15 Solar Collectors

Mr. Yogendra Singh Singh

1. Introduction

Heat from sun’s rays can be harnessed to provide heat to a variety of applications. But in general, sun’s rays are too diffuse to be used directly in these applications. So solar concentrators are used to collect and concentrate sun’s rays to heat up a working fluid to the required temperature. Therefore, a solar concentrating collector is defined as a solar collector that uses reflectors, lenses or other optical elements to redirect and concentrate solar radiation onto a receiver.

The solar heat can be used as hot water, air or steam that can be readily deployed for meeting numerous applications in different sectors such as industrial process heating, power generation on a large scale, community cooking and space cooling/ heating.

Solar collectors are classified as low, medium or high temperature collectors. Low – temperature collectors are used for smaller non-intensive requirements. Medium-temperature collectors are used for heating water or air for industrial and commercial use. High-temperature collectors concentrate sunlight using mirrors or lenses and are used for fulfilling heating requirements up to 400ᴼC/20 bar pressure in industries.

Table 1 Temperature range of solar thermal technologies

1.1  Types of Solar Thermal Technology

As mentioned above, solar thermal technologies use various collectors to generate heat. A collector is a device for capturing solar radiation. Solar radiation is energy in the form of electromagnetic radiation from the infrared (long) to the ultraviolet (short) wavelengths. Solar collectors are either non-concentrating or concentrating. In the non-concentrating type, the collector area (i.e., the area that intercepts the solar radiation) is the same as the absorber area (i.e., the area absorbing the radiation). In these types the whole solar panel absorbs light. Concentrating collectors have a larger interceptor than absorber.

Non concentrating solar thermal collectors are generally used for low and medium temperature requirements. Solar water heating is the perfect example of a non – concentrating type of solar thermal application.

FIGURE 1 CLASSIFICATION OF SOLAR THERMAL TECHNOLOGIES

1.1.1   Non-Concentrating Technology

Non concentrating solar thermal collectors are generally used for low and medium energy requirements. Solar water heating is the perfect example of a non – concentrating type of solar thermal application. A solar water heater is a combination of an array of collectors, an energy transfer system, and a thermal storage system. In an active SWH (solar water heating) system, a pump is used to circulate the heat transferring fluid through the solar collectors. The amount of hot water produced from a solar water heater critically depends on design and climatic parameters such as solar radiation, ambient temperature, wind speed etc. Common collectors used for solar water heaters are – Flat Plate Collectors and Evacuated Tube Collectors.

Flat Plate Collectors –consist of a thin metal box with insulated sides and back, a glass or plastic cover (the glazing) and a dark colour absorber. The glazing allows most of the solar energy into the box whilst preventing the escape of much of the heat gained. The absorber plate is in the box painted with a selective dark colour coating, designed to maximize the amount of solar energy absorbed as heat. Running through the absorber plate are many fine tubes, through which water is pumped. As the water travels through these tubes, it absorbs the heat. This heated water is then gathered in a larger collector pipe to be transported into the hot water system.

Evacuated Tube Collector – Evacuated tube collectors are more modern and more efficient in design. These can heat

water to much higher temperatures and require less area. However, they are also correspondingly more expensive. Instead of an absorber plate, water is pumped though absorber tubes (metal tubes with a selective solar radiation absorbing coating), gaining heat before going into the collector pipe. Each absorber tube is housed inside a glass tube from which the air has been evacuated forming a vacuum. The glass tube allows solar radiation through to the absorber tube where it can be turned into heat. The vacuum eliminates convective as well as conductive heat loss and virtually all heat absorbed is transferred to the water.

1.1.2 Brief on Concentrating Solar Technologies (CSTs)

These systems utilise solar radiation to generate heat – as hot water, air or steam that can be readily deployed for meeting numerous applications in different sectors such as industrial process heating, power generation on a large scale and space heating/cooling. These applications make use of solar energy collectors as heat exchangers that transform solar radiation energy to internal energy of the transport medium (or heat transfer fluid, usually air, water, or oil).The solar energy thus collected is carried from the circulating fluid either directly to the hot water or space conditioning equipment or to a thermal energy storage tank from which can it be drawn for use at night and/or cloudy days.

Unlike the non – concentrating solar thermal systems, concentrating solar thermal systems use mirrors and lenses to reach higher temperature mainly for various industrial processes. Under the concentrating type – there are imaging and non – imaging technologies. Imaging technologies have a smaller range of acceptance angle compared to that of non – imaging technologies. For example, imaging concentration gives about 1/3 of the theoretical maximum for the design acceptance angle, that is, for the same overall tolerances for the system. Non – imaging optics, which have a larger acceptance angle range, can be used to approach the theoretical maximum.

Imaging technologies, which make use of mirrors and lenses, are divided in to two types each based on the focus and receiver type. Line Focus collectors track the sun along a single axis and focus the irradiance on a linear receiver. Point focus collectors track the sun along two axes and focus the irradiance at a single point receiver which allows for higher temperatures. Fixed receivers are stationary devices that remain independent of the plant’s focusing device which eases the transport of collected heat. Mobile receivers move together with the focusing device which enables more energy to be collected. The different types of solar thermal technologies are depicted in the figure below.

A detailed explanation of Solar Collectors technologies – its concept, benefits, industrial applications– follows in the next section.

2. Concentrating Solar Thermal (CST) Technology

Industrial heat is characterized by a wide diversity with respect to temperature levels, pressures and production processes to meet many different industrial process demands. Concentrated Solar Technologies (CSTs) track the sun’s incoming radiation with mirror fields, which concentrate the energy towards absorbers, which then transfer it thermally to the working medium. The heated fluid or steam may reach high temperatures and may be used for various processes requiring heat.

CSTs can produce a range of temperatures, between 50°C and up to over 400°C, which can be used in a variety of industrial and commercial heat applications. The industries showing good potential for implementation of solar concentrators are food processing, dairy, paper and pulp, chemicals, textiles, fertilizer, breweries, electroplating, pharmaceutical, rubber, desalination and tobacco sectors. Any industrial/commercial establishments currently using steam/hot water for process applications can also employ CSTs with a minimum tinkering to the existing setup.

CST Technologies

The most mature concentrated solar technologies are –

A. Point Focus Technology:

1. Fixed focus automatically  elliptical dish (Scheffler dish)tracked

2. Fresnel Reflector based dish (ARUN dish)

3. Dual axis tracked paraboloid dish

B. Line Focus Technology:

4. Parabolic troughs collectors (PTC)

5. Linear Fresnel Reflector (LFR)

C. Non-Focussing Technology

6. Compound Parabolic Collectors(CPC)

3.1 Fixed Focus Elliptical Dish (Scheffler)

Fixed Focus Elliptical Dish, often called a Scheffler concentrator, is a small lateral section of a paraboloid which concentrates sun’s radiation over a fixed focus. The dish comprises of a large number of mirrors to reflect the sun rays to a fixed receiver which contains a working fluid to be heated. The Scheffler dish system works on the following principles:

1. The parabolic reflective dish turns about north-south (N-S) axis parallel to earth’s axis, tracking the sun’s movement from morning (East) to evening (West).

2. The parabolic reflector also performs change in inclination angle while staying directed to sun, in order to obtain sharp focal point. Adjustments for the seasonal variations in the sun’s position (N – S direction) have to be made manually every few days by an operator.

3. Focus always lies at the axis of rotation. It remains at a fixed position, where concentrated heat is captured and transferred to water through the receiver to generate hot water or high pressure steam.

4. Water from header pipe passes to receiver (thermosyphon principle). At the receiver, the hot water or steam generated water and collected in the header pipe flows to the end use application.

3.2 Fresnel Reflector Based Dish (ARUN Dish)

Underlying principle

Fresnel Reflector Based Dish (ARUN Dish) is made from panels of flat mirrors mounted on a frame such that the incident sunlight is reflected on to a cavity receiver which is specially designed to reduce heat losses. The receiver which is insulated on the outside is held in a fixed position in relation to the reflectors by means of a suitable structure. The entire array of panels with the receiver move to track the sun. The cavity receivers allow energy to be intercepted by a small aperture or opening which results in low losses. The inside of the cavity may be specially coated to increase its absorption of the sunlight that falls on it. There are certain mechanisms to prevent or reduce convective heat losses from the receiver. Usually Fresnel Dishes are large and could have an aperture area of 100 m2 or 160 m2.

The reflector and the receiver mounted on top are moved to track the Sun such that the reflectors faces the Sun at all times throughout the year. These systems, because of their size, require careful structural design to withstand their heavy weight and strong winds. Fresnel Dishes have high efficiencies throughout the year.

Schematics of ARUN Dish

Key Design Variant

Presently ARUN Solar Dish is available in three design variant, namely

Paraboloid Dish

Underlying Principle

Paraboloid Dish consists of mirrors mounted on a truss structure such that the incident sunlight is reflected on to a cavity receiver which is specially designed to reduce convective and radiation heat losses. The receiver which is insulated on the outside is held in a fixed position in relation to the reflectors by means of a suitable structure. The entire array of mirrors and receiver move to track the sun. Cavity receivers allow energy to be intercepted by a small aperture or opening which results in low losses. The inner part of the cavity is specially coated to increase its absorption of the sunlight that falls on it, and there are mechanisms to prevent or reduce convective heat losses from the receiver.

The reflector and receiver mounted on top are moved to track the sun such that the reflector faces the sun at all times throughout the year. These systems have light structures. They can be mounted wherever space permits and therefore are suitable for retrofitting in congested layouts. Paraboloid Dishes have high efficiencies throughout the year.

Schematic of Paraboloid dish

3.4 Parabolic Trough Concentrator (PTC)

Parabolic Trough Concentrators (PTC) are troughs made from shaped metal and coated with a reflecting material such as highly polished metal (usually aluminium) or metallised plastic which can withstand sunlight as well as rain and the elements. These surfaces reflect the incident sunlight on to a metallic collector pipe (the receiver) that runs axially along the trough. The pipe is specially coated to increase its absorption of the sunlight that falls on it and is encased in a glass tube to reduce convective heat losses from the receiver. Sometimes, to improve the insulation effect, the space between the receiver and the glass envelope is evacuated and the ends are sealed. Several such PTCs can be connected in series on a common axis.

The two common methods of mounting PTCs are with the focal axis:

• In a horizontal E-W direction and the trough is adjusted continuously so that the incident sunlight has the least angle of incidence.
• In the horizontal N-S direction with the troughs being moved to track the sun from east to west from morning to evening. These systems are preferable when a large contiguous area is available and relatively large quantities of heat are needed.

3.5 Linear Fresnel Reflector (LFR)

Linear Fresnel Reflecting Concentrator (LFRC) is made from multiple strips of straight reflecting material which are mounted on specially designed frames. The mirrors are arranged in a fresnelized manner which reflects the incident sunlight on to a focal line of metallic collector pipe (the receiver) that runs axially above the array of reflectors. The pipe may be specially coated to increase their absorption of the sunlight that falls on them and is encased in a glass tube to reduce convective heat losses from the receiver. Sometimes, to improve the insulation effect, the space between the receiver and the glass envelope is evacuated and the ends are sealed. The upper portion of the receiver is often insulated to prevent heat loss. The reflector strips are moved such that they always reflect the incident sunlight on to the receiver tube.

Several such LFRCs can be connected in series on a common axis. The two common methods of mounting LFRCs are with the focal axis:

• In a horizontal E-W direction and the reflectors are adjusted continuously so that the incident sunlight has the least angle of incidence.
• In the horizontal N-S direction with the troughs being moved to track the sun from east to west from morning to evening. These systems are preferred when a large contiguous area is available and relatively large quantities of heat are needed. They have efficiencies similar to those of PTCs, but can be made more robust.

Schematic

3.6 Non Imaging Concentrator

To make an image, all the parallel light rays should be concentrated at a single point. Non-Imaging collectors are the collectors which can’t make the image (of sun).Non-Imaging Concentrators (NIC) are also called Compound Parabolic Collectors (CPC). The NIC consists of a specially coated absorber tube that is enclosed in a concentric glass cover to reduce convection losses. The annulus between the tube and its cover is evacuated. The tube is placed at the focal plane of two reflectors shaped as parabolic troughs. The axes of the two parabolas are inclined at the acceptance angle which is the angle by which direct light may deviate from the normal and yet allow the reflected light to reach the receiver; a high acceptance angle requires fewer tracking adjustments.

Usually such concentrators are made of panels which are 1 to 6 m wide and can go up to 60 m of collector area. The receiver carries the fluid to be heated which could be water or a thermic fluid, and are connected to each receiver tube by means of a header. The most common method of NIC panel mounting is in the E-W direction with the panel facing south and inclined from the ground at an angle of latitude + 10°.

4. Concentrated Solar Power (CSP) Technology Principle of CSP Technology

Concentrated solar power (CSP) is also a solar thermal technology. Here the light energy of the sun is concentrated by using reflective mirrors to generate heat, which in turn produces steam to run a turbine. The generator coupled with the turbine rotates and produces electricity.

CSP Technologies:

1. Parabolic Trough Collector (PTC)

2. Linear Fresnel Reflector (LFR)

3. Heliostat Field Collectors (HFCs)

4. Dish Sterling Engine

4.1 Heliostat Field Collectors (HFCs)

Heliostat field collectors (HFCs) also known as “Power Tower” or “Central Receiver Plant”. Heliostats are named helio for sun and stat for the fact that the reflected solar image is maintained at a stationary position throughout the day. They are nearly flat mirrors (some curvature is required to focus the sun’s image) that collect and concentrate the solar energy on a tower-mounted receiver located 100 to 1000 meters distant.To maintain the sun’s image on the solar receiver, heliostats tracks the sun throughout the day.

The concentrated heat energy absorbed by the receiver is transferred to a circulating fluid that can be stored and later used to produce power. Central receivers have several advantages:

1. They solar energy is collected optically and transferred to a single receiver point, thus minimizing thermal-energy transport requirements;

2. They typically achieve concentration ratios of 100–1500 and so are highly efficient both in collecting energy and in converting it to electricity;

3. They can conveniently store thermal energy;

4. They are quite large (generally more than 10 MW) and thus benefit from economies of scale.

Each heliostat at a central-receiver facility has from 50 to 150 m2 of reflective surface. The heliostats collect and concentrate sunlight onto the receiver, which absorbs the concentrated sunlight, transferring its energy to a heat transfer fluid. The heat-transport system, which consists primarily of pipes, pumps, and valves, directs the transfer fluid in a closed loop between the receiver, storage, and power-conversion systems. A thermal-storage system typically stores the collected energy as sensible heat for later delivery to the power-conversion system. The storage system also decouples the collection of solar energy from its conversion to electricity. The power-conversion system consists of a steam generator, turbine generator, and support equipment, which convert the thermal energy into electricity and supply it to the utility grid.

Schematic of Heliostat Field Collector:

4.2   Dish Sterling Engine

Dish/engine systems use a parabolic dish of mirrors to direct and concentrate sunlight onto a central engine that produces electricity. The engine is placed at the focus of the parabolic dish. The dish/engine system is a concentrating solar power (CSP) technology that produces smaller amounts of electricity than other CSP technologies—typically in the range of 3 to 25 KW. The two major parts of the system are the solar concentrator and the power conversion unit.

Solar Concentrator

The solar concentrator, or dish, collects the solar energy coming directly from the sun. The resulting beam of concentrated sunlight is reflected/ focussed onto a thermal receiver that collects the solar heat. The dish is mounted on a structure that tracks the sun continuously throughout the day to reflect the highest percentage of sunlight possible onto the thermal receiver.

Power Conversion Unit (Engine)

The power conversion unit includes the thermal receiver and the engine/generator. The thermal receiver is the interface between the dish and the engine/generator. It absorbs the concentrated beams of solar energy, converts the energy to heat, and transfers the heat to the engine/generator. A thermal receiver can be a bank of tubes with a cooling fluid—usually hydrogen or helium—that typically is the heat-transfer medium and also the working fluid for an engine. Alternate thermal receivers are heat pipes, where the boiling and condensing of an intermediate fluid transfers the heat to the engine.

The engine/generator system is the subsystem that takes the heat from the thermal receiver and uses it to produce thermal to electric energy conversion. The most common type of heat engine used in dish/engine systems is the Stirling engine. A Stirling engine uses the heated fluid to move pistons and create mechanical power. The mechanical work, in the form of the rotation of the engine’s crankshaft, drives a generator and produces electrical power.

Schematic of Dish Sterling Engine

5. Efficiency of CST System

Performance of CST system depends on the following factors:

• Direct Normal Solar Irradiance, Ibn
• Latitude/ Incident effect
• Required Temperature
• Wind Velocity
• Ambient Temperature

5.1 Optical Efficiency

Optical efficiency describes the system’s ability to absorb radiation that strikes normal to the concentrator aperture. When direct solar radiation reaches the surface of solar collectors, a significant amount of it is lost due to several different factors. The total loss can be divided into three types, which are as follows:

Optical losses – solar radiation incident upon the collector that is not converted to heat energy due to following factors.

a) Reflectivity,

b) Transmissivity of the glass tube, τ

c) Intercept factor, γ, (losses due to tracking, geometry, heat removal factor of fluid)

d) Absorptivity of the absorber selective coating,

Thermal heat losses – solar radiation that is converted to heat, but lost before it can be used. Losses are due to three modes of heat transfer within the collector.

a) Radiative heat transfer loss

b) Convective heat transfer loss

c) Conductive heat transfer loss

Geometrical Losses-due to the incidence angle,, of direct solar radiation on the aperture plane of the collector. The incidence angle of direct solar radiation is a very important factor, because the fraction of direct radiation that is useful to the collector is directly proportional to the cosine of this angle, which also reduces the useful aperture area of the solar collector. The effect of the incidence angle on the optical efficiency and useful aperture area of a PTC is quantified by the incident angle modifier (explained later), because this parameter includes all optical and geometric losses due to an incidence angle greater than 0ᴼ.

5.2  Basic Performance Equation

The performance of CST based solar collector operating under steady state conditions can be described by the following equation:

5.2 Evacuated Tube Collector (ETC)

In flat plate collectors, significant heat is lost mostly by convection and re-radiation through the top surface of the collector. This heat loss increases as the water temperature in the collector gets hotter during the day. So while the collector is highly efficient at the beginning of the day (e.g. 70%), the efficiency decreases as the water circulating through the collector gets hotter.

In evacuated tube systems, this heat loss is reduced by almost totally eliminating conduction and convection heat losses. This is because the space between the absorber and the glass outer tube is evacuated. With little air to move and transfer heat by conduction and convection, heat loss is further reduced. Radiation losses are reduced by incorporating a selective surface on the absorber, similar to flat plate collectors. As a result, evacuated tube collectors can operate at temperatures above 100ºC, compared with about 100ºC for flat plate collectors.

The principle of operation is similar to a flat plate collector in that solar radiation (both direct and diffuse) enters through the glass tube and is absorbed by the absorber plate, which transfers the heat into a heat transfer fluid inside the collector tube.