Active Solar HVAC Systems

An Active Solar HVAC System consists of a number of collectors that use the sun as its primary source of energy.  For maximum exposure and effectiveness the collector panels are south facing.   

There are many factor in Active solar Heating, below are the following topics which will be discussed:

Typical Uses

Residential Buildings

Commercial Building

• Any building with a lot of sun exposure

Pool heating

Water heating process

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The System

These systems consist of three components;

• A collector

• A storage system

• A distribution system

 Collectors

 The collectors obtain and secure as much energy as possible from the sun.  There are three types of collectors; flat plate, concentrating and evacuated tubing.

 Flat Plate Collectors – These are stationary flat plates whose effective surface area changes with the time of the day.  They receive maximum solar radiation when the collectors face the sun directly but receive much less energy when the sun radiation penetrates at an angle.  The collectors heat either air or a liquid.  One example of this process is as follows:

“The transparent cover admits solar radiation.  The absorber plate, a flat panel with tubing through which the water is circulated and heated, collects the radiant energy.  The plate and tubing are typically steel, aluminum, plastic, or copper and are often black to increase energy absorption.  The radiation that passes through the transparent cover has a short wavelength.  The absorber plate reradiated some of the received energy outward in longer wavelengths, which does not pass back through the cover plate.  The transparent cover also minimizes convected heat loss to the surrounding air.  Insulation surrounds the other sides of the absorber plate to reduce heat losses there.  A flat plate collector collects that heats air is constructed similarly to one that heats liquids, but functions in a slightly different way.  The absorber plate no longer includes circulating tubing.  Instead, air is heated directly as it is circulating through the space behind the absorber plate.  Absorber plates sometimes have fins to increase the heat transfer surface”

• This is the typical process used, if different collectors are used the system is altered slightly.*

http://www.flasolar.com/active_dhw_flat_plate.htm

http://www.flasolar.com/active_dhw_flat_plate.htm

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Collector Specifications

For more information click here

Collector Specifications

For more information click here 

Concentrative Collectors (also known as focusing collectors) – They concentrate solar radiation on concaved surfaces.  The shape allows the collectors to produce a much higher temperature of about 400 degrees Celsius which is much higher than the flat plate collectors.  These collectors often receive maximum radiation because they are usually designed to track the sun and move along its path.  While these collectors are more effective than the flat plates, their initial cost are extremely higher causing them to become less popular.

For more information click here

Evacuated Tube Collectors – These collectors are composed of smaller tubes inside larger tubes.  Solar radiation is transmitted thru the outer, transparent tube.  Liquid circulates in the inner tube, waiting to be heated.  The space between the two tubes is a vacuum that reduces convection heat losses from the collector.  This system is nearly twice as efficient as the flat plate collectors, and has the capability to heat water to temperatures to 300 degrees Celsius.  Production of this system is becoming more affordable, which will lead to greater use, increasing its popularity among collectors.

http://www.sunutility.com/html_pg/cross_section.html

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Storage Systems 

The storage system collects excessive heat and that energy is used when the sun is not shining.  There are two common types of storage systems.  One is a large storage tank which heats and stores water until there is a demand.  The second is a bin filled with pebbles through which hot air from the collector pass the pebbles store the heat, and air circulates and delivers it to the building. 

Liquid Storage

Air Ventilation

As precautionary measures, a back up system is required for climates prone to cloudy weather.  The most typical back up systems is a hot water boiler or a warm air furnace. See website for a more detailed explanation for storage system along with other conventions

Storage Systems 

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 Distributing System

The distribution is similar to that of a conventional fuel-fired heating system.  The active solar system uses ductwork or piping to distribute the heated air of liquid to its final destination.

For more information click here

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Limitations 

Most system can be oriented and positioned for maximum efficiency.  The maximum exposure occurs between 9:00 am to 3:00 pm.  The system most be located in unshaded areas were there is an absence of obstacles that may block the path of the sun.  This system requires an average of 2 gallons of water for every square foot of collector area.  Large open spaces are required for these systems to be effective.  One benefit is that the solar collectors are sizable allowing them to fit the appropriate building.

            Collector Area =   __________(Energy Demand)__________

                                         ((Solar Energy/Area) x Collector Efficiency)

Change of weather can dramatically affect these systems, for example during the winter season when temperatures are under freezing, the collectors’ efficiency decrease.  This issue can be alleviated with the installation of an antifreeze solution.  Rain, snow, ice and even a dirty environment can reduce the efficiency by 50% or more.  Collectors must be cleaned periodically.  The average efficiency of these systems is around 40-75%.

http://www.flasolar.com/active_dhw_flat_plate.htm

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Cost

Initial costs are usually high, but energy costs of the systems are much lower than conventional HVAC systems.  When long term benefits are considered, a client can expect a positive cash flow in about 4 to 10 years, depending on the efficiency of the system and the cost of utilities in the area. 

A typical solar heating system consisting of a flat panel collector and storage tank can cost anywhere from $3,200 to $4,800 depending on the collector area and tank capacity.  Cost of the collectors range from $30 to $80 per square foot, but costs tend to decrease as the size of the overall size of the panel increases.  Pumps range from $100 to $400, controls and sensors from $50 to $200, heat exchangers $200 to $300 and freeze protection from $50 to $100. 

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