The FPC (Flat-Plate Collectors) is the heart of any solar energy collection system designed for operation in the low-temperature range (less than 60°C) or in the medium temperature range (less than 100°C).
Overview
It is used to absorbed solar energy, convert it into heat, and then to transfer that heat to stream of liquid or gases. They use both direct and diffuse solar radiations, do not require tracking of the Sun, and require little maintenance. They are mechanically simpler than concentrating collectors. The major applications of these units are in solar water heating, building heating, air conditioning, and industrial process heat. An FPC generally consists of the following components :
Components
- Glazing consists of one or more sheets of glass or other radiation-transmitting material.
- Tubes, fins, or passages to conduct or direct the HTF from the inlet to the outlet.
- Absorber plates are flat, corrugated, or grooved plates, to which the tubes, fins, or passages are attached. The plate may be integral with the tubes.
- Headers or manifolds to admit and discharge the HTF.
- Insulation to minimize heat loss from the back and sides of the collector.
- Container or casing to surround the other components and protect them from dust or moisture. FPCs have been built in a wide variety of designs from many different materials (Fig. below). Their major purpose is to collect as much solar energy as possible at lowest possible total cost.
The collector should also have a long effective life, despite: the adverse effects of the Sun’s ultraviolet radiation; corrosion or clogging because of acidity, alkalinity, or hardness of the HTF; freezing or air binding in the case of water, or deposition of dust or moisture in the case of air.
Types
Glass has been widely used to glaze FPCs because it can transmit as much as 90% of the incoming shortwave solar radiation while transmitting very little of the longwave radiation emitted outward from the absorber plate. Glass with low iron content has a relatively high transmittance for solar radiation (0.85–0.90 at normal incidence).
Plastic films and sheets also have high shortwave transmittance, but because most usable varieties also have transmission bands in the middle of the thermal radiation spectrum, their longwave transmittance may be as high as 0.40.
The glass generally used in solar collectors may be either of single-strength (2.2–2.5 mm) or double-strength (2.92–3.38 mm). For direct radiation, the transmittance varies markedly with the angle of incidence θ, as shown in Table below.
Incident Angle of Transmittance and Absorptance
Variation With Incident Angle of Transmittance and Absorptance
| Incident Angle θ (°) | Transmittance τ | Absorptance α (for Flat-Black Paint) | |
| Single Glazing | Double Glazing | ||
| 0 | 0.87 | 0.77 | 0.96 |
| 10 | 0.87 | 0.77 | 0.96 |
| 20 | 0.87 | 0.77 | 0.96 |
| 30 | 0.87 | 0.76 | 0.95 |
| 40 | 0.86 | 0.75 | 0.94 |
| 50 | 0.84 | 0.73 | 0.92 |
| 60 | 0.79 | 0.67 | 0.88 |
| 70 | 0.68 | 0.53 | 0.82 |
| 80 | 0.42 | 0.25 | 0.67 |
| 90 | 0.00 | 0.00 | 0.00 |
Antireflective coating and surface texture can improve transmission significantly. The absorber plate absorbs as much of the radiation as possible through the glazing, while losing as little heat as possible up to the atmosphere and down through the back of the casing.
The absorber plate transfers retained heat to the transport fluid. The absorptance of the collector surface for shortwave solar radiation depends on the nature and color of the coating and on the incident angle, as shown in Table for a typical flat-black paint.
By suitable electrolytic or chemical treatments, selective surfaces can be produced with high values of solar radiation absorptance α and low values of longwave emittance ε. Selective surfaces are particularly important when the collector surface temperature is much higher than the ambient air temperature.
Materials most frequently used for absorber plates are copper, aluminum, and stainless steel. UV-resistant plastic extrusions are used for low-temperature applications. Potential corrosion problems should be considered for any metals.
Materials most used for insulation are mineral wool, glass wool, and foam glass. Polyurethane foam is used for low-temperature application and styropor is used very rarely. The main properties of insulation materials are summarized in Table below.
Insulation Materials
Main Properties of Insulation Materials
No. | Materials | Accepted Temperature (°C) | Density (kg/m3) | Thermal Conductivity (W/(m K)) |
1 | Mineral wool | >200 | 60–200 | 0.040 |
2 | Glass wool | >200 | 30–100 | 0.040 |
3 | Foam glass | >200 | 130–150 | 0.048 |
4 | Polyurethane foam | 30–80 | 0.030 | |
5 | Styropor | 30–50 | 0.034 |
FPC is usually permanently fixed in position and requires no tracking of the Sun. The collectors should be oriented directly toward the equator, facing south in the northern hemisphere and north in the southern. The optimum tilt angle of the collector is equal to the latitude of the location with angle variations of 10–15°C, more or less depending on the application.
Solar Air Collector
Solar air collectors have the operating principles similar to FPCs (Fig. below).
The difference is that instead of liquid fluid, an electric fan pumps air through the collector. The main components are: absorber plate (1), glass cover (2), insulation (3), casing (4), and exhaust fan (5).
This collector type is not very common in Europe (covers only 1–2% of the solar liquid collector market). Reasons for this might be that on the one hand, the lack of experience and the lack of knowledge of the end users, and on the other hand, this collector type cannot be used directly for DHW production, which dominates the market today.
Energy Balance
In Fig. below is illustrated energy balance of a standard FPC. From total solar radiation IT incident on glass cover, a part (τ · IT) determined of transmittance τ reaches the absorber plate where it is transformed in heat.
The glass cover reflects in space radiation (ρ · IT) and absorbs radiation (α · IT), where ρ and α are the glass cover reflectance and absorptance, respectively. One part of radiation (τ · IT) incident on absorber plate is reflected and the most part of this radiation is transformed in heat. The sum of the coefficients τ, ρ, and α is
