Liquid-Based Active Solar Heating 101

Solar Hot Water US & Canada

Liquid solar heating systems can be installed virtually anywhere in North America for a variety of energy saving uses.

A solar heating system is a source of on-site renewable energy that is proven to be one of the most cost-effective for homeowners, whether in cold climates such as in New Jersey, Iowa, Delaware, and even Alaska or in warmer regions of the U.S.  When you go green with an effective heating system such as liquid-based solar heating, you also make a large contribution to achieving a cleaner environment, which is priceless.

All forms of solar heating systems use energy from the sun to heat either liquid or air and then transport the solar heat directly into an interior space or to a storage system for future use.  If and when the solar system fails to provide adequate space heating, additional heat is provided by an auxiliary system.

Liquid-Based Active Solar Heating Basics

Liquid-based solar heating systems are used most often when storage is included.  Liquid systems can supplement forced air heating systems and are well suited for:

  • Boilers which have hot water radiators,
  • Absorption heat pumps and coolers
  • Radiant heating systems.

Liquid Solar Collectors

Liquid solar collectors come in a variety of sizes and types; and various system types require different levels of maintenance.  The most common types are flat-plate collectors, concentrating and evacuated tube collectors.  Periodically, a circulating pump operated by a controller moves the fluid through the collector.

Flat Plate Collectors are comprised of an absorber enclosed with and insulated and glazed casing. They capture the sun’s energy by means of the flat absorber plate. The flat plate heat exchanger utilizes narrow, alternating channels to allow efficient heat transfer between two fluids. Flat Plate Collectors are able to absorb 75% of the available solar radiation. As the liquid rapidly flows through the collector, it increases in temperature. The warmed liquid flows to a heat exchanger for immediate use or to a storage tank.

Liquid SolarOther liquid-based active solar heating components include:

  • Pumps
  • Piping
  • Valves
  • An expansion tank
  • Heat exchanger
  • Storage tank
  • Controls

Tank Storage in Liquid Systems

There are many options for tank storage in liquid-based active solar heating.  Depending on overall system design, tanks can be pressurized or unpressurized. You can choose a storage tank based on cost, durability, size, whether it will be placed outside or in the basement, and installation requirements.  It’s sometimes necessary to build a tank on-site, if the necessary size of the tank won’t fit through existing doorways.

There are local plumbing, building, and mechanical codes which must be met.  In addition, tanks have limits for pressure and temperature.  Care needs to be taken in providing necessary insulation, to prevent heat loss.  Corrosion and leaks need to be avoided with protective sealing or coating.

When large storage requirements are involved, custom tanks may be needed.  These are usually fiberglass, high temperature plastic, or stainless steel.  Hot tub tanks constructed of wood and concrete are another option.  The simplest type of storage system you can use is standard domestic water heaters, which are lined to inhibit corrosion, are easy to install, and meet building codes related to pressure vessel requirements.

Heat Distribution

Distributing solar heat can be achieved with a central forced-air system, a radiant floor, hot water baseboards, or radiators.

Radiant Floor System

radiant floor sysyem

The radiant heat lines will be covered with light weight cement and then used to heat the room with solar energy.

In a radiant slab system, there is a system of pipes embedded in a thin concrete slab floor which is typically covered with tile or lightweight concrete.  The solar-heated liquid circulates through those pipes, which radiates heat into the room.  Because it performs well at moderately low temperatures, radiant floor heating is an ideal choice for liquid solar systems.

Once radiant slab heating systems are operational, a consistent level of heat is provided when cared for properly.  The system may be less effective if the floor is covered with rugs or carpeting.

It’s possible that a separate heat storage tank may not be required for a radiant floor, if the system is carefully designed.  A supply of back-up heat can be supplied by a standard domestic water heater or a conventional boiler.

Radiators and Hot-Water Baseboards

Water at temperatures between 160° and 180°F is required to effectively heat a room with radiators and hot-water baseboards.

Forced-air Heating System

The basic design for incorporating a liquid heating system into a forced-air heating system is to place a heating coil (liquid-to-air heat exchanger) in the main room air-return duct before it reaches the furnace.  As air returns from the living space, it is heated by passing over the solar heated liquid in the heat exchanger.  When additional heat is necessary, it’s supplied by the furnace.  To ensure effectiveness, it’s important that the coil be large enough to transfer a sufficient amount of heat to the air at the lowest operating temperature of the collector.

To find out more information about active solar heating in your area or to speak with your local North American Solar Store solar specialist, find the nearest NASS member store to you.


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