chap3 sts till 29-nov12

7/29/2019 Chap3 STS Till 29-Nov12

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Department of Mechanical EngineeringHITEC University Taxila

1

Components of Solar Thermal Systems

Components and Subsystems of Solar Thermal Installation

Collectors

Storage Plan

Heat Exchangers

Pumps and Expansion

Chambers

Controller

Other Components




2

Typical Solar Installations With Main Components


Solar Collector mounted on the roofconverts the light that penetrates its glasspanes (short-wave radiation) into heat. Thecollector is therefore the link between thesun and the hot water user

Forced Circulation

Fig. 3.1

image shows a solar heating installationproviding hot water in a house

It is an indirect, closed loop, pumped system

fluid that flows through the collectors is isolatedfrom the potable water, which permits use ofantifreeze and anti-corrosive agents; thesereduce freezing and corrosion problems andtherefore increase the durability and reliability ofthe system.




3



Forced Circulation—Contd--

Fig. 3.1

As the system has pumped circulation, a largepart of the system, including the tank, can beinstalled inside of the house, with the additionaladvantage of lower thermal loss and increaseddurability

Auxiliary system is installed in series with thepotable water, and as it is an instantaneouswater heater, it achieves higher final output withlower consumption.

In most solar water heating systems – This

configuration is by far the most commonly usedtype of solar thermal systems




4



Natural Circulation or Thermosiphon Type

This solar heating installation has advantagesand disadvantages over the previous system

Fig. 3.2

It has Natural or Thermosyphon circulation. It

can be an indirect system or a direct system,which is only feasible for certain water qualities,and when no risk of freezing exists

Systems are designed based on the principlethat hot water rises. These are calledthermosyphon systems , and the storage tank isalmost always located outdoors, directly on top

of the solar collector




5



Natural Circulation or Thermosiphon Type

Main advantage: Simpler and cheaper.Main disadvantages: Less integration (and

therefore higher visual impact) as the tank ismounted above the collector, and normallyoutside. Less durable as it is controlled less,with the possibility of over-heating during lowusage and high solar radiation.

Fig. 3.2




6

Components of a Solar Thermal Installation – Forced Circulation


components in a typical solarinstallation:

Collector area

Tank

Pump

Piping

Expansion vessel

Purge unit (air vent)

Various types of valves

Temperature and pressure

sensors

Fig. 3.3




7

Components of a Solar Thermal Installation – Natural Circulation


Components in a typicalthermosiphon solar installation:

Collector area

Tank

Piping

Expansion vessel

Purge unit

Various types of valves

Temperature and pressure

sensors.

Fig. 3.4




8

Solar Energy Collectors


Solar energy collectors are special kinds of heat exchangers that transform solarradiation energy to internal energy of the transport medium

There are basically two types of solar collectors: non-concentrating or stationary andconcentrating

A non-concentrating collector has the same area for intercepting and absorbing solar

radiation, whereas a sun-tracking concentrating solar collector usually has concavereflecting surfaces to intercept and focus the sun’s beam radiation to a smaller receivingarea, thereby increasing the radiation flux




9

Concentration ratio is defined as the aperture area divided by the receiver/absorber area of the collector

Types of solarcollectors





10



Different definitions of area are used in the manufacturers’ literature to describe the

geometry of the collectorso Gross surface area (collector area) is the product of the exterior length and width of the

collector and defines for example the minimum amount of roof area required formounting.

o Aperture area corresponds to the light entry area of the collector – that is, the areathrough which the solar radiation passes to the collector itself

Fig. 3.5




11



o Absorber area (also called the effective collector area) corresponds to the area of the

actual absorber panelo When comparing collectors, Reference area is important – surface area from which the

collector’s characteristic values are drawn. Reference area is equal to either theaperture area or the absorber area.

Fig. 3.6




12



Three main types of collectors fall into this category of non-concentrating collector

1. Flat-plate collectors (FPCs)

2. Stationary compound parabolic collectors (CPCs)

3. Evacuated tube collectors (ETCs)

Flat Plate Collectors (FPCs)

Main components of a flat-plate collector are:

o COVER / GLAZING : One or moretransparent covers made of glass orplastic that let in solar energy, mostly ofwhich is in visible range and block thelong wavelength thermal radiation

emitted by enclosed heated materials

o ABSORBER : Dark surfaces inside ofhigh absorptivity, called absorber platesthat soak up heat. plate is usually coatedwith a high-absorptance, low-emittancelayer

Fig. 3.7




13



o

INSULATION: material at the back andsides of the collector box to preventconduction heat losses from the backand sides to the environment

o FLUID PASSAGEWAYS: Vents or pipes

connected to or integrated into theabsorber plates to carry the heatedfluid to where it can be stored or used

o CONTAINER: The casing surroundsthe aforementioned componentsand protects them from dust,

moisture, and any other material

Fig. 3.7




14

When solar radiation passes

through a transparent cover

and impinges on the

blackened absorber surface ofhigh absorptivity, a large

portion of this energy is

absorbed by the plate and

transferred to the transportmedium in the fluid tubes, to

be carried away for storage or

use

Glazing

Casing

Back and side

Insulation Absorber Plate with

Selective Coating

Incident radiation

(visible range)

Emitted radiation by absorber (Infrared range)

Heat Loss to Ambient

Reflection

Fluid Carrying

passage



Flat Plate Collectors (FPCs)—contd--

Fig. 3.8




15




Liquid tubes can be welded to the absorbing plate or they can be an integral part of theplate

Liquid tubes are connected at both ends by large-diameter header tubes

HEADER AND RISER COLLECTOR is the typical design for flat-plate collectors

Alternative is the SERPENTINE DESIGN does not present the potential problem of unevenflow distribution in the various riser tubes of the HEADER AND RISER DESIGN

Fig. 3.9




16




Absorber plate can be a single sheet on which all risers are fixed, or each riser can befixed on a separate fin

Transparent cover is used to reduce convection losses from the absorber plate throughthe restraint of the stagnant air layer between the absorber plate and the glass

Fig. 3.10




17



Flat Plate Collectors (FPCs) — contd--

Transparent cover reduces radiation losses from the collector because the glass istransparent to the shortwave radiation received by the sun, but it is nearly opaque tolongwave thermal radiation emitted by the absorber plate (GREENHOUSE EFFECT).

FPCs are inexpensive to manufacture, theycollect both beam and diffuse radiation

FPCs are permanently fixed in position, so notracking of the sun is required

Optimum tilt angle of the collector is equal to thelatitude of the location, with angle variations of 10°

to 15° more or less, depending on the application For solar cooling the optimum angle is latitude -10°

so that the sun will be perpendicular to thecollector during summertime, when the energy willbe mostly required. For space heating, optimalangle is latitude +10°




18




Collector should also have a long effective life, despite the adverse effects of the sun’s ultraviolet radiation and corrosion and clogging because of acidity, alkalinity, or hardness of the heat transfer fluid, freezing of water, or deposition of dust or moisture on theglazing and breakage of the glazing from thermal expansion, hail, vandalism, or othercauses

Glazing Materials

Requirements for the transparent cover are:

ohigh light transmittance during the whole service life of the collector

o low reflection

o protection from the cooling effects of the wind and convection

o protection from moisture

o stability with regard to mechanical loads (hailstones, broken branches etc.).




19




Glazing Materials—contd--

Glass has been widely used to glaze solar collectors

Glass transmit as much as 90% of the incoming shortwave solar irradiation whiletransmitting virtually none of the longwave radiation emitted outward by the absorberplate

Glass with low iron content has a relatively high transmittance for solar radiation(approximately 0.85 –0.90 at normal incidence), but its transmittance is essentially zero forthe longwave thermal radiation (5.0 –50 μm) emitted by sun-heated surfaces

For direct radiation, transmittance of glass varies considerably with the angle of incidence

Antireflective coatings and surface texture can improve transmission significantly. The

effect of dirt and dust on collector glazing may be quite small

Plastic films and sheets also possess high shortwave transmittance

Most usable varieties also have transmission bands in the middle of the thermal radiationspectrum, they may have long wave transmittances as high as 0.40




20

Only a few types of plastics can withstand the sun’s ultraviolet radiation for long periods

Plastics are generally limited in the temperatures they can sustain without deteriorating or

undergoing dimensional changes



Glazing Materials—contd--

Plastics are not broken by hail or stones, and in the form of thin films, they are completelyflexible and have low mass

ABSORBER

Core piece of a glazed flat-plate collector

Consists of a heat conducting metal sheet with a dark coating

When the solar radiation hits the absorber it is mainly absorbed and partially reflected

It consists of a heat conducting metal sheet with a dark coating

Heat is created through the absorption and conducted in the metal sheet to the heattransfer medium tubes or channels





21


ABSORBER — contd--



To maximize the energy collection, the absorber of a collector should have a coating thathas high absorptance for solar radiation (short wavelength) and a low emittance for re-radiation (long wavelength) → Such a surface is referred as a SELECTIVE SURFACE

Copper Sheet Black Paint Black Chrome TINOX

By suitable electrolytic or chemical treatment, surfaces can be produced with high valuesof solar radiation absorptance (α) and low values of longwave emittance (ε)

Selective surfaces are particularly important when the collector surface temperature ismuch higher than the ambient air temperature

Fig. 3.11




22

Flow passages




For fluid-heating collectors, passages must be integral with or firmly bonded to theabsorber plate

Materials most frequently used forcollector plates are copper,aluminum, and stainless steel

If the working fluid is a liquid , theflow passage is usually a tube that isattached to or is a part of absorberplate. If the working fluid is air , theflow passage should be below theabsorber plate to minimize heatlosses

Further Details-Self StudyPages: 127-129, PDF: Solar Collectors

Fig. 3.12




23



Flat Plate Collectors (FPCs) — contd – Special Designs

Integrated Collector Storage (ICS) or Batch Collectors

Collector and water store form a construction unit

components omitted are:oHeat exchanger,opiping for the solar circuit,ocontroller and circulation pump

heat water in dark tanks or tubes within aninsulated box, storing water until drawn.

It is a self-contained Integration of solar Collectorand solar heated water Storage

ICS systems are similar to thermosyphon systemsin that they heat water passively without pumpsand controller systems




24



Integrated Collector Storage (ICS) or Batch Collectors — contd –

It is prone to freezing, the system mustbe drained in winter months in colderclimates.

Efficiency is also limited in cold weatherand at night due to the significant loss ofheat

A batch solar water heater is somewhatlimited in size, so typically no more thanfour people can benefit from the system

Limitations/Disadvantages:





25




Hybrid Collectors

Hybrid collectors are a combination of solar panels (PV) with liquid based collectors aswell as with air-based collectors

combination with solar panels is reasonable, because during the solar electricityconversion only about 12% (with crystalline silicon) of the solar radiation converts into

electricity, whereas the remainder converts into heat. This heat is used in the hybridcollector to either heat up a liquid or air

The central problem with hybrid collectors is the high and in part more persistenttemperature load of the solar cells in the event of stagnation




26



Compound Parabolic Collectors (CPCs)

CPCs are made up of two parabolicreflectors and an absorber which is placedat the bottom of the collector

(CPC) are usually non-tracking and non-imaging collectors

Compound parabolic concentrators canaccept incoming radiation over a relativelywide range of angles

Fig. 3.13

By using multiple internal reflections, any radiation entering the aperture within the

collector acceptance angle finds its way to the absorber surface located at the bottom ofthe collector




27

The most commonly studied system is adoubled walled concentric glass tube placed at

the focus of a compound parabolic concentrator



Compound Parabolic Collectors (CPCs)—Contd--

Fig. 3.14

The absorber can take a variety ofconfigurations. It can be flat, bifacial, wedge,or cylindrical

Compound parabolic collectors should have agap between the receiver and the reflector toprevent the reflector from acting as a finconducting heat away from the absorber

For higher-temperature applications a trackingCPC can be used




28



Compound Parabolic Collectors (CPCs)—Contd--

Compound parabolic collectors can be manufactured either as one unit with one openingand one receiver (see Figure 3.14) or as a panel (see Figure 3.15). When constructed as apanel, the collector looks like a flat-plate collector

Fig. 3.14

T P j




29

Term Project

Project Group Members

Solar Cooking Technology Sidra Akbar

Sadaf Saghir

Solar Drying Technology Imran Sajid Shahid

Usman Manzoor

Saad Malik

Introduction

Historical Background

A brief review of available technologies/methods

oPros and cones of each technology/methodoTechnical Aspects ( Typical Temperatures, heat requirements etc.)

A simple case study (Theoretical design of a system for a certain application

Submission Date: 03-01-2013




30



Vacuum Collectors

Benefits of simple conventional FPCs are greatly reduced when conditions becomeunfavorable during cold, cloudy, and windy days

Weathering influences, such as condensation and moisture, cause early deterioration ofinternal materials, resulting in reduced performance and system failure

To overcome the above mentioned factors and due to the technical difficulties to evacuatethe space between the absorber and cover in FPC and to withstand the resulting externalpressure, evacuated tube collectors are used

In order to completely suppressthermal losses throughconvection, the volume enclosed

in the glass tubes must beevacuated to less than 10 –2 bar (1kPa)

Additional evacuation (up to 10-6 bar) prevents losses throughthermal conduction

Fig. 3.15

1bar 10-2 bar 10-6 bar

f l h l




31

Radiation losses cannot be reduced by creating a vacuum, as no medium is necessary forthe transport of radiation. They are kept low, as in the case of glazed flat-plate collectors,by selective coatings (small ε-value).



Vacuum Collectors—contd--

absorber is installed as either flat or upward-vaulted metal strips or as a coating applied toan internal glass bulb in an evacuated glass tube

An evacuated tube collector consists of a number oftubes that are connected together and which arelinked at the top by an insulated distributor orcollector box, in which the feed or return lines run.

There are two main sorts of evacuated tube collector:the Direct Flow-through Type and the Heat-pipe Type.

C f S l h l S




32

b) Absorber: flat, metal with coating. Passage: metal U-pipe

a) Absorber: flat, metal with coating. Passage: metal coaxial pipe

Glass

Absorber

Glass

Absorber

Glass

Absorber

c) Absorber: flat, metal with coating. Passage: metal, circular pipe straight through g) Absorber: cylindrical, metal with coating. Passage: metal U-pipe

Glass

Absorber

f) Absorber: cylindrical, glass with coating. Passage:

glass coaxial pipe

Glass

Absorber

e) Absorber: cylindrical, glass with coating. Passage: glass

Glass

Absorber

Schematic diagrams of different configurations of directflow type evacuated tube collectors




Fig. 3.16

C f S l Th l S




33




C t f S l Th l S t




34




Sydney collector

A particular design of direct flow-through evacuated tubecollector marketed in some countries is the Sydney collector

Collector tube consists of:

o

vacuum-sealed double tubeoinner glass bulb is provided with a

selective coating of a metal carboncompound on a copper base

oInto this evacuated double tube isplugged a thermal conducting plate

in connection with a U-tube to whichthe heat is transferred.

To increase the radiation gain thecollector is fitted with externalreflectors in the sloping roof version Fig. 3.17

C t f S l Th l S t




35




Heat-pipe Evacuated Tube Collectors

Heat pipe is an isothermal devise purely based on evaporation and condensation cycle ofthe working fluid

A selectively coated absorber strip, which ismetallically bonded to a heat pipe, isplugged into the evacuated glass tube

heat pipe is filled with alcohol or water in avacuum, which evaporates at temperaturesas low as 25°C

The vapor thus occurring rises upwards

At the upper end of the heat pipe the heatreleased by condensation of the vapor istransferred via a heat exchanger (condenser)to the heat transfer medium as it flows by Fig. 3.18

C t f S l Th l S t




36




Heat-pipe Evacuated Tube Collectors—contd--

The condensate flows back down into the heat pipe to take up the heat again

For appropriate functioning of the tubes they must be installed at a minimum slope of 25°

Fig. 3.19

C t f S l Th l S t




37




Integrated Compound Parabolic Collector

An evacuated tube collector in which, at the bottom part of the glass tube, a reflectivematerial is fixed

Either a CPC reflector, or a cylindrical reflector, is used

Fig. 3.19





38



oParabolic trough collector

oLinear Fresnel reflector

oParabolic dish

oCentral receiver

Sun-tracking concentrating collectors

Fresnel Collectors

Parabolic Dish Reflectors

Heliostat Field Collectors

Self Study Pages: 135-156,PDF: Solar Collectors





39



Stagnation temperature

Stagnation Temperature is the temperature at a stagnation point in a fluid flow

oIf the circulation pump fails in the event of strong solar irradiance,

oAbsorber heats up until the heat losses through convection, heat radiation and heatconduction reach the thermal output of the absorber

oor if – for example in holiday times – no hot water is used so that the store is hot (60 –90°C; 140 –194°F) and the system switches off, no more heat is drawn from thecollector

Well-insulated Glazed Flat-plate Collectors achieve maximum stagnation temperatures of160 –200°C (320 –392°F) --- Evacuated Tube Collectors 200 –300°C (392 –572°F) or with areflector – as much as 350°C (662°F)

For Solar collectors STAGNATION CONDITIONS are considered to be any situation underwhich the solar collector can not adequately reject absorbed solar heat to its primaryheat transfer fluid, thereby resulting in the solar collector and/or its components(including the heat-transfer fluid contained within its flow passages) to increase in

temperature above a desired maximum level





40

Such extreme stagnation conditions lead to:o deterioration of collector materials, like loss of vacuum in ETCs (evacuated tube

collectors) de to out-gassing from collector materials,oheat transfer fluid may rapidly degrade or even boil,oexcessive pressures may occur in the solar collector heat-transfer loop



Stagnation temperature—contd--

Heat Storage

Energy storage in solar thermal applications is necessary, whenever there is mismatchbetween the available solar radiation and demand---Because of the seasonal, diurnal andintermittent nature of solar radiation

Optimum capacity of an storage system depends on:

o the expected time dependence of solar radiation availabilityo the nature of loads to be expected on the processo The manner in which auxiliary energy suppliedo An Economic analysis that determines how much of the load should be carried by

solar and how much by the auxiliary energy source





41


Heat Storage

Process Loads and Solar Collector Outputs

Further details: Pages:382-384,

Book: by Duffie and Beckmann

G T, Q u and L as a function of time for 3-day period

Energy added to or removed from storage

Integrated values of the G T, Q u and L for the same 3 day period

GT: Available Solar Radiation

Qu: Useful Heat GainL: LoadsLA: Auxiliary energy supplied

A major objective of thesystem’s performance

analysis is to find long-termvalues of LA ,i.e amount ofenergy that must bepurchased

Times of excess energy

Energy withdrawn

from the storage





42


Heat Storage

Types of Thermal Energy Storage

Sensible-Heat Storage

Sensible heat Q is stored in a material of mass m and specific heat C p by rising thetemperature of the storage material from T 1 to T 2 and is :

2

1

2

1

T T

T

p

T

p dT VcdT mcQ

Most Common sensible heat storage materials are water, organic oils, rocks, ceramics,and molten salt----Water has the highest specific heat value of 4190 J/kg. C

Most common medium for storing sensibleheat for use with low and mediumtemperature solar systems is Water---it’s cheap and abundant

Water is the standard storage medium for solar-heating and cooling systems for buildingstoday





43


Heat Storage

Types of Thermal Energy Storage—contd--

Sensible-Heat Storage—contd--

)()()('

.

asssu

s

s p T T UA LQdt

dT

mC

)]()([)(

'

asssu

s p

ssT T UA LQ

mC

t T T

An energy balance on the unstratified storage tank ofmass m operating at time-dependent temperature T s in ambient temperature T a

’

• Q u Rate of addition or removal of energy from the collector• Ls Rate of addition or removal of energy from the load and

• T a ’ Ambient temperature for the tank (which could be different from that of collector)

3.1

3.2

Eq. (3.1) is to be integrated over time to determine the long term performance of the

storage unit and the solar processUsing Euler integration (i.e. rewriting the temp derivative as [(T s

+ - T s ) /∆t] and solving forthe tank temperature at the end of time increment:





44


Heat Storage



)]()([)(

'

asssu

s p

ssT T UA LQ

mC

t T T

3.2

Temperature at the end of an hour is calculated from that at the beginning, assuming Q u ,

Ls , and the tank losses do not change during that hour

Terms in Eq. (3.1) are rates and in Eq. (3.2), they are integrated quantities over an hour

Once the tank temperature T s is know, other temperature dependent quantities can beestimated

Solutions to the governing system equations when integrated over long periods (usually a

year) provide information on how much solar energy is delivered to meet load.





45


Heat Storage


Sensible-Heat Storage—contd-- Example

A fully mixed water tank storage containing 1500 kg of

water has a loss coefficient area product of 11.1 W/oC

and is located in a room at 20 oC. At the beginning of a

particular hour the tank temperature is 45 oC. During the

hour energy Qu is added to the tank from a solar

collector, and energy Ls is removed from the tank and

delivered to a load as indicated in the table. Using Euler

integration, calculate the temperature of the tank at the

end of each of 12 hours.

)]()([)(

'

asssu

s p

ssT T UA LQ

mC

t T T





46


Heat Storage


Sensible-Heat Storage—contd-- Example—contd--




Department of Mechanical EngineeringHITEC University Taxila 47


Heat Storage



Narrowness of the Hot Water Store

A general schematic of the Upright modelsolar tank with auxiliary heat exchanger isshown

Hot waterextraction

Heating feed(Auxiliary)

Heating Return(Auxiliary)

Solar CircuitFeed

Solar CircuitReturn

Cold WaterSupply

Every time a tap is turned on, cold waterflows into the lower area of the store, →

cold, warm and hot water are found in theone store at the same time

Because of the different densities, a

Temperature Stratification effect forms,‘lighter’ hot water collects at the top, the‘heavier’ cold water in the lower tank area

Stratification Effect has a positive effect onthe efficiency of a solar system






Heat Storage


Sensible-Heat Storage—contd-- Narrowness of the Hot Water Store

As soon as hot water is drawn off, cold water flows in: thisshould not mix with the hot water. The slimmer and taller thestore, the more pronounced the temperature stratification will be

Upright stores have the best stratification; the recommendedheight –diameter ratio for optimum stratification is at least 2.5:1

Coldest possible lower zone ensuresthat, even with low irradiance, the solarsystem can still operate with highefficiency at a low temperature level

Stable Stratification

No stratification, Additional Heat Required

For interested readers:1- Chapter-8, Energy Storage, Book: by

Duffie and Beckmann2- Chapter-2, Book: Planning and

installing Solar Thermal Systems






Heat Storage


Latent Heat Storage

Thermal energy can be stored as latent heat in a material that undergoes phasetransformation at a temperature that is useful for the application

If a material with phase change temp. T m is heated from T 1 to T 2 such that T 1 < T m < T 2 , the

thermal energy Q stored in a mass m of the material is given by:

3.3 ʎ = heat of phase transformation

Four types of phase transformation useful for latent heat storage are:oSolid ⇌ Liquid

oLiquid⇌ VaporoSolid⇌ Vapor

oSolid⇌ Solid Some common phase change materials (PCMs) used for thermal storage are: Paraffin

waxes, nonparaffins, inorganic salts (both anhydrous and hydrated).






Heat Storage

Types of Thermal Energy Storage—contd-- Latent Heat Storage—contd--






Heat Storage


Thermo Chemical Heat Storage

Heat storage in this mode is based on the Reversible Chemical Reaction according to theformula

A + B⇌ AB + Heat

Reaction in the forward direction is exothermic and endothermic in the reverse direction

Amount of heat stored in a chemical reaction depends on the heat of reaction and theextent of conversion as given by:

a r : fraction reacedm : mass∆H : Heat of reaction per unit mass

3.4

During the load phase, the substance AB is supplied with heat, which dissociates thecomponents A and B

In order to recover the heat, both components A and B are allowed to react with one other

As long as a reaction between A and B is prevented, the heat stored in form of chemicalenergy cannot be set free





Heat Storage


Thermo Chemical Heat Storage—contd--

Energy densities of water: approx. 50 kwh/M3

Energy densities obtainable with thermo chemical heat storage: approx. 120-140 kwh/M3)

means a large amount of heat can be stored in small quantity of material

chap3 sts till 29-nov12

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