Active Solar Heating Systems

Introduction

As fossil fuel reserves available to us for our energy needs continue to dwindle, other energy sources need to be utilized. One of these alternative energy sources is energy from the sun, called solar energy. Enough solar energy hits the United States every twenty minutes to supply our needs for an entire year. An active solar heating system is one way to utilize this energy. Active systems are available in two categories, liquid-based and air-based . The type of system it is depends on what fluid is circulated through the solar collector to be heated.

Liquid-based solar heating systems are typically used for space heating, water heating, and heating pool water. Freezing is the principle cause of failure because the collector radiates heat to the cold night sky and the water in the collector can freeze at temperatures above 32°F. A liquid system is further classed as direct or indirect. A direct liquid system runs city or well water through the solar collector, thus requiring fewer heat exchangers. The indirect liquid system has the collector isolated from the water supply system. This type of system offers several advantages over a direct system. An isolated collector can use fluids that will not freeze at normal temperatures, such as ethylene or propylene glycol, hydrocarbon oils, or common refrigerant oils. When these liquids are used, mineral buildup in the piping is not a problem.

Space heating is the most common use for an air-based solar heating system since the heat does not have to be transferred to another medium before being used. Air can also be used to preheat domestic hot water. Using air for the collection fluid requires no freezing, overheating, or corrosion protection. The drawbacks of an air system are that ducts require more space than pipes, and air has poorer heat transfer qualities than liquids. Poor heat transfer means that more collector surface is required for the same amount of heat collected.

While a solar heating system is cost efficient during the life of the system, its initial costs are much higher than a conventional heating system. This is because a standard heating system must be installed with the solar heating system. A solar heating system cannot provide all the heating needs since heat is not collected at night or during days with overcast skies.

The control system of a solar heating system causes the solar radiation to be collected, stored, and distributed. The main components of a solar heating system are the solar collectors for collecting solar radiation, the solar storage system for storing the heat, and heat exchangers for distributing the heat throughout the building.

Solar Collectors

Flat plate solar collectors are typically used for solar energy collection. This type of collection system is capable of developing fluid temperatures of 150-200°F. Flat plate collectors have a simple design, are easy to repair, are low cost, and require no tracking mechanisms.

Liquid-based solar collector

Figure 1 Solar Flat Plate Collector for Liquid Source: 1996 ASHRAE (American Society of Heating, Refrigerating, and Air Conditioning Engineers) Systems and Equipment Handbook (ASHRAE, Inc., 1996): 33.3.


A flat plate solar collector (Figures 1 and 2) consists of a metal box that houses the components. The bottom and sides are insulated with an aluminum foil backed fiberglass insulation. The insulation reduces heat loss to the surrounding environment. Above the insulation is an absorber plate, which absorbs the radiation from the sun. Absorber plates are made of copper, aluminum, or steel, and are painted with a good quality flat black paint. A poor quality paint will give off gas at high temperatures and will discolor the transparent covers. The covers are single or double pane tempered glass with a low iron content. The glass is transparent to incoming ultraviolet radiation and opaque to outgoing heat radiation. Heat radiation loss can be further reduced by using glass that has been etched with acid, called stippled glass.

Air-based solar collector

Figure 2 Solar Flat Plate Collector for Air Source: 1996 ASHRAE Systems and Equipment Handbook (ASHRAE, Inc., 1996): 33.3.


Solar collectors are arranged in banks and must be positioned properly to maximize the radiation that is collected. The collectors should face as close to due south as possible, but deviation of as much as 30° will result in a decrease in performance of only five percent. The collectors are also tilted to face the sun. The angle of tilt is equal to the location latitude plus 15°. For example, Dayton, Ohio is at 40° latitude, so the angle of tilt relative to horizontal is 40° + 15° or 55°.

In a liquid-based system (Figure 1) the collecting fluid is pumped into the collector through the plumbing fitting, and then through the tubes attached to the absorber plate. The fluid is piped out of the collector and into the next collector in the array until it has traveled through the entire array. An air-based system collector (Figure 2) has air ducted into the internal manifold at the bottom of the collector. The air then travels over and under the absorber plate to pick up heat. After covering the entire plate, the air moves out of the collector through a manifold at the top.

Liquid-based storage

Figure 3 Multiple Storage Tank Arrangement for a Liquid-Based Solar Heating System Source: 1996 ASHRAE Systems and Equipment Handbook (ASHRAE, Inc., 1996): 33.13.


Heat Storage

Water filled tanks made of insulated steel or concrete are used for storage in a liquid system (Figure 3). The collecting fluid from the collector passes into the ‘hot’ tank, then into the ‘warm’ tank, and finally back to the collector array. Most of the heat will be transferred to the water in the first tank, leaving it at a higher temperature than the second tank. Heat is removed from the storage tanks in the reverse order that it was added. The cold water is preheated in the warm tank and is heated further in the hot tank before going to the building as hot water.

Rock bin heat storage

Figure 4 Insulated Concrete Rock Bin for an Air-Based Solar Heating System Source: Raymond A. Havrella, Heating, Ventilating, and Air Conditioning Fundamentals (McGraw-Hill Book Company, 1982): 202.


An insulated concrete rock bin (Figure 4) is used to store heat in an air-based solar heating system. The rock bin is placed in the basement of the building so any heat lost will be recovered by the building above. Air is ducted into the bottom of the bin and into a plenum formed by the concrete blocks and hardware cloth. The hardware cloth keeps the rocks from filling the spaces between the concrete blocks. The air then moves through, and transfers heat to the rocks. Washed rock fills the bin to within eight inches of the top. The volume of rock needed for efficient storage is 0.5-1.0 cubic feet of rock per square foot of collector surface. The air then passes out of the top and back to the collector. During heat removal, the air is forced into the top of the rock bin and out the bottom.

Liquid storage can be used for an air-based system, and a rock bin for a liquid-based system. To use these different storage methods, air-to-liquid or liquid-to-air heat transfer must be made, but the additional equipment will increase the costs to install and operate the system.

In-duct heat coil

Figure 5 In-duct Coil for Liquid-to-Air and Air-to-Liquid Heat Transfer


Heat Transfer

Liquid-to-air and air-to-liquid heat transfer is accomplished in the duct system (Figure 5). Hot water is pumped into the coil installed in the ductline. As air passes over the fins of the coil, heat is transferred from the fins to the air. Air-to-liquid transfer is accomplished by passing hot air over a coil filled with a cold liquid.

Liquid to liquid heeat exchanger cross section

Figure 6 Cross Section of Wraparound Shell Heat Exchangers Source: 1996 ASHRAE Systems and Equipment Handbook (ASHRAE, Inc.,1996):33.16.


Heat is transferred between liquids by means of a tank with a jacket around it (Figure 6A). The heat transfer liquid is circulated through the jacket around the tank. Potable, or drinking, water is pumped into the tank and receives heat through the tank wall. The building codes in some locales require separation of potable water and the heat transfer fluid by an air gap as in Figure 6B. If corrosion of the tank or heat exchanger walls occurs, the liquid will leak out of the bleed hole. This will keep the potable water from being contaminated with the heat transfer fluid, which would make it dangerous for human use.

Liquid-based solar heating system

Figure 7 Schematic Diagram of a Liquid-Based Solar Heating System Source: 1989 ASHRAE Fundamentals Handbook (ASHRAE, Inc., 1989): 28.29.


Liquid -Based System Operation

In a liquid-based solar heating system (Figure 7), a Differential Temperature Controller (DTC) senses and compares the temperatures of the storage tank and the top of the collector array. While the collector temperature is higher than the storage temperature the pump sends the fluid through the collector to be heated. The fluid’s heat is then transferred to the storage tank directly, or indirectly using a heat exchanger. This fluid loop is protected with a thermal expansion tank to allow for the expansion of the fluid and a vacuum relief valve to open if the pressure inside the loop becomes lower than the atmospheric pressure. If a vacuum is created, the piping can collapse causing the fluid to flash into steam, damaging the system.

Water for domestic use is pumped into the preheat tank, and heat is added through the tank’s heat exchanger. After the water is heated it moves into the service hot water tank for storage until it is used. If the water is not adequately heated in the preheat tank, the auxiliary heat supply is used to heat the water to the required temperature.

To heat the space, fluid from the storage tank is circulated through the coils in the building duct system. If the air cannot be adequately heated by the solar storage system, auxiliary heat is added by the furnace.

Air-based solar heating system

Figure 8 Schematic Diagram of an Air-Based Solar Air Heating System Source: 1989 ASHRAE Fundamentals Handbook (ASHRAE, Inc., 1989): 28.29.


Air System Operation

Air in the air-based solar heating system (Figure 8) is pulled through the collector, preheat tank heat exchanger, and damper A. Dampers are used in a ductline to stop or redirect airflow in the duct, just as valves are used in piping to stop water flow. The fan then pushes the air to damper B. If the building requires heat, the air is directed to the building, otherwise, the air goes to the rock bin for heat storage. The air used to heat the building, or from the rock bin, then cycles through the collector again. If the collector is not receiving sunlight and the building requires heat, damper C will close and the air will come from the building and into the rock bin to be heated. The air then is blown into the building. When the collector and rock bin are both too cool to heat the air, auxiliary heat is added by the furnace.

Domestic hot water is heated by pumping cold water into the preheat tank. The water is pumped through a heat exchanger to be heated before passing into the water heater. The water is heated by the auxiliary heat supply if it is insufficiently heated in the preheat tank. The water is then stored in the water heater until needed.

Written April 20, 1998

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