Solar hot water collector efficiency
The efficiency of the solar collector is directly related to heat losses from the surface of the collector. Heat losses are predominantly governed by the thermal gradient between the temperature of the collector surface and the ambient temperature. Efficiency decreases when either the ambient temperature falls or when the collector temperature increases. This decrease in efficiency can be mitigated by increasing the insulation of the unit by sealing the unit in glass (for flat collectors), or providing a vacuum seal (for evacuated tube collectors). Figure 3.18 shows efficiency curves of these collectors. When comparing collector efficiencies, it is important to assume the same type of area (net vs. gross), and the same irradiation level.
The thermal performance of solar hot water collectors is characterized by:
- The power curve as shown in Figure 3.18, parameters: η0, a1 and a2
- Incidence Angle Modifier (IAM) because of the optical efficiency of the collector
- Thermal capacity (Ceff), which is the measure of thermal response to heating and cooling.
- The quantity of heat input into the collector to heat it by -457.87 °F (1 °K). This information would be available from the collector manufacturer. This value is used in the solar collector simulation computer programs as it relates to the small time steps in the program to the estimated heat removed. The larger the Ceff, the more energy that will be lost when switching off and on the solar heat transfer pump, which can happen as the weather changes during the day.
Power curve
For the power curve, collector performance is measured at different operating temperatures and with perpendicular insolation of G > 29,360 Btu/hr/sq ft (800 W/m2). The collector’s performance is represented by:
Figure 3.19 shows the power curves of four low* temperature collectors. A “rule of thumb” is to select a collector type that achieves an efficiency η ~ 50% for the working temperature range.
For use in water and low temperature space heating, both flat plate collectors (with solar glass and selective coating) and evacuated tube collectors are applicable. Both have certain advantages in specific applications especially when freezing, leaving water temperature, available space and installation cost are concerns. For applications with large collector fields, these two collector types should always be considered, and the final decision to select one technology over the other should be based on annual simulation results.
Incidence angle modifier
The IAM [-] describes the modification of the conversion factor 0 ηof the collector for nonperpendicular solar incidence angles. By definition, an IAM equal to 1 is for normal incidence. The IAM has a significant effect on the performance of stationary installed collectors as the incidence angle changes throughout the day and the year. Incidence angles less than 50 degrees do not have a significant effect on the solar thermal collector efficiency while an Incidence angle of 90 degrees is equal to a total reflection of the sun rays. Figure 3.20 shows the IAM curves of a typical FPA and a typical ETC.
The longitudinal IAM (i.e., in the direction parallel to the tubes for the ETCs) of the ETCs is similar to the flat plate collector’s while the transversal IAM of most ETCs shows a characteristic increase at intermediate angles.
Collectors with a flat absorber surface, which includes some types of evacuated tubes, only have 100% efficiency at midday. Other evacuated tube collectors collect solar radiation in a perpendicular fashion over a longer period of the day since the collecting absorber surface is cylindrical. This feature can be enhanced by placing an optimally designed reflective compound parabolic concentrator (CPC) mirror behind the collectors, causing the sunlight to strike the collector at a perpendicular angle for a great percentage of the day. This provides most of the advantages of tracking systems while avoiding their high costs. The advantages of the CPC ETC include:
- longer usable daylight time
- more continuous power in the course of the day
- high target temperatures over the entire day
- higher daily and yearly energy yields.
Thermal capacity
The thermal capacity ( Ceff ) of the collector has an effect on the system performance. Every time the collector heats up, energy is absorbed by the collector. This energy is not fully recovered as useful energy. In simulations, this factor is taken into account when calculating the annual energy yield. The thermal capacity ( Ceff ) of the collector is expressed in kWh per m2 collector area per degree K. The influence of this parameter is comparably small. Nevertheless, the lower the thermal capacity of the collector, the better.