Domestic Hot Water
If you leave a black plastic bag full of water in the sun for a few hours, you will have a bagful of solar heated water whose temperature will range some where between pleasantly warm and much too hot to touch. If you add a hose with a shut-off valve to that bag, and suspend the whole unit higher than the outflow end of the hose, you can have solar heated water on demand, from the time the water warms up in the morning until early evening. If you add a second hose to the bag that introduces a fresh supply of unheated water as heated water is drawn off, you can use a slow but steady stream of solar heated water nearly all day.
If you want to be sure of a hot water supply all night as well as all day, you might simply wrap your water bag in a blanket of insulation every day in the late afternoon, and turn off the cold water influx at the same time. Or you might provide your system with an auxiliary water heater in case you’re faced with several days of heavy cloud cover in a row. If you want to use a lot of hot water at once from time to time (a lot meaning, here, more than your bag can hold), you can simply add an insulated storage tank, and pipe your heated water there by way of your outflow base. When you want it, you can demand hot water from the storage tank.
Despite our technological progress, the simplicity of the principles behind Clarence Kemp’s 1891 water heater re main. and even at its most complex level, water heating is less expensive than space heating, and since hot water is used year-round, the cost is amortized more rapidly.
The average daily consumption of hot water in the United States is about 10 to 15 gallons per person. This use includes washing machines, dishwashers, and bathtubs, which consume a disproportionately high percentage of our hot water, and which operate principally in the morning and evening hours—before and after any non-storage solar hot water system can be effective.
As with all other solar energy adaptations for the house, conservation should be your first step because it requires the least effort or expenditure in proportion to the savings it returns. With solar heated water the savings are reflected directly and immediately in the size and complexity—and therefore in the cost— of the system you install. Less use translates into a need for smaller storage capacity and smaller collector area relative to the proportion of hot water sup plied by the system. By the same token, greater use is reflected in a need for greater collection ability, and facilities for storing more heated water, or storing it for longer periods of time.
Bread Box Collector
The simplest form of solar water heater, such as the black plastic bag described above, is known as a storage heater, or bread box collector. In the bread box collector, as in the black plastic bag, the solar heat is collected and stored in the same unit. There is no transfer from one part of the system to another. However, all storage heaters lose at night the heat they collect by day. Storage heaters are essentially passive systems except at the use, or distribution, stage; an elementary mechanism allows user control.
The bread box has been around for half a century, in one form or another, but its basic design is always the same: one or more water tanks painted black and contained in an insulated box covered at the top (and sometimes on the south face as well) by double or triple glazing. The box is opened (usually by hand) in the morning to expose the tank(s) and glass to the sun; it is closed at night to minimize heat loss. The tank(s) can be filled by some in dependent method such as a hose, or connected to the general water supply of the house. The insulated solar heated water in the tank(s) is unlikely to freeze; however, if your area is subject to deep winters, you probably shouldn’t use this system. In any case, insulate the pipes that carry water from or to the heater; this will guard against freezing and against ordinary heat loss.
With storage heaters, the collector / storage tank is ordinarily located above water outlets. This configuration keeps the system as simple as possible, and also keeps the cost down.
Bread Box Solar Collector: Reflective surface; Black-painted tank
Thermosiphoning Systems
Solar heat can also be collected at a point below the storage tank, and transported by thermosiphoning to the higher storage. As mentioned earlier, thermosiphoning actually relies on gravity, taking advantage of the fact that warm air or liquid rises, and cool air or liquid falls.
In thermosiphoning systems, cool water circulates starting at the bottom of the collector (usually a flat-plate collector), is warmed, rises, and is piped into the top of the storage tank. As it cools, the water falls to the bottom of the tank and reenters the collector. Water for use in the house is tapped from the top, which is the warmest part of the storage tank.
The continuous cycle raises the temperature of solar heated water as long as the sun shines on the collector. How long the solar heated water remains warm after the sun ceases to shine on the collector depends on the capacity of the storage tank and how effectively the tank is insulated.
Next: Active Hot Water Systems