Solar collectors are characterised by their:
- Optical losses
- Heat losses
These losses can
be used to calculate the global efficiency of the collectors, in a given
Solar collectors are characterised by their optical efficiency (coefficient B).
The optical efficiency of the collector represents the percentage of the sun’s radiation power which will actually be absorbed by the solar collector.
efficiency is sometimes referred to as solar efficiency. This should not be
confused with the global output or efficiency of the collector which will be
defined later, and which can only be calculated in a given operating mode.
What is the optical efficiency B of the solar collector below?
The above collector converts 1,000 [W/m²] of solar radiation to 800 [W/m²] of heat for the heat transfer fluid.
Its optical efficiency B is therefore:
(800 x 1,000) x 100 = 80%
When the heat transfer fluid in the solar collector warms up, some of the received heat is lost by convection and radiation.
These losses are specified by a coefficient k (or a1) expressed in [W/m².K].
The coefficient k indicates these thermal losses in watts per 1 [m²] of the collector and [K] as the temperature difference between the heat transfer fluid and the exterior air temperature.
Well insulated flat plate collectors have a coefficient k of approx. 3 [W/m².K].
collectors have a coefficient k of approx. 1 [W/m².K].
They lose around 4 times less power through convection and conduction for the same temperature difference between the fluid and the exterior air as the flat plate collectors.
- The coefficient k (or a1) of the collector refers to the front surface of the collector.
- The European norm, EN 12975-1-2006 indicates the minimum specifications for collectors manufactured in Europe.
An example of collector specifications:
Calculate the actual power recovered by the collector below, knowing that its optical efficiency is 85% and its coefficient k is 3 [W/m².K].
The solar energy flow received is 1,000 [W/m²].
The surface area of the collector is 1 [m²].
It therefore receives 1,000 [W].
The optical efficiency of the collector is 85%.
Hence 850 [W] is recovered by the absorber at the bottom of the collector.
However, part of this power escapes towards the exterior through the thermal losses of the collector due to convection and radiation.
The front surface area of the collector is 1 [m²].
The temperature difference between the interior and exterior of the collector is:
70 – 30 = 40 [K] (104°F)
P = k x S x ΔT = 3 x 1 x 40 = 120 [W].
The power recovered by the collector is therefore:
850 – 120 = 730 [W]
Global efficiency rate:
The overall or global efficiency rate of a solar collector is calculated from the optical and thermal efficiency under given operating conditions of the collector.
We can define the global efficiency rate of a collector as being the ratio between the amounts of energy recovered during a given lapse of time and the solar energy received by the collector during the same time period, under given stable operating conditions.
What is the global efficiency of the solar collector studied in the previous exercise?
The collector receives a solar energy flow of 1,000 [W], but owing to optical and thermal losses, only 730 [W] is actually recovered in the end by the heat transfer fluid.
Under these operating conditions the global efficiency rate is 73%.
For 1 [m²] of collector surface area we can state:
- Coefficient B: optical efficiency
- Solar radiation in [W/m²]
- Losses: thermal losses of the collector in [W/m²]
- k : coefficient of thermal losses in [W/m².K]
- Aft : Average heat transfer fluid temperature in [°C]
- Text : exterior temperature in [°C]
Using the efficiency calculation formulafor solar radiation of 1,000 [W/m²], an optical efficiency of 85% and a coefficient k of 3 [W/m².K], calculate the global efficiency rate of the collector defined in exercise n°1.