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Efficient Solar Collector Technology

The solar collector is the main component of a solar thermal system that converts solar radiation into heat. Discover its operation and types.

Solar thermal is a system that harnesses energy from the sun to produce thermal energy. At the core of this system lies a solar collector that gathers solar energy as heat and transfers it to a fluid, which then conveys it to domestic hot water. The solar collector is the key component of a solar thermal system and, like other components, it needs to be sized according to specific requirements: a tailored project is necessary.

For this task, the use of software for designing solar thermal panels is essential to support you in the design and sizing of a forced circulation solar thermal system, whether single or double tank.

Solar Collector: What Is It?

The term “solar collector” is the technical expression used to refer to solar thermal panels. It is the main component of a solar thermal system that enables the conversion of solar radiation into thermal energy, which is then stored and used later on. The hot water produced in this way is used for sanitary purposes or for heating indoor spaces.

How Does a Solar Collector Work?

A solar collector consists of a radiator/absorber, typically made of high thermal conductivity materials like copper. This component is designed to capture solar heat and transfer it to the water tank.

The operation of a solar collector can be compared to what happens in a greenhouse. When sunlight hits the glass surface, a small portion is reflected, while the rest passes through the glass and is absorbed by a absorber plate usually black in color.

As the plate heats up, it emits energy in the form of infrared radiation. Since the glass acts as opaque to infrared radiation, it traps the heat inside the panel (greenhouse effect), causing the primary heat transfer fluid to warm up. Subsequently, the fluid moves through a coil to the tank using either natural or forced circulation.

The main difference between natural and forced circulation systems lies in the use of a water circulation pump that allows the water to flow through the piping from the collector to the storage and vice versa. In a natural circulation system, liquid flows naturally without the need for electricity, while a forced circulation system employs a circulation pump to push the heat transfer fluid.

Difference Between Solar Thermal Panel and Photovoltaic Panel

The distinctions between a solar thermal panel and a photovoltaic panel can often be confusing. Solar thermal panels convert solar energy into heat for water heating, while photovoltaic panels utilize solar energy to generate electricity.

Other differences include:

  • material: photovoltaic panels are mainly composed of silicon, while solar thermal panels are made of metal (aluminum, copper, steel) and glass;
  • efficiency: solar thermal panels can achieve efficiency up to 80%, while photovoltaic panels generally have a variable efficiency between 17% and 25%, depending on the panel type;
  • installation: while a single solar thermal panel may suffice for heating domestic hot water, photovoltaic panels require a larger size to achieve equivalent electricity performance. Both require a south orientation, but unlike photovoltaic panels that need ample space, solar thermal panels are more flexible in placement.

Solar Thermal Collector: Types

There are 3 main types of solar collectors:

  • flat-plate collectors;
  • evacuated tube collectors;
  • concentrating collectors.

The flat-plate collectors can be further divided into 2 categories: glazed flat-plate collectors and unglazed flat-plate collectors.

Flat-Plate Collectors

Flat-plate panels are the most common and cost-effective, mainly used for domestic hot water production. They are easy to install and require minimal maintenance. They have a glass top cover, a surrounding structure insulated with material, and an internal heat absorber that heats the heat transfer fluid in copper tubes.

Evacuated Tube Collectors

Evacuated tube panels are considered high-efficiency collectors, capable of operating even at very low external temperatures and low solar radiation. They are more expensive than flat-plate panels and can reach high temperatures close to 100°C. They consist of a series of glass tubes, within which there is a second glass tube and in between a vacuum that acts as insulation and reduces heat loss. This is a reliable technology, suitable for integration with a radiant panel heating system.

Concentrating Collectors

Concentrating solar collectors are concave collectors designed to optimize the concentration of solar energy at a specific point (focus). They are effective only with direct sunlight as they need to follow the sun’s movement. This type of collector, capable of reaching high temperatures like 400-600 °C, is a logical choice for solar generators or solar-electric power plants.

The main types of collectors are 4 and differ in placement, shape, and focus position:

  • linear parabolic trough collector: have a parabolic shape and are mounted aligned with each other. Along their focal center runs a tube containing the heat transfer fluid that, once heated, flows into the turbines connected to the power generators. They are equipped with an automated system that adjusts their orientation to follow the sun’s inclination according to the time of day;
  • linear Fresnel reflector collector: are positioned linearly and rotated based on the sun, presenting a flat shape rather than parabolic. The focal point is thus shifted upwards, and the liquid positioning is elevated compared to the previous type;
  • circular parabolic dish collector: are independent, each with its own receiver placed at the focal center, the central point of the parabola itself;
  • solar power tower: are independent mirrors characterized by a single focal point for all panels. They autonomously orient towards the receiver, placed on a tower, where a tank containing the heat transfer fluid to be heated is located.

Glazed and Unglazed Flat-Plate Collectors

Glazed collectors are primarily composed of a durable glass cover designed to withstand adverse weather conditions such as rain, wind, hail, and cold temperatures. This cover also serves to filter sunlight and create a greenhouse effect that retains heat. Inside, there is a thermally insulated absorber plate at the bottom and laterally enclosed within a metal or plastic container.

The absorber plate can be:

  • non-selective surface absorber: the heat absorber is simply painted black, a color that helps capture and retain sunlight better and longer. This type of panel is recommended for residences primarily used during short summer periods;
  • selective surface absorber: the heat absorber is enhanced by a surface that allows the panel to better retain solar heat by reflecting the infrared rays it emits internally, while reducing reflection. This type is made possible by a glass surface that allows the passage of sunlight but reflects the infrared rays coming from the collector. Although more expensive, these panels are much more efficient and allow for year-round use.

Unglazed flat-plate collectors (also known as non-glazed) have a coating made of plastic material. They are very affordable but not very efficient, suitable only for the summer season and areas with high solar radiation. For example, they are used to provide DHW to establishments and campsites or to heat the water of an outdoor pool.

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