Lecture 5a solar thermal.pdf

SOLAR POWER

SOLAR ENERGY

SUN-193, 000,000 Miles Away from Earth

No deadly radioactive waste

Free-Renewable Fuel

Already heating Earth (No added Heat)

Dependable-Sustained Clean Energy Natural Power

Source of Solar Energy The sun radiates energy at the rate of 3.9 × 1026 W (watts) and has been doing so for several billion years.

The fusion reaction in the sun is a multistep process in which hydrogen is burned into helium, hydrogen being the "fuel" and helium the "ashes” as per following overall reaction.

The fusion reactions in the solar core take place because of the very high temperatures ( ) in this region of the Sun.

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108K

SOLAR SPECTRUM Solar radiation received at the top of earth’s atmosphere comprises of :

Ultraviolet (UV) radiation : 9% Visible radiation: 40% Infra Red (IR) radiation: 51%

Roughly half of it reaches the surface of earth.

Much of the UV is absorbed by Oxygen, Nitrogen and Ozone in upper part of earth’s atmosphere.

Some of the infrared rays are also absorbed by water vapours, carbon dioxide and methane in the lower atmosphere.

Diffuse and Direct Solar Radiation As solar radiation passes through the atmosphere, part of the radiation is absorbed, scattered and reflected by the following:

•Air molecules •Water vapor •Clouds •Dust •Pollutants

The above component is called diffuse solar radiation or diffuse insolation.

The solar radiation that reaches the earth's surface without being diffused is called direct solar radiation or direct insolation.

The sum of the diffuse and direct solar radiation forms global insolation.

Diffuse component depends on the clarity of the sky. Atmospheric conditions can reduce direct beam radiation by 10% on clear, dry days and by 100% during thick cloudy days.

Solar energy balance for Earth Total Incident solar radiation is distributed as follows: 30% is reflected back to the space by atmosphere, clouds and earth’s surface. This component is called albedo.

19% is absorbed by atmosphere and clouds. Remaining 51% of the incident radiation is absorbed by earth’s surface.

The relatively constant temperature of earth is the energy balance between the incoming and outgoing radiations.

HARNESSING SOLAR ENERGY

The range of solar technologies can be split into three categories:

(i) Solar thermal devices for direct heat applications.

(ii) Concentrating solar power (CSP) thermal

devices which use heat for electricity production in a steam turbine.

(iii) Photovoltaic devices that produce

electricity directly from solar radiation (PV).

SOLAR HEATING SYSTEMS SOLAR COOKERS, SOLAR FURNACES, SOLAR STEAM BOILERS

Active solar systems: the solar heated fluid is circulated by a fan or pump.

Modern Solar Heating Systems

Example: Heating of Swimming Pools, Domestic water heating Systems

Passive solar systems: Uses no external power but allows the fluid heated by the sun to circulate by natural means.

Example: Passive space heating in buildings

Solar thermal devices for direct heat applications: Solar Cookers

The basic purpose of a solar box cooker is to heat things up - cook food, purify water, and sterilize instruments The interior of the box is heated by the energy of the sun. Sunlight, both direct and reflected, enters the solar box through the glass or plastic top. Single or multiple reflectors bounce additional sunlight through the glass and into the solar box. This additional input of solar energy results in higher cooker temperatures.

The temperature inside the box rises until the heat loss of the cooker is equal to the solar heat gain.

Example: Collector area of Solar Oven

Useful rate of heat energy (Power) needed for cooking is 150 W.

The solar oven is 20% efficient.

The noon time insolation rate is 900 W/m2.

85% of above insolation is received by the oven. The required area of the oven to intercept the desired energy is worked out as follows:

Rate of insolation received by the Solar Oven, P = 0.85×900 = 765W/m2.

Power needed for heating = P × Efficiency of Oven × Collector Area

⇒150=765×0.2×Area

⇒Collector Area=150/(765×0.2)=0.98m2

Examples of other Designs of Solar Cooker

Both Designs use auxiliary reflectors to reach high temperature. Heating of the pots inside the box is done by direct absorption and by convection.

Parabolic Reflector Design Telkes’s Oven: Design has fixed cooking pot and movable oven

SOLAR WATER HEATING SYSTEM

Most common use of solar heating systems today is to provide hot water for domestic purposes (DHW) or for swimming pools. 7% of the Solar water collectors sold today in US are for DHW and 93% for Swimming Pools. Swimming Pool Collectors operate at temperatures less than 430C DWH Collectors operate at temperatures varying from 60-820C.

SOLAR WATER HEATING SYSTEM Solar water heating systems use the sun's energy to heat water in liquid-based solar collectors. These are usually used along with conventional water heaters. Solar collectors for these systems are typically 3–6 m2 in area.

A typical solar water heating system can meet approx. 50% of the water heating requirements in a home.

SOLAR WATER HEATING SYSTEM Solar water heating systems include storage tanks and solar collectors. There are two types of solar water heating systems:

The storage tanks are well insulated.

Heart of The system is the SOLAR COLLECTOR

(i) Active, which have circulating pumps.

(ii) Passive, which are based on natural convection.

SOLAR COLLECTORS for Solar water Heating Systems

Three types of solar collectors are used for residential applications:

•Evacuated Tube Solar Collectors

•Batch Collectors

•Flat Plate Collectors

SOLAR COLLECTORS for Solar water Heating Systems

•Flat Plate Collectors •Evacuated Tube Solar Collectors

FLAT PLATE COLLECTOR

In an air based collector the circulating fluid is air, whereas in a liquid based collector it is usually water.

Flat plate collector.

These are best suited to applications that require heating during the winter months.

It has thin flat metal plate, painted black, to absorb sun’s radiation.

Absorber plate is in contact with fluid tubes. Fluids absorb heat and circulated by a pump to take away heat.

This set up is covered with one or two sheets of glazing.

Achievable temperature is 30-70°C (86-158°F)

Passive Batch Solar Water Heater

A batch collector is mounted on the ground or on the roof of the building .

Also known as a bread box system or integral collector storage system,

This solar collector consisting of one or more storage tanks placed inside an insulated box that has a glazed side facing the sun.

On an area basis, batch collector systems are less costly than glazed flat-plate collectors but energy delivered per year by them is less .

Choice of materials for surfaces on the tank(s): The surfaces of the tank should have good absorbers of solar infrared radiation and inhibit radiative loss.

Functioning of Passive Batch Solar Water Heater

Cold water enters a pipe and can either enter a solar storage/backup water heater tank or the batch collector, depending on which bypass valve is open.

Water, upon entering the tank, is heated in it.

Hot water from the batch collector is carried into the solar storage/backup water heater and thence to the house .

Evacuated Tube Collectors This type of solar collector can achieve high temperatures in the range 77°C to 177°C and under the clear sky conditions and proper orientation , work very efficiently.

Evacuated-tube collectors are, however, quite expensive, with unit area costs about twice that of flat-plate collectors.

They are well-suited to commercial and industrial heating applications and also for cooling applications.

They can also be an effective alternative to flat-plate collectors for domestic space heating, especially in regions where it is often cloudy.

Direct-flow evacuated-tube collectors

An evacuated-tube collector consists of parallel rows of evacuated glass tubes connected to a header pipe. This eliminate heat loss through convection and radiation.

A highly selective absorption coating is applied to the inner tube.

The water is circulated through the inner tubes and gets heated and sent to the header pipe.

The heat gained is conducted by special aluminum lamellas into copper tubes.

The collector header consists of two copper pipes. The lower pipe brings liquid into the collector, the upper pipe takes the warmed up liquid from the collector.

Passive Solar Space Heating System The house itself acts as the solar collector and the storage facility.

Heat energy flow is by natural means. No need of pumps and fans.

Sunlight enter through the south facing window and is stored in.

Principle of passive solar heating is that the amount of thermal energy transmitted in the house from the south facing window during a clear day is more than the thermal energy loss from house over 24 hours period.

This is achieved by enhancing thermal mass i.e., using such materials which have high heat absorption capacity. Ex: Concrete, water and stone.

Essential elements of Passive Space Heating System

Schematic illustration of direct solar heat gain

1. Maximising solar heat gain: It depends on:

(a) The solar radiation available at the location of the building.

(c) The characteristics of the collection areas (their solar transmittance/absorption and heat transfer).

(b) Orientation of the building (South facing windows).

(a)Applying thermal insulation of high quality.

(c )avoiding thermal bridges.

(d) providing air tightness.

(b) installing multiple-glazed windows.

2. Minimising heat losses

Heat losses may be minimised by following methods:

These methods help to reduce heat transmission and air infiltration which are the main avenues for heat transport through the building envelope.

Essential elements of Passive Space Heating System

Passive systems are of three types

1. Direct Gain

2. Indirect Gain

3. Attached Solar green House

Indirect Gain passive solar Space Heating System (Trombe Wall)

System collects and stores heat in one part of the house and uses natural heat transfer (conduction and convection) to distribute the heat to the rest of the house.

A massive concrete wall is placed 10cm behind a south facing glass area. Solar radiations are absorbed by the wall, which reradiates heat in the space between glass and wall and heats up the air.

Warm air rises and circulates in the room through vents and is replaced by cooler air from the bottom.

Attached Green House

Green house is attached on the south side of the house.

It acts as extended thermal wall.

Serve dual purpose (food production and space heating).

Thermal Energy Storage One important requirement of a solar energy heating system is its ability to store energy for night time use and for cloudy days.

An important criterion in selecting a storage medium is the specific heat of the substance. Good Example : Water

Q=mc∆T

Q=Heat added to a mass m whose specific heat is c.

∆T= Rise in temperature.

Large thermal capacity is achieved by using a material of large specific heat. Hence, drums filled with water serve as good storage device.

Principles of passive cooling

(b)Increasing thermal protection.

Features of passive cooling 1. Minimizing solar heat gain by

(a) Increasing the building mass.

(e) Air tightness of the building.

(c) Reflective coating (white) on exposed surfaces.

(d) Curtailing solar radiations using shading devices.

Principles of passive cooling 2. Removing unwanted heat

(b) Unwanted heat in hot and dry climates could be removed by (i) evaporative cooling

(ii)Nocturnal ventilation

(iii) in hot humid climate, a thermo-active ceiling could be installed (see figure), which would however need a pump. BUT as no energy for cooling is required, such systems are usually classed as passive cooling.

(a) Use of technologies for cooling of buildings.

(iv) By providing adequate cross ventilation on buildings.

Estimation Of Collector Size for Space Heating

How much collector area is required to provide the thermal energy needed to heat a house for one day when the average heat load is 21.1×106 J/h. Take mean daily insolation on the collector surface as 20.4×106 J/ m2/day and the collector’s efficiency is η=50%.

Heat needed for 1 day= Incident heat (Insolation)× efficiency×Area

Thermal energy needed will be provided by total radiation falling on the area A

21.1×106 × 24= 20.4×106 ×0.5× A

⇒A=49.65 m2 (about roof area of a two bed room HDB house)

Electricity Production from Solar Thermal Power Plant

Power plants use curved, mirrored troughs (collectors) which reflects the direct solar radiation onto glass tubes containing a fluid running along the length of the trough and positioned at the focal point of the reflectors.

The hot fluid is transported to a turbine where about a third of the heat is converted into electricity.

The fluid (also called heat transfer fluid) becomes very hot. Common fluids are synthetic oil, molten salt and water.

parabolic trough (PSA) solar tower (SNL)

linear Fresnel (Solarmundo) parabolic dish (SBP)

SOLAR THERMAL POWER PLANTS

SOLAR COLLECTORS

Eurotrough Reflector

The focus pipe has flexible pipes on the ends to allow rotary motion to track the sun, while the absorber pipe is kept at the focus.

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SOLAR POWER TOWER

Flat mirrors are aimed to focus sun at the receiver target to melt salt

SOLAR POWER TOWERS capture and focus the sun's thermal energy with thousands of tracking mirrors (called heliostats) in roughly a two square mile field.

A tower is located in the centre of the heliostat field.

The heliostats focus concentrated sunlight on a receiver which is mounted atop of the tower. Within the receiver the concentrated sunlight heats molten salt to over 5500C.

SOLAR POWER TOWER

The heated molten salt then flows into a thermal storage tank where it is stored, maintaining 98% thermal efficiency, and eventually pumped to a steam generator.

The steam drives a standard turbine to generate electricity. This process, also known as the "Rankine cycle" is similar to a standard coal-fired power plant.