Prime Solar Company

Ways to use the energy of the sun. heating.

Obviously the simplest way to use the heat of the sun is to heat something. Some of the things that are commonly heated are; water for domestic use, water for pools and homes.

Solar panels are used to heat water. They are very different from PV panels. Water or some other heat exchange fluid is circulated through the panels. The panels are often insulated and have a sheet of glass over them. The heat is then collected in some type of container. This can be a large water tank, perhaps 120 gallons, a pool or a spa. The water can be circulated with a pump, an active system, or if the tank is above the panels the warmer water will rise into the tank, this is called a thermosiphon system.

In this part of the country there is very little active solar heating of homes. This is because most of the time we want heat the sun is not shining. The cost to store large amounts of water or otherwise store heat is prohibitive. On the other hand passive heating can be very effective. Roof overhangs that allow sun into a series of south facing windows can warm a home. If the sun falls on a concrete slab and warms it, the home may stay comfortable all night. Using curtains or other window coverings at night can help keep the warmth in. There are many variations of this idea that can be designed into new construction or remodels that will help homes stay comfortable.

Well insulated homes consume less energy and are more comfortable. Vented attics help keep a home cooler in the summer. A little known fact is that setting the heating thermostat 5 degrees lower can save as much as 50% on heating bills. Radiant heating is a great idea for new construction. In a home with a slab floor the heat radiating off of the slab warms your body even when the air is a little bit cool. This means that you can set the thermostat lower saving energy. These systems are also silent and are better for people with allergies as dust and pollens are not blown around like with a forced air heater. They can be used in conjunction with solar water panels.

Solar for heating water can be very cost effective. In homes using electricity or propane to heat the water the payback can be under four years. As the price of natural gas goes up the payback is approaching that. A good solar water heater will last 20 years with minimal maintenance. The new rebates will include one for solar water heating that offsets the electric usage of an electric water heater. The details are being worked in the pilot program in San Diego and the rebate should be available by the end of the year. There is also a 30% capped at $2000, federal tax credit.

Typically a large storage tank, 80 to 120 gallons, is used to store hot water. When a faucet is opened pressurized cold water from your water utility company forces the warm water from the solar tank into the standard water heater and then out your faucet. Many systems have bypass valves that allow you to take the standard water heater out of circuit during warmer months so that you can use the solar water directly.

The solar water can be very hot. A valve should be installed with any solar water heater for domestic use that automatically mixes cold water with the hot if it is too hot.

The water in the storage tank is heated by solar panels. There is a pump with a control on all active systems. When the water in the panels is a preset temperature above the water at the bottom of the tank the pump comes on. It stays on until the temperature of the panels drops to where it will no longer effectively heat the tank. On a good sunny day the pump will run all day. On a marginal day it will cycle on and off. The pump is small enough that it is not a large electricity drain.

Freezing panels is also a consideration in this area. The two most common forms of freeze protection are freeze recirculation and freeze dribble. With a recirculation system the pump comes on and circulates water from the tank up to the panels. In this area the heat loss is not too great. The other is a freeze dribble, a valve on the panels opens up allowing a small amount of water to escape, this water is replaced by warmer water in the tank keeping the panels from freezing. The two methods are sometimes used on conjunction so that in a power outage on a cold night the panels will not freeze. The freeze dribble valve is completely mechanical.

Spas can be heated with either glazed or unglazed panels as their desired temperature is not nearly as hot as domestic hot water. Pools are almost never heated with glazed panels.

Often the desired temperature of the pool is lower than the daytime air temperature. This means that even if the solar panels were not in the sun they would still heat the pool. Insulating the panels would decrease their efficiency and the glass does reflect some heat. The speed at which the water is circulated through the pool panels is fast enough that the panels are only a few degrees over the temperature of the pool.

A pool contains a large volume of water, in the thousands of gallons. The typical collection area of pool panels is usually about 75% of the surface area of the pool. For instance with a 20 by 40 foot pool you have 800 square foot. This would require 600 square feet of solar absorber. To do this with glazed panels would be prohibitively expensive in most applications. Pool panels are a great way to heat a pool and will usually add three month or more to the unheated swimming season.