1.Problems related to solar energy

Sunlight is a universal source of energy that is free and available to everyone. The sun energy source is practically inexhaustible. Solar thermal energy can be used for both heating and cooling. Key applications of solar energy include domestic water heating, space heating, pool heating and certain industrial processes. There are however some problems which hinder more widespread use of solar energy. The most serious one is that solar energy is available only during sunny days. At any given time the amount of the available solar energy depends on weather conditions, location, and the time of year. Solar energy received at a given location may vary considerably within an hour or even minutes. During the winter months, when the demand for thermal energy is the highest, relatively low amount of the solar radiation can be received. In turn, the bulk of the solar energy available in the summer period cannot be fully used due to the lower demand for heat in this period.

2.Types of solar collectors

Solar collectors transform solar radiation into heat and transfer that heat to a medium (water, solar fluid, or air). Currently the market offers two basic types of the solar collectors: flat-plate collectors, and vacuum tube collectors. A typical flat-plate collector consists of an absorber plate, transparent cover, rigid frame and insulation (Fig 1). Frame is usually made of aluminium alloy or galvanized steel. A low-iron solar safety glass is typically used as a transparent cover. An absorber plate is a sheet of metal of high-thermal conductivity such as copper or aluminium. Top surface of the absorber plate, which is exposed to the sun, is coated to maximize absorption of radiant energy and to minimize radiant emission. Heat from the sun that strikes the absorber is transferred to a fluid that circulates through the collector tubes. Flat collectors can be mounted in a variety of ways, depending on the type of building, application, and size of collector. Options include mounting on a roof, in a roof, on walls or as freestanding units. Flat-plate collectors are commonly used in domestic water-heating systems (DHW), for heating swimming pool water and in solar space heating.

(Fig 1) flat-plate collector (Fig 2) vacuum tube collector

In vacuum tube collectors, the absorber is located in an evacuated and pressure proof glass tube (Fig 2). The absorber is similar to a thermos flask in that it is set into a glass tube that is under vacuum pressure. The vacuum features excellent thermal insulative properties. Consequently, heat losses are lower than for flat-plate collectors, particularly at low temperatures. Evacuated tube solar collectors are very efficient and can achieve very high temperatures. They can be an effective alternative to flat-plate collectors for domestic space heating, especially in areas where it is often cloudy. A vacuum tube collector is generally more expensive than a flat-plate collector but it is generally more efficient in operation.

3.Collector yield

The collector yield is a vital variable for sizing and operating the solar thermal system. It can be described as the annual energy output from the collector per collector area (kWh/m2) for defined operating conditions. The collector yield depends on many different factors such as the solar irradiation at the location (Fig 3), the tilt angle and orientation of the collector working surface (Fig 4), any shading of the collectors and their average temperature.

(Fig 3) map of the solar irradiation (Fig 4) collector yield vis inclination (and orientation)

4.Collector efficiency

The solar system efficiency describes the ratio of the annual amount of the solar energy converted into available heating energy and the irradiation that strikes the collector surface: The operational efficiency of the collector is dependent upon the operating conditions of the system, in particular on ambient and the collector temperatures. When collector heats up as a result of insolation, some of that heat is transferred to the ambience through thermal conduction of the collector material, thermal radiation and convection. These losses can be calculated with the heat loss correction values: k1 and k2 and the temperature differential ΔT between the absorber and the ambient (Fig 5). Maximum efficiency is achieved when the differential between the collector and the ambient temperature is zero and there are no energy losses to the environment. The efficiency of the collector falls with the increasing the ambient temperature. Operational reliability and overall efficiency of the solar systems is related significantly to the implemented technologies and designs of solar collectors.
  Flat-plate collector ;   optical_efficiency about 80%,   k1 about 4 W/(m²K),   k2 about 0,01 W/(m²K²)
  Vacuum tube collectors ;   optical_efficiency about 80%,   k1 about 1,5 W/(m²K),   k2 about 0,005 W/(m²K²)

(Fig 5) formula for the collector efficiency

5.Solar coverage

The solar coverage is the next essential parameter required for designing a solar thermal system. The solar coverage parameter describes the percentage of the energy required annually for the intended use (i.e. for DHW applications) that can be covered by the solar heating system. The higher the selected solar coverage, the more conventional energy is saved. For a solar system designed (i.e.) to cover 100% of the energy demand for DHW applications in the winter, there will be a net energy surpluss during the summer months. Thus the higher solar coverage means the lower specific yield per square metre of a collector area and lower collector efficiency. A good compromise between solar coverage and solar yield is needed for every solar thermal system. Such a compromise also means a good trade-off between investment costs for a solar thermal system and profits related to conventional energy savings. It Germany, solar systems with a solar coverage of 50 to 60 percent are designed for DHW heating in detached houses. Coverage of less than 50% is generally appropriate in multifamily buildings. The solar coverage is largely dependent on the building energy characteristics including level of energy consumption, thermal insulation and air-tightness of a building. To secure a reliable supply of heat, solar thermal systems are often combined with additional heat sources i.e. gas or oil boilers. It allows for optimising both the heating system as a whole and costs related to the residential hot water supply.

6.Solar systems

The successful operation and the entire efficiency of a solar thermal system depend not only on the collector configuration but also on the quality of all components used in the installation and degree to which they are correctly sized and matched to each other. Even in the case of a simple dual-model of a solar system (Fig 6) it is evident how many components can be responsible for operational conditions of the solar installation. The overall efficiency of the system is influenced by the efficiencies of particular components such as a heat exchanger, hot water storage tank and DHW circulation system but also a control system for operational parameters. Heat losses from non-insulated pipes or a poorly insulated hot water storage tank can reduce efficiency significantly. For example, the losses from a 300 l hot water tank can amount to 4 kWh/day x 365 days =1 460 kWh. The compensation for a 50 percent loss in a solar coverage will demand an additional collector of 1 m2 area and an additional consumption of 50 litres of fuel oil or equivalent other form of fuel. For a well-insulated hot water storage tank and DHW circulation system the total losses are typically rated at the level of about 30%.

(Fig 6) simple solar system

Prepared on the basis of Solar Technical Guide - Solar Thermal Systems (Viessmann)

Technical detailed informations are accessible on VIESSMANN webside   "Solar Technical Guide"   or
on BUDERUS webside   "Solar Design Guide"