Page 107 - GRIHA Manual Volume III - Introduction to National Rating System
P. 107
100 GRIHA Manual: Volume 3
There is an Indian Test Standard (IS12933) for FPC collectors approved by the Bureau of Indian
Standards (BIS), which essentially characterizes the performance of FPC. The characterization
parameters for FPC are namely (a) heat loss factor (F U ) and (b) optical efficiency factor (F h ). In
R o
R
L
Indian conditions, the acceptable values of F U varies from 4.0–6.0 W/m / C and F h from 0.60–
2 o
L
R
R o
0.70 respectively.
Evacuated tube collectors (ETC): Evacuated glass tubes are the key components of evacuated tube
collector based water heating system. These evacuated glass solar tubes (EGT) have vacuum jacket
as insulation, which allows perfect sunlight penetration but cuts off thermal heat transfer. Each EGT
has two concentric borosilicate glass tubes, with the outside surface of the inner tube having a
selective coating. The jacket between the cover glass tube and the inner glass tube is evacuated
and permanently sealed off to form a vacuum. In order to maintain vacuum in the jacket between
the two tubular glass tubes, a barium getter (gas absorbent) is used at the bottom of the cover
tube. During the manufacturing process, this getter is briefly exposed to high frequency magnetic
fields that generate high temperatures instantly. Due to this, the bottom of the evacuated tube gets
coated with a layer of pure barium. During operation, this barium layer actively absorbs any CO, CO ,
2
N , O , H O, and H from the tube, thus, helping the vacuum tube to have a long life span of up to
2
2
2
2
15–20 years.
As the heat losses are lesser, the evacuated tube collectors are more efficient than flat-plate
collectors and can perform better at higher temperatures.
Hot water tank
The storage tanks are made of stainless steel, mild steel or copper. These have a heat-resistant
anti-corrosion protective coating on the inside. To minimize heat losses, the tank is covered with
insulation such as rockwool pads or polyurethane foam. The insulation is covered with aluminium
sheet cladding, reinforced fibre glass, fibreglass reinforced plastic (FRP) cover or suitable grade
plastic cover. The tanks are commercially available in a variety of sizes and shapes.
Plumbing
Plumbing forms an important component of the solar water heating system. The pipes carrying hot
water should be well insulated in order to minimize heat losses. The thickness of insulation depends
upon the thickness of the pipes used. Most commonly used pipes for this purposes are G I pipes
and fittings, which are BIS-approved class ‘B’ or higher. In case pipes are hidden inside the walls,
insulating them ensures that the expansion of pipes will not lead to cracks in the wall.
Controls and instrumentation
Gate valves or ball valves are used to manage water flow. Problems like pressure buildup or vacuum
formation are taken care of by providing pressure and vacuum release valves or open vents.
Temperature gauge assists in measuring temperature of the water. In active systems, a temperature
difference controller is provided to manage the start and stop of the pump. The controller starts the
pump when the temperature at the outlet of the solar collector is higher than the water temperature
at the bottom of the storage tank by a preset value. Similarly, the controller stops the pump as soon
as this temperature difference drops below the specified limit.
There is an Indian Test Standard (IS12933) for FPC collectors approved by the Bureau of Indian
Standards (BIS), which essentially characterizes the performance of FPC. The characterization
parameters for FPC are namely (a) heat loss factor (F U ) and (b) optical efficiency factor (F h ). In
R o
R
L
Indian conditions, the acceptable values of F U varies from 4.0–6.0 W/m / C and F h from 0.60–
2 o
L
R
R o
0.70 respectively.
Evacuated tube collectors (ETC): Evacuated glass tubes are the key components of evacuated tube
collector based water heating system. These evacuated glass solar tubes (EGT) have vacuum jacket
as insulation, which allows perfect sunlight penetration but cuts off thermal heat transfer. Each EGT
has two concentric borosilicate glass tubes, with the outside surface of the inner tube having a
selective coating. The jacket between the cover glass tube and the inner glass tube is evacuated
and permanently sealed off to form a vacuum. In order to maintain vacuum in the jacket between
the two tubular glass tubes, a barium getter (gas absorbent) is used at the bottom of the cover
tube. During the manufacturing process, this getter is briefly exposed to high frequency magnetic
fields that generate high temperatures instantly. Due to this, the bottom of the evacuated tube gets
coated with a layer of pure barium. During operation, this barium layer actively absorbs any CO, CO ,
2
N , O , H O, and H from the tube, thus, helping the vacuum tube to have a long life span of up to
2
2
2
2
15–20 years.
As the heat losses are lesser, the evacuated tube collectors are more efficient than flat-plate
collectors and can perform better at higher temperatures.
Hot water tank
The storage tanks are made of stainless steel, mild steel or copper. These have a heat-resistant
anti-corrosion protective coating on the inside. To minimize heat losses, the tank is covered with
insulation such as rockwool pads or polyurethane foam. The insulation is covered with aluminium
sheet cladding, reinforced fibre glass, fibreglass reinforced plastic (FRP) cover or suitable grade
plastic cover. The tanks are commercially available in a variety of sizes and shapes.
Plumbing
Plumbing forms an important component of the solar water heating system. The pipes carrying hot
water should be well insulated in order to minimize heat losses. The thickness of insulation depends
upon the thickness of the pipes used. Most commonly used pipes for this purposes are G I pipes
and fittings, which are BIS-approved class ‘B’ or higher. In case pipes are hidden inside the walls,
insulating them ensures that the expansion of pipes will not lead to cracks in the wall.
Controls and instrumentation
Gate valves or ball valves are used to manage water flow. Problems like pressure buildup or vacuum
formation are taken care of by providing pressure and vacuum release valves or open vents.
Temperature gauge assists in measuring temperature of the water. In active systems, a temperature
difference controller is provided to manage the start and stop of the pump. The controller starts the
pump when the temperature at the outlet of the solar collector is higher than the water temperature
at the bottom of the storage tank by a preset value. Similarly, the controller stops the pump as soon
as this temperature difference drops below the specified limit.
