Solar Thermal Energy and Concentrated Solar Power

Introduction

Solar thermal technology converts solar radiation into useful heat. That heat may be used directly for domestic hot water, space heating, drying, industrial processes, desalination, or electricity generation. Unlike photovoltaic systems, which convert sunlight directly into electricity, solar-thermal systems first produce thermal energy and may then use a heat engine.

The central engineering challenge is to collect sufficient solar energy, transfer it efficiently to a working fluid, store it when necessary, and deliver it at the required temperature and reliability. Collector selection depends strongly on temperature: unglazed collectors are suitable for low-temperature applications, flat-plate and evacuated-tube collectors commonly serve hot-water systems, and concentrating collectors are preferred for medium- and high-temperature processes and power generation.

Learning Objectives

After studying this chapter, the reader should be able to:

1. Explain the construction and operation of flat-plate, evacuated-tube, and concentrating collectors.
2. Apply energy-balance equations to estimate useful collector heat gain and efficiency.
3. Distinguish optical efficiency, thermal efficiency, and overall system efficiency.
4. Describe thermal-storage media, heat-transfer fluids, and control strategies.
5. Design a preliminary solar water-heating system.
6. Compare parabolic-trough, solar-tower, dish, and linear-Fresnel power plants.
7. Relate concentrated solar power systems to Rankine, Brayton, and combined cycles.
8. Evaluate practical, environmental, economic, and safety constraints.

1. Solar Radiation and Thermal Conversion

The solar resource incident on a collector consists of direct-beam radiation and diffuse radiation. Non-imaging concentrating collectors require direct normal irradiance (DNI), because mirrors must redirect rays arriving from a specific direction. Flat-plate collectors can use both beam and diffuse radiation.

The useful heat delivered by a collector is the absorbed solar energy minus optical and thermal losses. A general steady-state energy balance is

where Q̇_u is useful heat-transfer rate (W), Q̇_abs is absorbed solar power (W), and Q̇_loss is heat loss to the surroundings (W). The equation applies to a control volume around the collector under steady or quasi-steady conditions.

Collector efficiency is

where η_c is collector efficiency, A_c is aperture or gross collector area (m²), and G is incident solar irradiance (W/m²). Both numerator and denominator have units of watts.

For a conventional flat-plate collector, a useful engineering expression is

Here F_R is the heat-removal factor, τ is glazing transmittance, α is absorber absorptance, G_T is irradiance on the collector plane, U_L is the overall heat-loss coefficient (W/m²·K), T_i is inlet-fluid temperature (°C or K), and T_a is ambient temperature. Temperature differences may be expressed in °C or K because their increments are identical. The equation assumes approximately steady conditions, uniform properties, and a collector operating below stagnation.

The product τα represents optical absorption. The optical efficiency under reference conditions is often approximated as

where ρ_m is mirror reflectance, γ is the intercept factor, ϕ accounts for geometric or shading effects, α is receiver absorptance, and τ is any receiver-cover transmittance. The factors are dimensionless. In concentrating systems, incidence-angle modifiers and cosine losses may significantly reduce η_opt.

2. Non-Concentrating Collectors

2.1 Flat-Plate Collectors

A flat-plate collector normally contains a dark absorber plate, fluid passages, a transparent cover, insulation, a casing, and headers. Solar radiation passes through the glazing and is absorbed by the plate. Heat is conducted into tubes or channels and carried away by water or a water–glycol mixture.

Flat-plate collectors generally operate at approximately 30–80 °C above ambient, depending on design and flow. Their strengths are simple construction, acceptance of diffuse radiation, low tracking requirements, and suitability for domestic hot water and space heating. Their limitations include increasing heat loss at high operating temperature and reduced performance during cold, cloudy, or windy conditions.

The heat-removal factor F_R incorporates the relation between actual fluid temperature rise and the maximum possible rise. A higher flow rate normally increases F_R, but excessive flow increases pumping power and may provide little additional thermal benefit.

2.2 Evacuated-Tube Collectors

An evacuated-tube collector places an absorber inside a glass tube from which most air has been removed. The vacuum suppresses convection and gas conduction, greatly reducing heat loss. Heat may be transported by a heat pipe to a manifold or directly by fluid through a U-tube.

Evacuated tubes are advantageous where high delivery temperature, low ambient temperature, or limited roof area is important. They usually perform better than flat plates at large temperature differences, but they are more sensitive to tube breakage, may have higher capital cost, and require careful protection against overheating and freezing.

The useful heat equation remains an energy balance, but U_L is generally lower than for a flat plate. Performance is still affected by absorber quality, tube spacing, manifold losses, incidence angle, and operating temperature.

2.3 Collector Heat Transfer

Heat transfer within a collector occurs by radiation from the Sun to the absorber, conduction through absorber materials, convection from absorber to fluid, and radiation and convection from the outer surfaces to the environment. The heat-transfer rate carried by a fluid is

where ṁ is mass-flow rate (kg/s), c_p is specific heat capacity (J/kg·K), T_o is outlet temperature, and T_i is inlet temperature. For water near ordinary operating temperatures, c_p is approximately 4.18 kJ/kg·K, although design calculations should use temperature-appropriate property data.

The temperature rise should not be maximized without considering efficiency. A very low flow rate produces a high outlet temperature but increases collector losses; a high flow rate lowers the temperature rise and may increase pump electricity. System design therefore targets the required delivery temperature at acceptable efficiency.

3. Concentrating Solar Collectors

Concentrating collectors use mirrors or lenses to focus solar radiation onto a receiver. The concentration ratio is

where A_ap is aperture area and A_rec is receiver area. Geometric concentration reduces receiver area relative to aperture area, lowering thermal losses per unit of collected solar power. High concentration requires accurate tracking and primarily direct-beam sunlight.

Parabolic Trough

A parabolic trough uses a long, curved mirror to focus sunlight onto a linear receiver located along the focal line. A heat-transfer fluid, commonly synthetic oil, molten salt, or water/steam, flows through the receiver. Single-axis tracking follows the Sun across the sky.

Trough plants are modular and commercially mature. Their normal operating temperatures support steam generation through heat exchangers and a Rankine cycle. Thermal storage is commonly coupled through molten salt, allowing electricity production after sunset.

Central Receiver or Solar Tower

A solar tower uses a field of two-axis-tracking heliostats to reflect sunlight toward a receiver at the top of a tower. Molten salt is a common heat-transfer and storage medium. Higher temperatures than trough systems are possible, improving the potential efficiency of the power cycle.

Tower plants require sophisticated optical control, heliostat calibration, flux management, receiver protection, and land-use planning. Receiver tubes can experience severe thermal gradients, so startup and transient control are important.

Parabolic Dish

A parabolic dish focuses sunlight onto a point receiver. Dishes generally use two-axis tracking and can achieve high concentration and temperature. A Stirling engine or small Brayton engine may be integrated at the focus, producing a modular generator without a central steam plant.

Dishes have high optical performance but face challenges involving cost, wind loading, maintenance, and the absence of inexpensive large-scale thermal storage in many configurations.

Linear Fresnel Reflector

A linear-Fresnel system uses rows of nearly flat or mildly curved mirrors to focus sunlight onto an elevated linear receiver. The design can reduce structural and manufacturing cost, but it generally has lower optical efficiency than a parabolic trough because of cosine, blocking, shading, and spillage losses. Linear Fresnel systems are attractive for industrial steam and moderate-temperature applications.

4. Thermal Efficiency, Exergy, and Performance Testing

Optical efficiency describes how much incident solar radiation reaches and is absorbed by the receiver before thermal losses. Thermal efficiency describes the fraction of incident solar power delivered as useful heat. System efficiency additionally includes pumps, tracking drives, controls, heat exchangers, and parasitic electricity.

A collector efficiency curve is often expressed as

where η_0 is zero-loss efficiency, a_1 and a_2 are empirically determined loss coefficients, T_m is mean fluid temperature, and G_T is irradiance. This relation is an empirical test model and should not be extrapolated far outside its tested temperature and irradiance range.

Thermal efficiency alone does not measure the quality of heat. Heat delivered at 40 °C has less ability to perform work than heat delivered at 400 °C. Exergy analysis accounts for this temperature quality. For heat Q̇ supplied at absolute temperature T, the ideal maximum work potential relative to ambient absolute temperature T_0 is approximately

This is a reversible upper bound; real equipment produces less work because of irreversibility. Exergy analysis is especially useful when comparing low-temperature solar water heating with high-temperature power production.

5. Thermal Storage and Heat-Transfer Fluids

Storage improves solar utilization by separating collection from demand. Sensible storage stores energy through temperature change:

where m is storage mass (kg), c_p is specific heat capacity (J/kg·K), and T_1 and T_2 are initial and final temperatures. Water is inexpensive and has high heat capacity, but its practical temperature range is limited by boiling, pressure, scaling, and material compatibility.

Latent storage uses phase change, such as melting a salt or wax. Its approximate energy is

where T_m is phase-change temperature, L is latent heat (J/kg), and c_p and c_p' are heat capacities on either side of the phase change. The expression applies when the storage passes through the phase transition.

Thermochemical storage stores energy in reversible chemical reactions and can offer high energy density, but reactor complexity and material durability remain important research issues.

Common heat-transfer fluids include water, water–glycol mixtures, thermal oils, molten nitrate salts, and pressurized gases. Selection depends on operating temperature, freezing point, viscosity, corrosion, toxicity, pressure, cost, and compatibility with pumps and seals. Thermal oils can decompose at excessive temperature; molten salts can freeze in pipes; glycol can degrade if stagnation temperatures are high.

6. System Controls and Safety

A solar water-heating system normally includes collectors, a storage tank, circulation pump, expansion vessel, check valves, heat exchanger where required, temperature sensors, pressure relief devices, and a controller.

A differential controller starts circulation when the collector is sufficiently warmer than the tank and stops it when the temperature difference becomes too small. A simple control rule is

where T_c is collector temperature, T_t is tank temperature, and ΔT_on is greater than ΔT_off to create hysteresis. Hysteresis prevents rapid switching when temperatures fluctuate near a threshold.

Controls should also provide high-temperature shutdown, freeze protection, maximum-tank-temperature protection, pump-failure detection, pressure monitoring, and safe stagnation management. In larger plants, supervisory control and data acquisition systems monitor irradiance, flow, temperatures, valve positions, storage state of charge, and alarms.

7. Preliminary Design of a Solar Water-Heating System

A first estimate of daily hot-water demand is

where m_d is daily water mass (kg/day), T_use is required water temperature, and T_cold is cold-water temperature. If the solar fraction is f_s, the required solar-delivered energy is

A preliminary collector area is then

where H_d is daily solar irradiation on the collector plane (kWh/m²·day) and η_sys is the assumed daily system efficiency. This sizing equation is approximate: it does not replace hourly simulation, weather data, pipe-loss analysis, or storage optimization.

Worked Example

An institutional residence requires 500 L/day of water. Cold water is 20 °C and delivery temperature is 60 °C. Assume a solar fraction of 0.60, daily plane-of-array irradiation of 5.0 kWh/m²·day, and daily system efficiency of 0.45. Take water density as 1 kg/L and c_p = 4.18 kJ/kg·K.

The daily load is

Since 1 kWh = 3,600 kJ,

The solar contribution is 0.60 × 23.22 = 13.93 kWh/day. Therefore,

A practical design might select approximately 6.5–7 m² after considering pipe losses, shading, seasonal variation, and available commercial module sizes. A backup heater remains necessary during periods of insufficient solar energy.

8. Thermodynamic Cycles in Concentrated Solar Power

In a solar Rankine plant, solar heat produces steam in a steam generator. The turbine expands the steam and produces work; a condenser rejects heat; a pump raises feedwater pressure. The ideal thermal efficiency is

where W_net is turbine work minus pump work and Q_in is heat supplied to the cycle. Actual efficiency is lower because of turbine, pump, heat-exchanger, pressure-drop, and heat-loss irreversibilities.

A Brayton cycle compresses a gas, adds heat at high pressure, expands it through a turbine, and rejects heat. Solar towers can supply high-temperature air or indirectly heat a gas-turbine system. Combined cycles join Brayton and Rankine cycles: exhaust heat from the gas turbine produces steam for a bottoming Rankine cycle, increasing efficiency.

Dish systems may use Stirling engines. The external heat source is applied to a closed working gas, and pistons convert cyclic pressure changes into mechanical work. Stirling systems can achieve high efficiency at suitable temperature differences but require precision engines and effective maintenance.

CSP plants may also supply industrial process heat, district heating, desalination, and fuels. Electricity generation is not always the best application; direct use of heat avoids conversion losses when a nearby industrial load exists.

9. Illustrative Case: Solar Water Heating for a University Residence

Background. A university residence with 120 occupants uses electric resistance heaters for morning and evening hot-water demand. The roof receives good annual solar exposure, but winter mornings are cool and partly shaded.

Problem. The university seeks to reduce electricity consumption without compromising water temperature, hygiene, or availability.

Analysis. A load survey identifies approximately 500 L/day of hot-water demand. A shading study excludes a portion of the roof. Evacuated-tube collectors are considered because winter temperature differences are substantial, while flat plates have lower initial cost and easier maintenance. The design team estimates a target solar fraction of 55–65%, rather than attempting 100% solar supply, which would cause summer overheating and excessive cost.

Approach. The selected system uses approximately 7 m² of collectors, a 600-L insulated storage tank, a closed water–glycol collector loop, a heat exchanger, differential control, a backup electric heater, a mixing valve, expansion protection, and temperature and pressure relief. The backup heater is controlled to maintain hygienic delivery temperature, while the mixing valve prevents scalding. The tank is located close to the main demand points to reduce distribution losses.

Outcome. The preliminary energy estimate indicates that about 14 kWh/day of useful solar energy can be supplied under design conditions. Actual annual savings depend on weather, occupancy, control settings, and maintenance. Metering of collector heat, backup electricity, water volume, and temperatures is installed to verify performance.

Learning points. Good design begins with a measured load profile rather than collector area alone. Storage and control are as important as the collector. Backup equipment is part of a reliable renewable system, not evidence of design failure. Safety, hygiene, maintainability, and measurement must be addressed at the design stage.

10. Challenges, Sustainability, and Professional Practice

Solar thermal systems have variable output, so designers must address cloudy weather, seasonal mismatch, stagnation, freezing, scaling, corrosion, leaks, and degradation. Concentrating systems additionally face mirror soiling, wind loads, tracking errors, receiver damage, and land or water constraints.

Environmental assessment should include material extraction, manufacturing, transport, cleaning water, end-of-life recycling, and replacement components. Molten salts, glycols, oils, and cleaning chemicals require appropriate handling and spill procedures. Occupational safety includes high-temperature surfaces, pressurized fluids, electrical equipment, moving trackers, glare, working at height, and confined spaces.

Standards and building regulations should govern pressure vessels, plumbing, electrical protection, structural loading, fire safety, potable-water separation, and worker protection. Local solar access, planning approval, glare assessment, and water availability may determine feasibility. Artificial intelligence can assist with irradiance forecasting, fault detection, predictive maintenance, and plant dispatch, but sensor quality, cybersecurity, explainability, and human oversight remain essential.

Best practice includes accurate resource assessment, shading analysis, appropriate collector orientation and tilt, insulated piping, accessible components, freeze and overheat protection, calibrated sensors, commissioning tests, documented operating procedures, and performance verification against a baseline model.

Chapter Summary

Solar thermal systems convert solar radiation into useful heat for water heating, buildings, industrial processes, and electricity generation. Flat-plate collectors are robust and economical for low-temperature applications, while evacuated tubes reduce convective losses and perform well at higher temperature differences. Concentrating collectors use optical systems to focus direct-beam radiation and include parabolic troughs, solar towers, parabolic dishes, and linear-Fresnel reflectors.

Useful heat is determined by absorbed radiation minus thermal losses. Collector efficiency depends on optical properties, irradiance, operating temperature, ambient temperature, and heat-removal performance. Storage may be sensible, latent, or thermochemical. Controls coordinate pumps, valves, backup heaters, and safety systems. Preliminary water-heating design requires a load estimate, solar fraction, solar irradiation, and realistic system efficiency.

CSP plants commonly use Rankine, Brayton, combined, or Stirling cycles. Technology selection must consider temperature, DNI, storage, cost, maintenance, water use, land, safety, and the value of direct heat versus electricity. Reliable systems result from integrated design, careful commissioning, measurement, and lifecycle thinking.

Key Terms

Solar thermal energy: Energy obtained by converting solar radiation into useful heat.

Flat-plate collector: Non-concentrating collector with a plane absorber and fluid passages.

Evacuated-tube collector: Collector whose absorber is enclosed in a vacuum-insulated tube.

Concentrating collector: Collector that focuses solar radiation onto a smaller receiver.

DNI: Direct normal irradiance; direct solar radiation measured normal to the Sun’s rays.

Optical efficiency: Fraction of incident radiation transmitted, reflected, intercepted, and absorbed by a receiver.

Thermal efficiency: Useful heat output divided by incident solar power.

Heat-removal factor: Factor relating actual useful heat removal to the maximum possible removal.

Stagnation: Condition in which a collector receives solar radiation without adequate fluid circulation.

Sensible storage: Storage based on temperature change without phase change.

Latent storage: Storage using energy absorbed or released during a phase change.

Heat-transfer fluid: Fluid that transports thermal energy through a collector or heat exchanger.

Parabolic trough: Linear-focus collector using a curved mirror and usually one-axis tracking.

Solar tower: Central-receiver plant using heliostats and a tower-mounted receiver.

Parabolic dish: Point-focus collector using a two-axis-tracking dish.

Linear Fresnel reflector: Linear-focus system using rows of mirrors and an elevated receiver.

Solar Rankine cycle: Steam power cycle supplied with solar-derived heat.

Solar fraction: Portion of total thermal demand supplied by solar energy.

Exergy: Maximum theoretical useful work obtainable relative to an environment.

Differential controller: Controller that operates circulation based on temperature differences.

Review Questions

Short Answer Questions

1. Why do concentrating collectors mainly require direct normal irradiance?
2. State two advantages and two limitations of evacuated-tube collectors.
3. Define optical efficiency and thermal efficiency.
4. What is the purpose of hysteresis in a differential temperature controller?
5. Name three common thermal-storage approaches.

Long Answer Questions

1. Explain the construction, operation, and applications of a flat-plate solar collector.
2. Compare parabolic trough, solar tower, parabolic dish, and linear-Fresnel systems.
3. Derive and explain the main energy flows in a solar water-heating system.
4. Discuss the role of thermal storage in improving CSP dispatchability.
5. Explain how a solar Rankine cycle converts collected heat into electricity.

Analytical / Application Questions

1. A building uses 300 L/day of water heated from 18 °C to 55 °C. Calculate the daily thermal load using c_p = 4.18 kJ/kg·K.
2. A collector receives 800 W/m² over 10 m² and delivers 5.6 kW of useful heat. Calculate its instantaneous thermal efficiency.
3. Select a suitable collector and storage arrangement for a hospital requiring hot water throughout the day. Justify the choice using temperature, reliability, safety, and backup requirements.

Multiple Choice Questions with Answers

Question 1

Which collector can generally use both direct and diffuse solar radiation without tracking?

A. Flat-plate collector

B. Parabolic dish

C. Solar tower heliostat

D. Parabolic trough

Correct Answer: A. Flat-plate collector

Question 2

In the relation Q̇ = ṁ c_p (T_o − T_i), what does ṁ represent?

A. Collector area

B. Mass-flow rate

C. Thermal conductivity

D. Solar irradiance

Correct Answer: B. Mass-flow rate

Question 3

A system receives 8 kW of solar power and delivers 5.6 kW of useful heat. Its thermal efficiency is:

A. 30%

B. 56%

C. 70%

D. 143%

Correct Answer: C. 70%

Question 4

Which CSP technology uses heliostats to focus radiation onto a receiver at the top of a tower?

A. Parabolic trough

B. Linear Fresnel

C. Parabolic dish

D. Solar tower

Correct Answer: D. Solar tower

Question 5

Why is molten salt useful in many CSP plants?

A. It eliminates all optical losses

B. It can transfer and store heat at high temperature

C. It requires no pumps or valves

D. It uses only diffuse radiation

Correct Answer: B. It can transfer and store heat at high temperature

Question 6

A 500 kg water store is heated by 20 K. Using c_p = 4.18 kJ/kg·K, the stored energy is:

A. 2.09 kJ

B. 10.45 kJ

C. 41.8 kJ

D. 41,800 kJ

Correct Answer: D. 41,800 kJ

Question 7

A differential controller commonly turns a solar circulation pump on when:

A. The tank is colder than the ambient air

B. The collector exceeds the tank by a specified temperature difference

C. Solar irradiance is exactly zero

D. The storage tank reaches its minimum volume

Correct Answer: B. The collector exceeds the tank by a specified temperature difference

Question 8

Which cycle is most directly associated with steam expansion through a turbine in a conventional CSP plant?

A. Rankine cycle

B. Otto cycle

C. Vapour-compression cycle

D. Refrigeration absorption cycle

Correct Answer: A. Rankine cycle

Question 9

If daily hot-water demand is 20 kWh and the target solar fraction is 0.60, the required daily solar contribution is:

A. 8 kWh

B. 12 kWh

C. 20.6 kWh

D. 33.3 kWh

Correct Answer: B. 12 kWh

Question 10

Which issue is especially important for a concentrating solar collector but less important for a flat-plate collector?

A. Need for direct-beam sunlight and tracking accuracy

B. Water density

C. Storage-tank insulation

D. Domestic hot-water demand

Correct Answer: A. Need for direct-beam sunlight and tracking accuracy