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8/9/2019 Solar water heater Components
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Basic Components of
Good Solar Hot WaterSystems
Tri Ratna Bajracharya, Ph.D.Center for Energy Studies (CES)
Institute of Engineering/TU
Pulchowk, Lalitpur
Tribhuvan UniversityInstitute of Engineering
Lecture 2: Solar Thermal Technology
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colle
ctor
heat storage
heattransportsystem
heat
exchanger
cold water supply
connection tohot water tap
pump
control unit
powersupply
with insulation
expansion
vessel
backup heater
thermostatair releasevalve
(blockable)
pressurereleasevalve
(8 atm)
ch
ec
k
va
lve
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Forced-flow versus natural circulation
SHW systemsforced flow+ better performance+ can be installed in large systems+ allows independent location of collector and hot water tank requires more components
needs electrical energy for pumping and control is more expensive
natural circulation (thermosiphon)+ simple and require less components+ work without active control equipment+ cheaper not suitable for large systems less efficient storage tank must be located above the collec
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Types of collectors and Selective coatings
1. Flat Plate
2. Vacuum tube
3. Concentratic tube
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Flat-plate collectors
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SWH System Locally Made
Collector
Storage tank
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Locally made SWH
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Absorber types
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Open- versus closed-loop SHW
systemsClosed loop+ better performance avoids air bubbles+ Prevents siltation of the collectors+ allows use of antifreeze Some temperature loss needs significantly more components more expensive more difficult to manufacture
Open loop+ simple and require less components
+ Requires less tecnical skill+ Cheaper and less sensitive to flaws not suitable for frost area not suitable for poor quality water
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There are basically two types of coatings
black solar paint
+ easy to handle
+ absorber can be painted locally
+ good absorption of solar radiation
+ low costs
high emission losses
selective coatings+ high absorption of solar radiation
+ low emission losses
+ prefabricated absorber stripes exist on the market
higher costsmust be handles with care
Construction of a solar collector
The absorber coating
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Construction of a solar collector
Casing
Materials
hardwood (resistant painting improves durability)aluminum profilessheet metal casing (aluminum, steel)Features
protects the collector from the elements (rain, dust)must be rigid to avoid the break of the glass coverall connections to the piping should be kept insulatedmust allow room for thermal expansionmust provide adequate ventilation to avoid condensation
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Construction of a solar collector
Insulationinsulation of the back side is very important toreduce heat losses and improve the efficiency
at least 5cm of a low conductivity insulator isrecommended
mineral wool and polyurethane foam is the mostcommon material
reflective lamination is often included betweenlayers of insulation
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Construction of a solar collector
transparent cover
Why is a transparent cover necessary?
protection of the absorber from the environment
reduction of heat losses
special solar glass (4mm)
+ high mechanical resistance
+ high transparency for solar
radiation
high weight and costs
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normal glass (4mm)
lower mechanical resistance
lower trasparency
high weight
+ lower costs
polycarbonate cover (3mm)+ high resistance
+ less weight
lower transparency
costs?
plastic foil
+ light and cheap
low resistance
short life span
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collectorarea reservoir size
4.5 m (3 panels) 250 l reservoir
7.5 m (5 panels) 400 l reservoir
10.5 m (7 panels) 600 l reservoir
Collector Area sizing Thumb Rule
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High efficiency collectors for process heat applications
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The evacuated tube collector - examples
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Evacuated tube collectors - examples
Compound parabolic concentrator (CPC) combined with
evacuated tube
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Conclusions
Characteristics of a good collector:
selective absorber coatinglow heat capacity, high heat transfer coefficientsiron-free glass coverstiff and light casingsufficient insulation of the back sideno thermal bridges at the connecting pipesTwo options available:flat-plate collectorsevacuated tube collectors
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