Here is an essay on the ‘Types of Solar Collectors’ especially written for school and college students.
Essay # 1. Flat Plate Collectors:
These are used for temperatures below 100°C. These can use both beam and diffuse solar radiation. They are installed in a fixed tilted position optimally oriented towards the equator.
The main components of a flat plate collector are shown in Fig. 8.3.
i. Transparent cover,
ADVERTISEMENTS:
ii. Absorber plate,
iii. Heat transfer fluid tube,
iv. Thermal insulation, and
ADVERTISEMENTS:
v. Casing.
Flat plate collectors used for water heating and space heating can produce 250 to 400 kWh per year and per square meter of useful heat. Evacuated tube collectors can produce useful heat at a temperature up to 250°C. The efficiency of the collector reduces with the increase of fluid temperature.
Evacuated Tube Collector:
The efficiency of a collector is calculated from intensity of solar radiation, optical and heat losses. Evacuated tube collectors are used for reduction of losses. The main applications are water heating, space heating, refrigeration and air-conditioning and process heat production. Energy in the range of 300 to 600 kW h /m2 per year can be attained.
Performance Evaluation of Flat Plate Collector:
ADVERTISEMENTS:
The useful heat output of a flat-plate solar collector is given by:
Qc = A[lcFR(τα)e-FRUc(Tin-Ta)] [W]
Where,
A = surface area of collector or absorber [m2]
ADVERTISEMENTS:
Ic = Intensity of solar radiation incident on the collector [W/m2]
FR = Heat removal factor of the collector
(τα)e = Effective product of transmissibility τ of the transparent cover and absorptivity a of the absorber.
Uc = Overall heat loss coefficient of collector [W/m2]
ADVERTISEMENTS:
Tin = Fluid inlet temperature [ °C]
Ta = Ambient air temperature [ °C]
The efficiency of the solar collector is defined as the ratio of the useful heat output of the collector and the solar energy flux incident on the collector.
The performance curve of a collector can be plotted between ƞc and X.
The following parameters can be found out from the performance curves:
1. At X = 0, ƞc = FR(τα)e = Effective optical efficiency.
2. The value of effective overall heat loss coefficient Fr Uc.
The outlet temperature of the fluid from the collector can be found out as:
Tout = Tin + Qc/ṁCp [ °C]
Where,
Tjn = Inlet temperature of fluid [ °C]
Qc = Useful heat output of collector [W]
ṁ = mass flow rate of fluid [kg/s]
Cp = Specific heat of fluid [J/kg-K]
The stagnation temperature is the temperature of the absorber when there is no fluid flow.
Designs of Flat Plate Collector:
Low temperature solar plants including various types of water and air heaters use flat plate collectors for collection and absorption of solar radiations and heating of working fluid.
The design of flat plate collector consists of the following:
1. Optical design of collector.
2. Thermal design of collector.
3. Selection of materials for critical components such as transparent cover, absorber plate with selective coating, fluid flow channels and thermal insulation.
1. Optical Design:
The flat plate collector works on the basis of green-house effect. A transparent cover plate admits the short wave length radiation from the sun and surroundings.
These radiations heat the absorber plate and reradiates long wave length radiation as the plate is at low temperature. The cover plate is opaque to these radiations.
The cover plate should have the following properties:
i. Radiation received on the surface may be partly transmitted, partly absorbed and partly reflected depending upon the properties of the cover.
ii. I = Iα + Iρ + Iτ
I = Amount of incident radiation
Iα = Amount of radiation absorbed
Iρ = Amount of radiation reflected
Iτ = Amount of radiation transmitted
Also Iα = α I
Iρ = ρ I
Iτ = τ I
α + ρ + τ = 1
where,
α = Iα/I = Absorptivity of cover plate
ρ = Iρ/I = Reflectivity of cover plate
τ = Iτ/I = Transmissibility of cover plate
iii. A perfectly transparent plate will have:
α = 1
ρ = α = 0
iv. Most real materials are only partially transparent. The transmittance is dependent both upon the reflection and absorption of radiation. Transmissibility can be calculated as
τ = 1 – ρ – α
It can also be calculated as follows:
(a) Calculate τρ considering reflection alone and taking α = 0
(b) Calculate transmissivity τρ considering absorption only and ρ = 0
(c) Calculate transmissivity τ taking care of both reflection and absorption as follows.
τ = τρ × τα
v. Calculation of τρ:
Transmittance of a single cover when α = 0
τρ1 = 1-ρ/1+ρ
Transmittance for a system of n covers,
τρ, n = 1 – ρ/1 + (2n – 1)ρ
where,
n = No. of covers of the same material.
vi. Calculation of τα:
Transmittance of a single cover when ρ = 0
τα = e-kL
Transmittance of n covers when angle of reflection = θ2.
where,
k = extinction coefficient
= 0.01/cm for clear and white glass
= 0.32/cm for poor quality glass
L = Actual path of the radiation through the cover plate.
vii. Transmittance Absorptance Product:
The radiation passing through the cover system strikes the absorber plate. Most of the radiation is absorbed but some part is reflected back.
The radiation absorbed = τ ∝
The radiation reflected back = (1 – α) τ.
This reflected radiation is mostly diffuse radiation which is again reflected back to absorber plate by the cover plate. This process continues. The sum of total radiations absorbed will be more than τ ∝.
Total radiation absorbed
where,
ρd = Diffuse radiation reflected by cover plate
= 0.16 for incidence angle of 60°.
2. Thermal Design:
The radiations striking the absorber plate raise its temperature. Under the plates, fluid carrier tubs are bonded through which working fluid flows and gets heated. The energy balance for the whole collector can be written to find out the useful heat output of the flat-plate solar collector.
QC = A[IC FR (τα)e – FR UC (Tin – Ta)] [W]
where,
A = surface area of collector or absorber [m2]
IC = intensity of solar radiation incident on the collector [W/m2]
FR = heat removal factor of the collector
<τα>e = effective product of transmissibility τ of the transparent cover and absapticity ∝ of the absorber.
UC = overall heat loss coefficient of collector [W/m2]
Tin = Fluid inlet temperature [°C]
Ta = Ambient air temperature [°C]
Here, absorbed energy = A IC FR <τα>e
Effective heat loss = A UC (Tp – Ta)
where,
Tp = Temperature of absorber plate
i. Evaluation of Overall Loss Coefficient (UC):
The energy loss from the collector plate consists of radiation and convection to the cover and edges and conduction through the back insulation. Thermal network of a flat plate collector is shown in Fig. 8.22.
Insulation Resistance (R1):
k = insulation thermal conductivity
x = insulation thermal thickness
Heat Loss Resistance from Absorber Plate (R2):
This constitutes convective resistance and radiation resistance.
R2 = 1/hp + hr
hp = heat transfer coefficient between two inclined parallel plates, i.e., absorber plate and cover plate
hR = radiation heat transfer coefficient
where,
σ = Stefan Baltzmann’s constant
Tp = Temperature of absorber plate (metal)
Tc = Temperature of cover plate (glass)
εp = Infra-red emissivity of the absorber plate
εc = Infra-red emissivity of the cover plate
Heat Loss Resistance from Top Cover to Surroundings (R3):
R3 = 1/hr + hω
hr = radiation conductance from top cover to sky
ii. Evaluation of Heat Removal Factor (FR):
Heat removal factor (FR) is defined as the ratio of actual useful energy collected by the fluid to the useful energy collected if the entire collector absorber surface were at the temperature of the fluid entering the collector.
Where,
ṁ = mass flow rate of fluid [kg/s]
cp = heat capacity of fluid [kW/kg-K]
F = collector efficiency factor
Material Selection of a Flat Plate Collector:
The performance and durability of a flat plate collector depends upon the proper selection of material for critical components.
1. Cover Plate:
The purpose of cover plate is:
(i) To transmit maximum solar energy to the absorber plate.
(ii) Heat loss from absorber plate to environment should be minimum.
(iii) To protect absorber plate from direct exposure to weathering.
(iv) To receive maximum solar energy for the maximum duration in a day.
The most important properties are:
(i) Transmissibility,
(ii) Strength,
(iii) Durability, and
(iv) Non-degradability.
The most common materials used are:
(i) Tampered glass with low iron content.
(ii) Plastics such as Lexian.
2. Absorber Plate:
The most important properties of absorber plate should be:
(i) High thermal conductivity.
(ii) High strength.
(iii) Good corrosion resistance.
The common materials used are:
(a) Copper due to high conductivity and resistance to corrosion.
(b) Aluminium.
(c) Steel.
(d) Thermoplastics.
3. Fluid Flow Channels:
(i) Tubes soldered or welded to the bottom of absorber metal plate.
(ii) Roll-Bond panel made of formed copper or aluminium sheets to provide fluid channels.
4. Thermal Insulation:
Thermal insulation of 5 to 10 cm thickness is placed behind the absorber plate to prevent heat losses from the bottom surface.
The insulation materials used are:
(i) Mineral wool.
(ii) Glass wool.
(iii) Heat resistance fibre glass.
5. Selective Coating:
An effective way to reduce thermal losses from the absorber plate is by using selective absorber coating. This coating will make absorber plate a perfect absorber of solar radiator of short wave lengths and perfect reflector or poor emitter of thermal radiation of long wave lengths.
Some common selective coatings with their properties are given below:
Other properties of selective coating should be:
i. Non-degradability due to exposure to humidity.
ii. Ability to withstand temperatures and thermal shocks.
iii. Resistance to atmospheric corrosion and oxidation.
iv. Reasonable cost.
Essay # 2. Concentrating Collectors:
These are used for medium and high temperature applications. Various types of concentrating solar collectors are shown in Fig. 8.4. It consists of a concentrating device which may be a reflecting mirror or Fresnel lense. The beam radiations are concentrated and focused on to the absorber. Different types of collectors have different values of concentration ratio.
The concentration ratio,
c = Aa/Ar
where,
Aa = Aperture area of the concentrator
A = area of absorber
Parabolic trough concentrator is a linear concentrator. The value of C may be 20 to 100.
Parabolic disk concentrator is a point focus concentrator. The value of C may be 100 to 4000 for parabolic disk and heliostat field concentrator.
The mirrors and Fresnel lenses can focus only beam radiation. These concentrators are rotated through tracking mechanism to follow the motion of the sun in the sky. Temperatures as high as 1000°C can be achieved. These are mainly used in solar plants for power generation and process heat supply.
Performance Evaluation of Concentrating Collector:
The useful heat output,
Qc = FR Aa [Ibc ƞopt – (Uc/C)](Tin – Ta) [W]
Where,
FR = Heat removal factor of the collector.
Aa = Unshaded aparture area [m2]
Ibc = Intensity of beam radiation on aparture of the collector (W/m2)
Ƞopt = Optical efficiency of the collector.
Uc = Total heat loss coefficient of collector [W/m2-K]
C = Concentration ratio of the collector
Tin = Inlet temperature of fluid [ °C]
Ta = Ambient temperature [ °C]
Optical Efficiency:
It is the ratio of solar radiation absorbed and the beam radiation on the concentrator.
ƞopt – ƞopt (0°) Copt
= ρ γ ταa Copt.
where,
ƞopt(0°) = Optical efficiency at 0° incidence angle of beam
Copt = Correction factor for deviation from 0°
ρ = Reflectivity of mirror
γ = Intercept factor
τ = Transmissivity of the cover (if available)
αa = Absorptivity of absorber
Efficiency of the collector,
Figure 8.6. Shows the efficiency of a collector as a function of absorber temperature for different concentration ratios.
There can be phase change of fluid in a concentrating collector.
The outlet enthalpy:
hout = hin + Qc/ṁ [J/kg]
Where,
hin = enthalpy of inlet fluid [J/kg]
Qc = Useful heat output [W]
ṁ = mass flow rate of fluid [kg/s]
Comparison of Performance of Different Collectors:
The technical and economic parameters of different collectors are tabulated in Table 8.1 for comparison.
Optical Design of Focusing (Concentrating) Collectors:
High temperature solar plants are solar-electric conversion systems where the collectors gather the sun’s energy and direct it onto receivers that contain the working fluid of the thermodynamic cycle.
The main designs are:
1. Solar-Thermal Central-Receiver System:
A large field of reflecting mirrors called heliostats receives and redirects the sun’s energy and concentrates it on a central receiver mounted on top of a tower. The heliostats are individually guided, since they cover a large field so that each plain mirror focuses the solar radiations on the central receiver at all hours of sun light.
In the receiver the concentrated solar energy is absorbed by a circulating fluid. The fluid could be water, which vaporises into steam that is used to drive a turbo-generator in a Rankine cycle, or an intermediate fluid that transports the heat to the steam cycle.
2. Parabolic Trough System:
Long troughs of cylindrical parabolic shape are lined with mirrors to collect and concentrate the solar radiation onto a focal linear conduit through which the primary fluid flows (Fig. 8.9). The fluid from different receivers is combined for the thermal-electric conversion.
3. Dish-Stirling Engine System:
A mirrored parabolic dish tracks the sun and focusses the captured energy on a receiver which is mounted at the focal point of the parabola.
The focussing collectors involve additional optical losses.
The optical efficiency,
Ƞopt = ργταa Copt
(a) Reflectance Loss:
It depends upon the reflectivity of mirror p and depends upon the nature and smoothness of the reflector surface.
(b) Intercept Factor (γ):
The energy received by the absorber depends upon the part of the reflected radiation that is intercepted by the receiver.
The intercept factor depends upon:
(i) The shape, size and orientation of reflector
(ii) The shape, size and positioning of receiver relative to the reflector/concentrator.
(c) Concentration Ratio (C):
The ratio of effective area of the aperture to area of the solar energy absorber is called concentration ratio.
C = Aa/Ar
The collector efficiency increases with the increase of concentration ratio.