Solar Collector: An over view
 
Solar collectors transform solar radiation into heat and transfer that heat to a medium (water, solar fluid, or air). Then solar heat can be used for heating water, to back up heating systems or for heating swimming pools.
 
A typical ETC solar is a metal box with a glass or plastic cover (called glazing) on top and a dark-colored absorber plate on the bottom. The sides and bottom of the collector are usually insulated to minimize heat loss.

Sunlight passes through the glazing and strikes the absorber plate, which heats up, changing solar energy into heat energy. In addition, the transparent cover prevents wind and breezes from carrying the collected heat away (convection). Together with the frame, the cover protects the absorber from adverse weather conditions. Typical frame materials include aluminum and galvanized steel; sometimes fiberglass-reinforced plastic is used.

The heat is transferred to liquid passing through pipes attached to the absorber plate. Absorber plates are commonly painted with "selective coatings," which absorb and retain heat better than ordinary black paint. Absorber plates are usually made of metal—typically copper or aluminum—because the metal is a good heat conductor. Copper is more expensive, but is a better conductor and less prone to corrosion than aluminum. In locations with average available solar energy, ETC solar collectors are sized approximately one-half- to one-square foot per gallon of one-day's hot water use. Flat collectors demonstrate a good price-performance ratio, as well as a broad range of mounting possibilities (on the roof, in the roof itself, or unattached).
 
Applications: The main use of this technology is in residential buildings where the demand for hot water has a large impact on energy bills. This generally means a situation with a large family, or a situation in which the hot water demand is excessive.

Commercial applications include hospitals, hotels, resorts, convents, etc. By installing a solar water heater, these establishments save hundreds of thousands of rupees due to excessive hot water demand.

Unglazed liquid collectors are commonly used to heat water for swimming pools. Because these collectors need not withstand high temperatures, they can use less expensive materials such as plastic or rubber. They also do not require freeze protection because swimming pools are generally used only in warm weather or can be drained easily during cold weather.

While solar collectors are most cost-effective in sunny, temperate areas, they can be cost effective virtually anywhere in the country so should be considered
 
Performance/Costs: The Hykon solar systems are certified by ISI for quality and performance – to compare costs of different systems often payback period is used. (The amount of time required - usually in years - for positive cash flows to equal the total investment costs. This is often used to describe how long it will take for energy savings resulting from using more energy-efficient equipment to equal the premium paid to purchase the more energy-efficient equipment.) – This varies widely, but for a well-designed and properly installed solar water heater, you can expect a simple payback of 2to 4years, depending on climate and utility costs. It is found that solar water heaters offer potential savings, compared to electric water heating, of as much as 85% in the water heating portion of the utility bill.

ETC Solar Water heaters range in price from Rupees 17000 to 18000 installed for residential systems (for 100-300Liters per day usage), and 30,000 and above for commercial systems. Industrial systems cover areas where process heating is required.

Any application where heating is a requirement solar heating is a possible alternative.


“A typical evacuated tube collector” All glass-evacuated tubes are the main component of Hykon ETC solar water heater collectors. The evacuated tube consists of two borosilicate glass tube, a glass with high chemical and thermal shock resistance. The outer side of the inner tube is coated with sputtered solar selective surface. This coated inner tube is closed at one end and sealed at he other end to the outer tube. The annual space between the outer tube and the inner tube is evacuated to virtually eliminate heat loss by conduction and convection.
 
Comparison : Flate Plate v/s ETC Solar Water Heaters
 
Features Conventional Flate Plate Solar heater HYKON ETC Solar water heater
Sun tracking Sun tracking not possible due to flat design. Auto sun tracking due to circular shapes of the vaccum tubes.
Water quality Highle suscepitable to hard water resulting in loss of efficiency. Glass tubes of higher diameter. Practically free of scaling effects.
Start up time 4 to 5 hours to heat. Less than 2 hours.
Space required Length of approx. 10 ft. System overall length only 6 feets.
Effect of weather conditions Much lesser or no hot water during rainy/cloudy days. Efficient even in rainy seasons.
 
Output of a solar collector

The efficiency of a solar collector is defined as the quotient of usable thermal energy versus received solar energy. Besides thermal loss there always is optical loss as well. The conversion factor or optical efficiency h0 indicates the percentage of the solar rays penetrating the transparent cover of the collector (transmission) and the percentage being absorbed.

Basically, it is the product of the rate of transmission of the cover and the absorption rate of the absorber.

 
   
CO² emissions from heating systems producin3,500 kWh/a (meets the warm water needs of a four to five person household) and with a solar coverage percentage of 65%

Efficiency graph of solar collector

The heat loss is indicated by the thermal loss factor or k-value. This is given in watt per m² collector surface and the particular temperature difference (in °C) between the absorber and its surroundings. The higher the temperature difference, the more heat is lost. Above a specific temperature difference, the amount of heat loss equals the energy yield of the collector, so that no energy at all is delivered to the solar circulation system.
A good collector will have a high conversion factor and a low k-value.

 
Type of Collector Conversion Factor Thermal Loss Factor in W/m² °C Temperature Range in °C
Absorber (uncovered) 0,82 to 0,97 10 to 30 Up to 40
Flat-plate collector 0,66 to 0,83 2,9 to 5,3

20 to 80
Evacuated-plate collector 0,81 to 0,83 2,6 to 4,3

20 to 120
Evacuated-tube collector 0,62 to 0,84 0,7 to 2,0

50 to 120
Reservoir collector

About 0,55

 About 2,4

20 to 70
Air collector 0,75 to 0,90

8 to 30

20 to 50
Collector selection

The desired temperature range of the material to be heated is the most important factor in choosing the correct type of collector. An uncovered absorber is certainly not suitable for producing process heat. The amount of radiation on that spot, exposure to storms, and the amount of space must all be carefully considered when planning a solar array.
Graph of efficiency and temperature ranges of various types of collectors (radiation: 1000 W/m²)

For INDIA as a tropical country ETC solar collectors are more suitable from the system life point of view. However for process heating applications evacuated tube collectors are preferred.

Collector /system efficiency is greatly dependent on many factors such as insolation levels, ambient temperature, collector tilt, system installation, system configuration etc. after all an efficient collector doesn’t mean an efficient solar system. It depends on proper system selection and proper configuration and usage.
Collector Performance

Solar water heater performance is normally expressed with respect to a set of three parameters

Following is the parameters to express efficiency of solar collectors.

These are namely

Conversion Factor: h0 = 0.0.75
Loss Coefficient: a1 = 1.2 W/(m2K)
Loss Coefficient: a2 = 0.0023 W/(m2K2)

The collector efficiency can be expressed based on gross area, aperture area or absorber area. Solar water heater performance is often presented as a graph,

Another factor required for the calculation of collector efficiency is the value (x)

Insolation level (G) in Watts/m2, ambient temperatures (Ta) and average manifold temperature (Tm) must be known to get the value x.
Typical collector efficiency

The capacity conversion is based on the following simplified typical collector efficiencies (based on aperture area):

1. Unglazed ETC solar collectors: h0 = 0,90, a1 = 20,0 W/(K.m²), a2 = 0,00 W/(K².m²)
2. Glazed ETC solar collectors: h0 = 0,78, a1 = 3,2 W/(K.m²), a2 = 0,015 W/(K².m²)
3. Evacuated tubular collectors: h0 = 0,76, a1 = 1,2 W/(K.m²), a2 = 0,008 W/(K².m²)

An example

Based on the ambient temperature, average manifold temperature and insolation level; firstly calculate the value for x.

Eg. At 2:00pm, the ambient temperature is 25oC, and the average water temp [(Tin+Tex)/2] is 50oC). The insolation level is 800Watts/m2.

X = (50-25)/800 = 0.03125

Now enter all the values into the formula:

h(x) = 0.717 - (3.15*0.03125) - (0.015*800*0.03125*0.03125)

h(x) = 0.717 - 0.0984 - 0.0117 = 0.6069

The solar conversion efficiency for that specific point in time and set of environmental conditions is 60.7%. That is: 60.7% of the energy provided by the sun is actually used to heat the water.

Based on the assumption that those three environmental factors (G, Tm and Ta) are stable for a period of one hour, then 800 x 0.607 = 485.6 Watts of energy per m2 of absorber area will be used to heat the water (485.6Watts is equivalent to 418kcal, which could heat 100L of water by 4.18oC). Normally the Delta-T values will be in the range of 20-50oC, with higher values present for high temperature heating such as for absorption cooling applications, or during very cold weather.

As can be seen conversion efficiency is highly dependent on solar insolation levels, with higher insolation results in greater levels of solar conversion.

In reality ambient temperature will fluctuate, and the manifold temperature will gradually increase as the water is heated. Furthermore insolation levels may fluctuate with intermittent cloud cover. In order to more accurately calculate energy output, a more complete set of environmental data must be considered and many (hourly) performance calculations throughout the day taken. Your local Hykon distributor can provide estimates of average monthly and annual performance, heat output and thus solar contribution for your location.

One factor, which is not considered in the straight performance calculations, outlined above, is the effect of transversal IAM values (Incidence Angle Modifier) on solar collector output throughout the day. Longitudinal IAM is useful when looking at installation angle, and the changes in performance throughout the year as angle of the sun above the horizon changes between winter and summer.
Installation requirements

The angle and direction of installation is also of great importance, as it will affect the efficiency of the solar collector. For maximum efficiency the collector should receive the maximum amount of sunlight each day and throughout the year. As a general rule if you are in the Northern Hemisphere then the collector should face South and if you are in the Southern Hemisphere then the collector should face North. See diagram below.
The angle at which you mount the collector should roughly correspond to the latitude of your location. For example:
You do not have to be too careful about mounting the collector at the exact angle suggested. If your roof angle is within 10o+/- of your desired angle you can just mount the solar collector flush against the roof surface. For stand-alone systems the angle has to be adjusted to the required tilt.
But a very accurate angle adjustment doesn’t guaranty a better performance.
Seasonal factor in heat generation

For systems designed to meet 90-100% of heat requirement, there will be seasonal changes in the heat production and consumption.

As it can be seen from the diagram, in the northern hemisphere sun will be low in the sky during winter and high in the sky during summer. As the hot water requirement is comparatively low during summer, a system designed for the winter requirement may produce excess hot water. This will result in energy loss and some times system damage also.

This problem can be reduced to a certain extent by designing the system considering this seasonal consumption pattern.

Also a tilt adjusting mechanism is highly effective in some cases facing the collectors perpendicular to the winter resulting in reduced tilt angle during summer and subsequently the heat production.

In some cases detaching one or more collectors from the system also results in reducing the total heat output and thereby less system losses.

A solar collector is only one part of a solar heating system. Storage tanks, pumps, piping, controllers, valves and various plumbing components are required to form a complete system.

In many cases a household or commercial building may wish to retrofit solar into the existing water heating system. Rather than replacing the existing water heating equipment, which may still be in good operating condition, an additional “solar tank” can be added to store the heat provided by the solar collectors.
Thumb rule for system selection
Design Element Design Value
1. Collector slope Latitude +100C
2. Collector azimuth 00+ /-150
3. Water flow rate through collector 0.6-1.6 Liters /Min. per M2 collector area
4. Storage tank size

50-70 Liters per M2 collector area
5.Collector size 3-4.5 M2 selective surface areas for a family of 4.
6. Tank size 25-50 liters per person.
 

Comparison between different Hot water heating equipment

System

Electr. Heater

 Gas heater Diesel fired Solar heater
1.Cost Rs. 5,400.00

Rs. 6,700.00

 Rs. 10,000.00 Rs.17, 000.00
2.Life 5~7Years 4~5Years 5~7Years

Up 15 Years
3.Fuel consumption Electrical LPG Diesel None
4.Fuel efficiency 0.9 0.6 0.7 0.6
5.Fuel cost Annual Rs.6,500.00 Rs.5,400.00 Rs. 10,000.00 Rs.300.00 (with back up)
 
Comparison of performance features:
 

Electr. Heater

 Gas heater Diesel fired Solar heater
1.Operation Easy

Easy

 Easy

Automatic
2.Security Short circuit/ power out. Gas leak/ Risk of explosion Risk of explosion

100% safe.
HYKON solar water heaters installation guidelines

· Select the site for installation in a shade free area.

· The system should be selected for the type of water source.

· The system should be firmly grouted and fixed on the surface.

· Collector angle has to be considered depending on the positions latitude.

· The collectors should face NOUTH in the northern hemi-sphere.

· Connect the inlet and outlet piping to the appropriate tank positions.

· Connect the hot water tank and the overhead tank appropriately.

· The overhead tank should be at least 1 foot above the solar tank.

· Air vent should be provided close to the hot/cold water tank projecting above the overhead tank.

· Insulate the hot water lines adequately and leak proof.

· Electric back up heater should be connected to 230 Volts AC in series wit the thermostat.

· Set the temperature of the thermostat between 50-70 0C.

· The hot water outlets should not be more than 1 or 2 for 100LPD, 2 or 3 for 200LPD, 3 or 4 for 300LPD.

· Recommended outlet piping are 1/2 “ forn100LPD and ¾” for 200 and above.
Safety precautions

· Should not connect solar water heaters to any electrical geysers.

· Use the electrical; backup only when required. Switch OFF when not in use.

· If any leakages are observed tighten the parts or call the service technician.

· Clean/ swipe the collector surface with fine cloths to remove dust.

· Do not install systems meant for normal pressure, to pipe line with pressure pump.

· Care should be taken not to drop/throw any hard objects on the glass cover of the collector.

· Paint the MS support stand once in a year to prevent possible rusting.
 
Trouble shooting guide
Sl.no
 Problem Check for
1

No water

 1.Check overhead tank water level
2.Check the valves.
2

No hot water

 1.Lack of sun light/cloudy weather.
2.Check the valves.
3.Check for air lock and release the air if locked.
4.Excessive water consumption during daytime.
3 Partial heating of hot water. 1.Lack of sun light/cloudy weather.

2.Mixing through mixer taps.
3.Collector in shady area.
4.Wrong system layout
5. Wrong connection.
6.Improper insulation of hot water lines.
7.Air lock in inter-connecting pipes.
8.Too long piping.
9. Leakage in the system
10. Excessive use of hot water.
 
REFERENCES
1. American Solar Energy Society, ases@ases.org · www.ases.org

2. Solar Energy Industries Association, www@seia.org

3. US department of Energy, Energy Efficiency and Renewable Energy (EERE)

4. The solar server -forum for solar energy.

5. ASHRAE – standard Users manual.

6. The Information Group, Solar Energy Center,

Ministry of Non conventional Energy Sources (MNES), Govt. Of INDIA.

7. World Energy Council- www. Worldenergy.org

8 - American Society for Heating, Refrigeration and Air conditioning Engineers. (ASHRAE)

9. Solar Heating and Cooling Program of the International Energy Agency(IEA SHC)

10.Solar Rating and Certification Corporation-SRCC, www.solar-rating.org
 
TEXTS
1. SOLAR ENERGY, principles of thermal collection and storage.2nd edition. S.SP Sukhatme.

2.Non- Conventional energy Sources- G.D Rai.
 
 
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