Liquid-Based Active Solar Heating
Solar liquid collectors are most appropriate for central heating. They are the same
as those used in solar domestic water heating systems. Flat-plate collectors are the
most common, but evacuated tube and concentrating collectors are also available.
In the collector, a heat transfer or "working" fluid such as water, antifreeze (usually
non-toxic propylene glycol), or other type of liquid absorbs the solar heat. At the
appropriate time, a controller operates a circulating pump to move the fluid through
the collector.
The liquid flows rapidly through the collectors, so its temperature only increases 10°
–20°F (5.6°–11°C ) as it moves through the collector. Heating a smaller volume of
liquid to a higher temperature increases heat loss from the collector and decreases
the efficiency of the system. The liquid flows to either a storage tank or a heat
exchanger for immediate use. Other system components include piping, pumps,
valves, an expansion tank, a heat exchanger, a storage tank, and controls.
The flow rate through the collector should be between 0.02 and 0.03 gallons per
minute per square foot of collector when water is the heat transfer fluid (0.82 to
1.22 liters per minute per square meter of collector). Other flow rates apply for
different heat transfer fluids. The total flow rate, used to size the collector pump, is
the product of the above flow rate times the total collector area. To learn more
about types of liquid solar collectors, their sizing, maintenance, and other issues,
see the solar water heating section.
Storing Heat in Liquid Systems
Liquid systems store solar heat in tanks of water or in the masonry mass of a
radiant slab system. In tank type storage systems, heat from the working fluid
transfers to a distribution fluid in a heat exchanger exterior to or within the tank.
Most storage tanks require 1–2 gallons (3.8–7.6 Liters) of water for each square
foot (0.093 square meter) of collector area. Tanks are pressurized or unpressurized,
and the type used depends on the overall system design.
Before choosing a storage tank, you should consider several factors, including cost,
size, durability, where to place it (in the basement or outside), and how to install it.
You may need to construct a tank on-site if a tank of the necessary size will not fit
through existing doorways. Tanks also have limits for temperature and pressure,
and must meet local building, plumbing, and mechanical codes. You should also note
how much insulation is necessary to prevent excessive heat loss, and what kind of
protective coating or sealing is necessary to avoid corrosion or leaks.
Specialty or custom tanks may be necessary in systems with very large storage
requirements. They are usually stainless steel, fiberglass, or high temperature
plastic. Concrete and wood (hot tub) tanks are also options.
Each type of tank has its advantages and disadvantages. All types require careful
consideration for their location, due to their size and weight. It may be more
practical to use several smaller tanks rather than one large one. The simplest
storage system option is to use standard domestic water heaters. They are
designed to meet building codes for pressure vessel requirements, are lined to
inhibit corrosion, and designed so it is easy to attach pipes and fittings.
Distributing Heat for Liquid Systems
There are different ways to distribute the solar heat: with a radiant floor, with hot
water baseboards or radiators, or with a central forced-air system. In a radiant floor
system, a solar-heated liquid circulates through pipes embedded in a thin concrete
slab floor, which then radiates heat to the room. Radiant floor heating is ideal for
liquid solar systems because it performs well at relatively low temperatures. A
carefully designed system may not need a separate heat storage tank, though
most systems do for temperature control. A conventional boiler or even a standard
domestic water heater can supply backup heat. The slab is typically covered with
tile.
Radiant slab systems take longer to heat the home from a "cold start" than other
types of heat distribution systems. Once they are operating, however, they provide
a consistent level of heat. Carpeting and rugs will reduce the system's
effectiveness. See the radiant heating section for more information.
Hot-water baseboards and radiators require water between 160° and 180°F (71°
and 82°C) to effectively heat a room. Generally, flat-plate liquid collectors heat the
transfer and distribution fluids to between 90° and 120°F (32° and 49°C).
Therefore, using baseboards or radiators with a solar heating system requires that
either the surface area of the baseboard or radiators be larger, that the solar-
heated liquid be heated more with the backup system, or that a medium-
temperature solar collector (such as an evacuated tube collector) be used.
It is possible to incorporate a liquid system into a forced-air heating system, and
there are different options for doing so. The basic design is to place a liquid-to-air
heat exchanger, or heating coil, in the main room-air return duct prior to the
furnace. Air returning from the living space is heated as it passes over the solar
heated liquid in the heat exchanger. Additional heat is supplied as necessary by the
furnace. The coil must be large enough to transfer sufficient heat to the air at the
lowest operating temperature of the collector.
Solar liquid collectors are most appropriate for central heating. They are the same
as those used in solar domestic water heating systems. Flat-plate collectors are the
most common, but evacuated tube and concentrating collectors are also available.
In the collector, a heat transfer or "working" fluid such as water, antifreeze (usually
non-toxic propylene glycol), or other type of liquid absorbs the solar heat. At the
appropriate time, a controller operates a circulating pump to move the fluid through
the collector.
The liquid flows rapidly through the collectors, so its temperature only increases 10°
–20°F (5.6°–11°C ) as it moves through the collector. Heating a smaller volume of
liquid to a higher temperature increases heat loss from the collector and decreases
the efficiency of the system. The liquid flows to either a storage tank or a heat
exchanger for immediate use. Other system components include piping, pumps,
valves, an expansion tank, a heat exchanger, a storage tank, and controls.
The flow rate through the collector should be between 0.02 and 0.03 gallons per
minute per square foot of collector when water is the heat transfer fluid (0.82 to
1.22 liters per minute per square meter of collector). Other flow rates apply for
different heat transfer fluids. The total flow rate, used to size the collector pump, is
the product of the above flow rate times the total collector area. To learn more
about types of liquid solar collectors, their sizing, maintenance, and other issues,
see the solar water heating section.
Storing Heat in Liquid Systems
Liquid systems store solar heat in tanks of water or in the masonry mass of a
radiant slab system. In tank type storage systems, heat from the working fluid
transfers to a distribution fluid in a heat exchanger exterior to or within the tank.
Most storage tanks require 1–2 gallons (3.8–7.6 Liters) of water for each square
foot (0.093 square meter) of collector area. Tanks are pressurized or unpressurized,
and the type used depends on the overall system design.
Before choosing a storage tank, you should consider several factors, including cost,
size, durability, where to place it (in the basement or outside), and how to install it.
You may need to construct a tank on-site if a tank of the necessary size will not fit
through existing doorways. Tanks also have limits for temperature and pressure,
and must meet local building, plumbing, and mechanical codes. You should also note
how much insulation is necessary to prevent excessive heat loss, and what kind of
protective coating or sealing is necessary to avoid corrosion or leaks.
Specialty or custom tanks may be necessary in systems with very large storage
requirements. They are usually stainless steel, fiberglass, or high temperature
plastic. Concrete and wood (hot tub) tanks are also options.
Each type of tank has its advantages and disadvantages. All types require careful
consideration for their location, due to their size and weight. It may be more
practical to use several smaller tanks rather than one large one. The simplest
storage system option is to use standard domestic water heaters. They are
designed to meet building codes for pressure vessel requirements, are lined to
inhibit corrosion, and designed so it is easy to attach pipes and fittings.
Distributing Heat for Liquid Systems
There are different ways to distribute the solar heat: with a radiant floor, with hot
water baseboards or radiators, or with a central forced-air system. In a radiant floor
system, a solar-heated liquid circulates through pipes embedded in a thin concrete
slab floor, which then radiates heat to the room. Radiant floor heating is ideal for
liquid solar systems because it performs well at relatively low temperatures. A
carefully designed system may not need a separate heat storage tank, though
most systems do for temperature control. A conventional boiler or even a standard
domestic water heater can supply backup heat. The slab is typically covered with
tile.
Radiant slab systems take longer to heat the home from a "cold start" than other
types of heat distribution systems. Once they are operating, however, they provide
a consistent level of heat. Carpeting and rugs will reduce the system's
effectiveness. See the radiant heating section for more information.
Hot-water baseboards and radiators require water between 160° and 180°F (71°
and 82°C) to effectively heat a room. Generally, flat-plate liquid collectors heat the
transfer and distribution fluids to between 90° and 120°F (32° and 49°C).
Therefore, using baseboards or radiators with a solar heating system requires that
either the surface area of the baseboard or radiators be larger, that the solar-
heated liquid be heated more with the backup system, or that a medium-
temperature solar collector (such as an evacuated tube collector) be used.
It is possible to incorporate a liquid system into a forced-air heating system, and
there are different options for doing so. The basic design is to place a liquid-to-air
heat exchanger, or heating coil, in the main room-air return duct prior to the
furnace. Air returning from the living space is heated as it passes over the solar
heated liquid in the heat exchanger. Additional heat is supplied as necessary by the
furnace. The coil must be large enough to transfer sufficient heat to the air at the
lowest operating temperature of the collector.
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