What is a Solar Hot Water Heater?

The sun can heat water well above the temperature needed for typical household and business purposes, such as bathing, washing dishes, and washing clothes. Solar hot water heaters, or solar thermal systems, provide a method for homes and businesses to efficiently heat water in an environmentally benign way.

These systems use the sun to heat a fluid in solar collectors, which are generally mounted to the roof. The heated fluid then transfers heat directly or indirectly to water stored in a tank.

Solar hot water heaters are cost-effective over the lifetime of the system since the fuel they use—sunlight—is free. Although the initial costs of a solar hot water heater are higher than those of a conventional water heater, after an initial payback period, all subsequent fuel energy for heating water is free. And despite Humboldt County’s reputation for cloudy skies, the solar resource here is more than adequate for the hot water needs of most homes and businesses.

People switch to solar hot water heaters for environmental reasons, too. Solar hot systems do not emit pollutants such as carbon dioxide, nitrogen oxide, and sulfur dioxide into the atmosphere. When an average solar hot water system replaces a conventional electric water heater, the electricity saved over 20 years represents a reduction of more than 50 tons of greenhouse gases. That significantly reduces the environmental impact of our domestic activities.

System Types

The basic components of most solar hot water systems are solar collectors, a storage tank, and, depending on the type of system, a pump. There are three different types of solar collector: flat plate, evacuated tube, and concentrating. (sidebar) Solar hot water heaters currently have two basic designs: active or passive. (diagram)

Active systems

Active systems use pumps to circulate water or a heat transfer fluid through the system. They are usually more expensive than passive systems, but are also more efficient. Of the active systems, there are two basic designs: open loop and closed loop.

• Active open loop, or direct circulation, uses a pump to circulate household potable water through the solar collectors, where they collect heat from the sun, and back into the house for use. These systems need a drain back valve to guard against freezing when the air temperature drops at night and during winter months.
• Active closed loop, or indirect circulation, uses a pump to circulate a non-freezing heat-transfer fluid, such as water glycol antifreeze, through the solar collectors where it is heated. The heat-transfer fluid then flows into a heat exchanger immersed in a storage tank, where the water is then heated. The heat transfer fluid never comes into contact with the potable water in the storage tank. This type of system offers very good freeze protection without a drain back valve, and is commonly employed in Humboldt County.

Passive Systems

Passive systems move household water through the system without a pump. With less moving parts, the passive system requires less maintenance, and lasts longer than active systems. Passive systems are less expensive than active systems, but are also less efficient. Two common types are Batch Heaters and Thermosiphon.

• Batch heater, bread box, or integral collector storage systems are the simplest thermal systems out there, consisting of a large water storage tank placed in an insulated box and glazed on the side facing the sun. Basically, it’s a big box of water on your roof or other surface that’s heated directly by the sun. These systems are simple, inexpensive, and require little maintenance, but they can require a strong roof, are inefficient, and do not offer freeze protection.
• Thermosiphon systems operate on the principle that warmer water rises due to natural convection. In this type of system the tank is installed vertically above the solar collectors. As water in the collector heats, it becomes lighter and rises into the tank. Meanwhile, cooler water sinks and flows down into the collectors. This causes a circulation of water through the system. These systems are reliable and inexpensive, but require careful planning to install in new construction, and are very difficult in retrofits. They do not offer freeze protection.

Types of Collector

Flat Panel Collector: This is the most common type of collector and usually consists of a large plate of blackened absorber material that is insulated and encapsulated in a weatherproof box or other enclosure.

Evacuated Tube Collector: This type of collector consists of rows of transparent glass tubes. Each tube consists of a glass outer tube and an inner absorber tube.

Integrated Collector Storage: This type of collector is most commonly used in passive systems, and is far simpler than its counterparts mentioned above. These collectors consist of one or more black tanks or tubes in a glazed insulated box.