Solar Thermal Energy System - Part 4
 
 
 
 Reference:
    1- Solar Domestic Hot Water Heating Systems - Design, Installation and Maintenance
        by Christopher A. Homola, PE
        		 
   
     Solar thermal energy (STE) is a form of energy and a technology for harnessing solar energy to generate thermal energy or electrical energy for use in industry, and in the residential and commercial sectors. The first installation of solar thermal energy equipment occurred in the Sahara Desert approximately in 1910 when a steam engine was run on steam produced by sunlight.
 
  1- Gallery, Solar Water Heating
 
       
       
       
 
 
  2- Thermosiphon / Thermosyphon
 
  Thermosiphon (alt. thermosyphon) is a physical effect and refers to a method of passive heat exchange based on natural convection, which circulates a fluid without the necessity of a mechanical pump. Thermosiphoning is used for circulation of liquids and volatile gases in heating and cooling applications, such as heat pumps, water heaters, boilers and furnaces. Thermosiphoning also occurs across air temperature gradients such as those utilized in a wood fire chimney, or solar chimney.

This circulation can either be open-loop, as when the substance in a holding tank is passed in one direction via a heated transfer tube mounted at the bottom of the tank to a distribution point-even one mounted above the originating tank—or it can be a vertical closed-loop circuit with return to the original container. Its purpose is to simplify the transfer of liquid or gas while avoiding the cost and complexity
Thermosyphon circulation in a simple solar water heater

    Schematic Diagrams

      
  Solar Energy ...

  Thermosiphons are used in some liquid-based solar heating systems to heat a liquid such as water. The water is heated passively by solar energy and relies on heat energy being transferred from the sun to a solar collector. The heat from the collector can be transferred to water in two ways: directly where water circulates through the collector, or indirectly where an anti-freeze solution carries the heat from the collector and transfers it to water in the tank via a heat exchanger. Convection allows for the movement of the heated liquid out of the solar collector to be replaced by colder liquid which is in turn heated. Due to this principle, it is necessary for the water to be stored in a tank above the collector
Solar heating system featuring a thermosiphon
 
 
  3- Solar Water Heating
 
  1- How Works ...
A solar hot water system is really quite simple. You’re using the sun’s energy to heat a solar collector full of solar fluid which then transfers that heat into your home’s hot water tank. The solar fluid in the system protects against freezing.
 

(1of Schematic) When there is sufficient heat to be drawn from the collectors, a controller automatically turns the system on and activates pumps. (2 of Schematic) Heated solar fluid is then circulated from the collector… (3 of Schematic) through a heat exchanger where its heat is transferred to… (4 of Schematic) water in the solar storage tank. (5 of Schematic) Solar heated water in the solar storage tank is then drawn into the main water heater.
 

This circulation loop will continue as long as there is heat to be drawn from the collector. When there is little or no sun, the main water heater will still function providing ample hot water for your home.
Schematic -  Solar Hot Water for Home
  2- Efficient
   Solar System should respect the following principles:
  1. Suitable inclination (between 35 and 45° in Lebanon)
  2. Avoid shading
  3. Number of panels suitable for hot water consumption volume
  4. Storage water tank should be very well insulated
  5. Hot water pipes should be very well insulated
  6. Domestic hot water recirculation inside the house (if applicable) should be controlled to minimize heat losse
  3- How to size ...
Solar water heating systems include storage tanks and solar collectors. There are two types of solar water heating systems: active, which have circulating pumps and controls, and passive, which don't. Most solar water heaters require a well-insulated storage tank. Solar storage tanks have an additional outlet and inlet connected to and from the collector.

A properly sized system will provide almost all of a home's hot water in the summer months, and partially hot water demand in winter season. To determine the storage tank size you need to consider how much water is needed for your household. On residential applications, an average person uses 50 liters of water per day. Take into consideration how many people are in your household and you can determine the size of the water storage tank.

Example: 4 persons x 50 liters per person = 200 liters tank 
 
 
  4- Types of Solar Heating Systems
 
  1- Thermosiphon systems

It is one of the most basic and well designed solar water heater systems. This is a tremendous investment in saving energy for heating waters for all usage - an investment that will pay for itself several times over through the years that it works. The monobloc thermosyphon solar water heater is one of the least complex, most affordable and most cost effective investments for selecting a solar energy application product.

The water tank of a thermosyphon system itself usually does not require any pumps and controllers to operate in its simplest form – using the pressure of the water connection in house would allow the system to operate effectively as an on-demand hot water system, heating the water in the tank to well over 60 degrees (under fair conditions) before it enters house to be used.
  2- Forced circulation systems
  Forced heating systems are the ideal solution for producing large quantities of hot water: for buildings with numerous apartments, nursing homes and for places that have a very high consumption of water such as hotels, restaurants, sports centers or gyms with showers…
 

Forced circulation systems use electrical pumps, valves, sensors and controller to circulate water or other heat-transfer fluids through the collectors. This enables a high degree of flexibility in the positioning of the system components. For example, the tank can be installed in a basement. In addition  the system can operate with a series of collectors to supply heated water to several large tanks, making it suitable for hotels, swimming pools and other commercial applications
  3- Drain back systems
Drainback systems are closed-loop, indirect, active systems. The water contained in an unpressurized, closed loop is pumped through the collectors and is separate from the end-use water being heated through a heat exchanger. When the pump is off, the heat-transfer fluid drains out of the properly sloped collectors and pipe, leaving them empty and protected from freezing.  The basic components of a drainback system are:

  1. A storage tank.
  2. One or more solar collectors.
  3. A differential controller, which monitors the water temperature in the tank and the collector temperature. When the collector temperature exceeds the storage temperature by a set differential (number of degrees), the controller activates the pump.
Drainback systems have many advantages compared to other types of SHW systems. Because it needs an air space in order to drain, the loop is not pressurized. Less stress is placed on solder joints, threaded fittings, and gaskets. Furthermore, there are no motorized valves to fail, and the system does not rely on electricity to maintain freeze protection. If the power goes out, the pump shuts off and the water drains from the collectors back into the reservoir.

Drain Back Tanks are designed to allow the solar collectors to drain all water from the collector and related pipe connections into the drain back tank reservoir to protect the system from freezing and overheating. Drain back systems are a smart choice when designing a solar thermal system to supplement central heating and domestic hot water heating components. Maintenance is reduced when compared to a pressurized glycol system.
 
 
  5- Types of Solar Collectors
 
  1- Flat Plste Collectors

   Flat Plate collectors are commonly used in solar water-heating systems in residential, commercial and industrial projects. A flat-plate collector consists basically of an insulated metal box with a glazed glass cover and a dark-colored absorber plate. Heat from the sun strikes the absorber plate and is transferred to a fluid that circulates through the collector in tubes.


   The collector housing can be made of plastic, metal or wood, and the glass front cover must be sealed so that heat does not escape, and dirt, insects or humidity do not get into the collector itself.
 

   Many collectors also have controlled ventilation, so as to avoid condensation inside the glass front cover. The collector housing is highly insulated at the back and sides, keeping heat losses low. However, there are still some collector heat losses, mainly due to the temperature difference between the absorber and ambient air, and these are subdivided into convection and radiation losses. The former are caused by air movements, while the latter are caused by exchange of heat by radiation between the absorber and the environment.

   Flat collectors can be mounted in a variety of ways, depending on the type of building, application, and size of collector. Options include mounting on a roof, in the roof itself, or free-standing. The type of thermal system, drain-back or closed loop, can also have a big impact on how the panels are mounted. A drain-back system must ensure that all water is removed from the panels and tubing, if these systems are used in climates where freezing can occur.
  2- Evacuated tubes collectors
    Evacuated tube solar collectors are very efficient and can achieve very high temperatures. An evacuated-tube collector contains several rows of glass tubes connected to a header pipe, or directly to the hot water tank. Each tube has the air removed from it (evacuated) to eliminate heat loss through convection and radiation. Inside the glass tube, a flat or curved aluminum or copper fin is attached to a metal pipe. The fin is covered with a selective coating that transfers heat to the fluid that is circulating through the pipe. There are two main types of evacuated tube collectors:

  Evacuated-tube collectors can get very hot, exceeding the boiling point of water and can cause significant issues in an existing domestic solar water system. You need to use your hot water every day to ensure the temperature doesn't overheat in the tanks.
 
 
 
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