Non-Concentrating Solar Collectors

Overview of Lecture 4: Non-Concentrating Collectors

• Lecture 4 emphasizes non-concentrating solar collectors, following up on energy basics and solar radiation calculations.
• Solar energy harvesting focuses on collection and storage of solar radiation, covered in detail over the next two weeks.
• Collection involves converting solar energy into total radiation (IT) on a surface, considering both horizontal and tilted surfaces.
• Collectors fall into two main categories: concentrating and non-concentrating, with this lecture highlighting the latter.
• Three major types of non-concentrating collectors are liquid flat-plate collectors (FPC), solar air heaters, and evacuated tube collectors (ETC).
• FPCs use liquid (water) or air as the fluid to harvest solar energy into usable heat.
• ETCs reduce convection losses through a vacuum sealed between inner and outer tubes.

Common Solar Thermal Collectors

• The theory of flat plate collectors, thermal analysis, and absorber coatings are key topics.
• Coatings increase the absorption of radiation, improving efficiency.
• Non-concentrating collector applications include solar cookers, solar stills, solar cooling, and refrigeration.
• Solar collectors transform solar radiation into thermal energy, transferring it to a medium like water, air, or phase change liquids (refrigerants).
• The heat generated is used for water heating, heating/cooling systems, and heating swimming pools.
• Non-concentrating collectors are suited for low-temperature applications, while concentrating ones are for higher-temperature needs.
• Solar thermal collectors are applicable to solar cooling technologies and solar power plants, which require higher temperatures typically achieved by concentrating collectors.
• Collectors are divided into non-concentrating and concentrating types based on how solar radiation falls on the absorber.

Concentrating vs. Non-Concentrating Collectors

• Liquid and air collectors fall in the non-concentrating category; concentrating collector terminologies include CPC, parabolic dish, parabolic trough, and solar tower types.
• Concentrating collectors reach higher temperature ranges using only beam radiation.
• Non-concentrating collectors serve low-temperature applications, utilizing both beam and diffused radiation.
• In non-concentrating collectors, the aperture area (area receiving solar radiation) is roughly the same as the absorber area, which the term "aperture" is typically omitted.
• Concentrating collectors have a much larger aperture area compared to the absorber area.
• Non-concentrating collectors are frequently used in residential and commercial buildings for space heating.

Non-Concentrating Types and Applications.

• Concentrating collectors are used in concentrated solar power plants to generate electricity by heating a heat transfer fluid to drive a turbine connected to an electric generator.
• Flat plate collectors and evacuated tube collectors are non-concentrating examples.
• Spherical parabolic, cylindrical parabolic (CPC), and solar towers are concentrating collector examples.
• Concentrating collectors use heliostats to reflect the sun's energy towards a central tower.

Flat Plate Collector Components

• Major FPC components include the absorber plate, cover glass, insulation housing, and working fluid.
• Fluid passages or tubes carry liquid through the absorber plate.
• Transparent covers and thermal insulation are other components of a typical FPC.
• Both diffusive and beam radiation reach the surface of the absorber plate.
• Absorber plates are often made of copper and coated to increase solar radiation absorption using selective surfaces (typically black).
• The absorber plate can be flat, corrugated, or grooved with attached tubes, fins, or passages.
• Plate thickness is normally between 0.2 to 0.7mm.
• Absorber tubes have a diameter of 1 to 1.5 cm, with a center-to-center distance (pitch) of 5 to 12 cm.
• Header pipes connecting the tubes have a larger diameter of 2 to 2.5 cm.
• Standard liquid FPCs have an area of around 2 square meters.
• Cover glass reduces convection and re-radiation losses from the absorber, protecting the collector from environmental disturbances while transmitting solar radiation.
• Low ferric oxide content is preferred in glass material; it is usually toughened because it forms a protective layer for trasmittance and reducing re-radiation losses.
• Protective glass has a thickness of around 4 to 5 mm, with a distance of 1.5 to 3 cm between the cover and the absorber plate
• Insulation material consists of wool or rock around 2.5 to 8 cm thick, covered with aluminum foil.
• The collector box or casing is covered with aluminum and some coating.
• The housing holds the absorber with insulation and cover plates, protecting it from dust and moisture.
• Tubes, fins, and passages direct the heat transfer fluid from inlet to outlet.
• The working fluid (water or air) circulates through the absorber plate, carrying solar energy to its point of use.
• Plastic tubes and absorber plates are used, facing challenges like low thermal conductivity and a high thermal coefficient of expansion.

FPC Operation and Characteristics

• The term "non-concentrating collectors" refers to FPCs due to the lack of optical concentration.
• Absorbing surfaces can be flat, grooved, or other shapes, attached to heat removal devices like tubes or channels.
• FPCs convert solar energy into heat at attainable temperatures of 40 to 100 degrees Celsius.
• The simple design requires minimal material and labor, attracting more applications.
• A dark surface absorbs radiation and transfers it to a working fluid through tubes attached to the absorber plate to give a useful heat gain.
• Applications: water heating, space heating, cooling, and low-temperature cycle power generation.
• The cut section demonstrates a setup, with a glass cover, the absorber plate, liquid pipes, a metallic pipe, header pipe.
• When exposed to solar radiation, metal sheet temperatures rise until energy received equals heat lost, demonstrating energy balance.
• Placement of heat insulating material protects the plate to reduce loss.
• Painting black increases the plate's solar radiation absorbing capacity and temperature can raise.

Key Concept and Operation of Delta T

• Incoming Flux measures A p * S
• If no useful heat is extracted, losses equal A p * S, which is the stagnation temp, and collection efficiency is zero.
• High flow of liquid leads to small del T, losses are very small and collection efficiency is high close to 100%.

Installation and Fluid Considerations

• Flat plate collectors face the equator, being south-oriented in the northern hemisphere and north-oriented in the southern hemisphere.
• The optimum tilt angle is close to the location's latitude; for year-round hot water, the optimum angle is latitude +5 degrees.
• Installation should be at latitude -10 degrees for solar cooling applications.
• The installation angle is latitude + 10 degrees for best solar heating.
• Water is a popular working fluid due to its high volumetric heat capacity and incompressibility, allowing for the use of small tubes and pipes.
• Its disadvantages include freezing during winter, causing collector and piping damage.
• Systems often drain the collector at low solar inputs and critical insulation levels, monitored by sensors.
• Complete draining raises concerns about air pockets blocking water flow, reducing system efficiency.
• Antifreeze mixtures (ethylene glycol or propylene glycol) prevent freezing in closed-loop systems and should be replaced every 5 years.
• Water is a working fluid in flat plate collectors, notwithstanding anti-freezing consideration.

Solar Air Heaters

• Solar air heaters are commonly used for space heating and crop drying.
• These systems require a fan or blower to circulate air, known as a forced circulation system.
• Some designs can facilitate passive air movement through thermal buoyancy, but natural convection is generally less efficient than forced circulation.
• Refrigerants are often preferred to phase change liquids because the latter are not being used much due to low heat transfer coefficient.
• Liquid flat plate collectors, which avoid freezing issues, can use refrigerants that do not freeze in winter and can change from liquid to gas at low temperatures.
• The phase change from liquid to gas as temperature increases requires analysis in the region of multi-phase flow and can provide a quick response to temperature fluctuations.
• Water, air, or phase change liquids can be used as working fluids in flat plate collectors.

Key Design Considerations for Flat Plate Collectors

• Maximize absorption using selective surfaces.
• Minimize reflection and radiation losses through glazing.
• Ensure effective heat transfer from the collector plate to the fluids via flow passages.
• Achieve a good thermal bond between the absorber plate and the tubes or ducts carrying the heat transfer fluids.

Methods of Component Attachment

• Thermal cement
• Soldered ring tubes with the absorber plate
• Clips, clamps, and bracing
• Mechanical pressure applicators
• The choice of method depends on labor, materials, and minimizing thermal resistance; the installation process can be challenging.

Construction Designs

• Pipe and fin type:
  • Water flows only in the pipes.
  • Has a comparatively low wetted area and liquid capacity.
  • Features an absorber plate with tubes, spaced center to center by a distance W.
• Rectangular or cylindrical full sandwich type:
  • Both the wetted area and the water capacity are high.
  • Fluid passage between the absorber plate.
• Roll bond or semi-sandwich type:
  • Intermediate between pipe and fin type and rectangular or cylindrical full sandwich type.
  • Has moderate wetted surface area and liquid capacity.

Glazing Options

• Double glazing (two glass covers) can decrease convective heat losses.
• Using two glass covers may reduce solar radiation reaching the absorber plate due to double reflection, so one glass cover is also a viable option.
• One must decide whether one is decreasing the convection losses or increasing the reflection when using 2 glass covers.

Absorber Plate and Flow Passages

• Some constructions integrate fluid channels into the absorber plate to maximize thermal conductance between components.
• Other modifications include tubes or channels soldered or cemented to the plate.
• Different designs include tubes within the absorber plate (rectangular type) or tubes soldered below the absorber plate.

Water Collector Designs

• Pipe and fin type
• Water sandwich type
• Semi-sandwich type

Air Collector Designs

• Finned plate
• Metal matrix type
• Corrugated plate with selective surfaces
• Tube in plate designs include fluid passages in the absorber plate, tubes bonded to the bottom or upper surface of the plate, and flow passages grooved in the absorber plate.

Attachment Methods for Flow Passages and Absorber Plate

• Bond type: using adhesive material, though this adds adhesive resistance.
• Tie type: involves a specific design for attachment.
• Clamp type: uses clamps for secure attachment.

Thermal Analysis

• The basic principle is conservation of energy under steady state or quasi-steady state conditions.
• Useful energy output of the collector is the difference between absorbed solar radiation and total thermal losses.
• Useful energy (Qu) = Absorbed solar energy (S * Ap) - Thermal losses (Ql)
• Instantaneous efficiency = Useful energy (Qu) / (Total flux falling on the collector (IT) * Collector area (Ac))
• Higher useful energy output from a particular flat plate collector design results in higher expected efficiency.
• Thermal efficiency provides the basis for comparison of different materials and modifications of the collector system.
• The goal is to maximize energy gain to the working fluid from the absorber plate.

Parameters

• IT (total flux falling on the collector) characterizes the external radiation and can be measured using instruments or calculated using correlations and formulae.
• Collector area is a set technical characteristic.
• Estimating useful energy (Qu) requires understanding the energy balance within the collector.
• Qu = Absorbed energy - Losses
• Qu = Ap * (S - UL * (Tpm - Ta)), where:
  • S is absorbed solar radiation on the absorber plate.
  • UL is the total loss coefficient.
  • Tpm is the mean temperature of the absorbing plate.
  • Ta is the temperature of the air.
  • Ap is the area of the plate surface.

Absorbed Energy Approximation

• Conveniently approximated as IT * (τ * α)average, where:
  • τ is transmissivity (solar radiation coming through after reflection at the glass-air interfaces and absorption in the glass to the radiation incident on the glass cover system).
  • α is absorptivity of the plate (how much of IT is absorbed on the absorber plate).
• (τ * α)average is approximately 0.96 * (τ * α)b, where (τ * α)b is the transmissivity-absorptivity product based on beam radiation.

Calculating (τ * α)

• Snell’s law is used to calculate the angles of reflection and refraction when radiation transmits through an interface between two mediums.
• Reflectivity (ρ) has two components, ρ1 and ρ2, related to the two components of polarization.
• Transmissivity due to reflection and refraction (τr) is calculated in relation to ρ1 and ρ2.
• Transmissivity due to absorption (τa) is calculated using Bouger’s law: τa = Il / Ibn = e^(-k * δc), where:
  • Il is the intensity of radiation after passing through the glass cover.
  • Ibn is the incident beam radiation.
  • k is the extinction coefficient of the glass.
  • δc is the thickness of the cover system.
• Total τ = τa * τr.
• (τ * α) product = τ * αp / (1 - (1 - αp) * ρd), where:
  • αp is absorptivity of the plate.
  • ρd is reflectivity due to diffusion, calculated with an incident angle of 60 degrees.
• Practical estimations for ρd: 0.14 for one glass cover, 0.21 for two glass covers.
• (τ * α) for diffusive radiation is calculated similarly, using an angle of incidence of 60 degrees.

Simplified Calculation of S

• Estimated by IT * (τ * α)average, using (τ * α)average ≈ 0.96 * (τ * α)b

Comprehensive Calculation of S

• S = Ibrb(τ * α)b + (Idrd + IbId)IGrr(τ * α)d
• (τ * α)d is calculated using θi = 60 degrees.
• Calculations may be unwarranted for incidence angles less than 45 degrees, where changes in transmissivity are minimal.
• Accurate calculation of parameters is crucial for comparing efficiency with new materials and designs.
• S can be calculated using τ average assumptions

Wave Theory of Glass Cover

• Cover system
• Total energy is "one"
• Reflection is "rho one"
• The remaining reaching the cover system is "one minus rho one"
• (τ * α) = (1 - ρ1)^2 + ρ1^2(1 - ρ1)^2 +... this is done for all components where τ = (1- ρ1) / (1+ ρ1)

Calculating Transmissivity- Reflection Refraction

• (τ * α) where I l / Ibn meaning what is I l
• I l = -k δc / cos of theta
• Where I l is when after it passes through the class cover of glass cover of thickness delta then that's intensity of radiation is is what l is (light) from this can calculate this due to absorption where l.
• τ α (tau alpha) can be found, representing flux observed in the absorber plate divided by flux on the cover system
• A cover system and absorber plate are needed
• Energy incident on the cover system transmits as τ (tau)
• What emerges from the absorber plate is τ α (tau alpha)
• If the energy reflected is τ(1 - α), the energy that comes out is τ α (1 - α)
• When it comes out of the absorber plate, it's multiplied by absorptivity into ρd (rho d), so the energy that is reflected back is τ(1 - α)^2
• When it comes back as reflected radiation, it would be τ(1 - α)^2 * ρd^2
• So, the amount reflected would be τ α (1 - α)^2 * ρd^2
• Total τ α (tau alpha) is τ α / (1 - (1 - α) * ρd), found through mathematical manipulation.
• ρd (diffuse reflectivity) is normally 0.21 for a 2 glass cover system
• ρd (diffuse reflectivity) is normally 0.15 for a single glass cover or 0.14 in some systems.
• To calculate ρd, use τa * (1 - τr) at a 60-degree angle of incidence.