers water to an insulated storage tank. These insulated and glazed panels contain water in rows of copper tubes mounted in a heat-absorbing black plate. Most systems use an electric circulator, but photovoltaic pumps are available also.
The thermosiphon system shown at right is an open-loop type that uses a collector and a storage tank, but it takes advantage of convection to move heated water through the system. In most cases, the insulated tank is located in the attic. From the attic tank, the water is piped to the household hot-water system.
If you live in an area that experiences occasional freezing, either type of open-loop system can be set up to circulate warm water from the storage tank to the collector when the temperature drops to prevent freezing. However, this is risky since it can wreak havoc with the system’s temperature sensors. Also, it wastes heat and uses electricity; hence, these systems are not a good fit for many areas of the United States.
Closed-loop systems are inherently more complex than open-loop types. In the system shown on the lead page, the heated antifreeze-water solution flows from the collector to a coil in a tank. Domestic water in the tank is heated by the coil. In the drainback system shown at right, just the opposite occurs. In it, heated water flows directly into the tank, transferring its heat to the house’s domestic water in the coil. The system, which uses distilled water or a blend of water and glycol, is designed so that the collector has water in it only when the circulator is running. When the circulator shuts off, water drains into the storage tank. The design is popular in cold regions because it prevents freeze damage to the system.
The Heat Collector
There are several types of heat collectors used in modern solar hot water systems. Each has advantages and disadvantages.
Flat plate collector
Consists of a thin absorber sheet (usually copper, to which a black or selective coating is applied) backed by a grid or coil of fluid tubing and placed in an insulated casing with a glass cover. Fluid is circulated through the tubing to remove the heat from the absorber and transport it to an insulated water tank, to a heat exchanger or to some other device for using the heated fluid.
As an alternative to metal collectors, some new polymer flat plate collectors are now being produced in Europe. These may be wholly polymer, or they may be metal plates behind which are freeze-tolerant water channels made of silicone rubber instead of metal. Polymers, being flexible and therefore freeze-tolerant, are able to contain plain water instead of antifreeze, so that in some cases they are able to plumb directly into existing water tanks instead of needing the tank to be replaced with one with extra heat exchangers.
Evacuated (or vacuum) tubes panel.
Evacuated tube collectors
Are made of a series of modular tubes, mounted in parallel, whose number can be added to or reduced as hot water delivery needs change. This type of collector consists of rows of parallel transparent glass tubes, each of which contains an absorber tube (in place of the absorber plate to which metal tubes are attached in a flat-plate collector). The tubes are covered with a special light-modulating coating. In an evacuated tube collector, sunlight passing through an outer glass tube heats the absorber tube contained within it. The absorber can either consist of copper (glass-metal) or specially-coated glass tubing (glass-glass).
Two types of tube collectors are distinguished by their heat transfer method: the simplest pumps a heat transfer fluid (water or antifreeze) through a U-shaped copper tube placed in each of the glass collector tubes. The second type uses a sealed heat pipe that contains a liquid that vapourises as it is heated. The vapour rises to a heat-transfer bulb that is positioned outside the collector tube in a pipe through which a second heat transfer liquid (the water or antifreeze) is pumped. For both types, the heated liquid then circulates through a heat exchanger and gives off its heat to water that is stored in a storage tank (which itself may be kept warm partially by sunlight). Evacuated tube collectors heat to higher temperatures, with some models providing considerably more solar yield per square metre than flat panels. However, they are more expensive and fragile than flat panels.
For a given absorber area, evacuated tubes can maintain their efficiency over a wide range of ambient temperatures and heating requirements. However, due to the design the absorber area only occupies about 50% of the collector panel. In most climates and for the majority of domestic hot water services, flat-plate collectors will generally be a more cost-effective solution than evacuated tubes. Unless employed in large arrays, the efficient but costly evacuated tube collectors have only marginal net benefit in winter and give little real advantage in the warmer months. They are most suited to extremely cold ambient temperatures or in situations of consistently low-light. They are also used in industrial applications, where high water temperatures or steam need to be generated.