Solar water heater Components

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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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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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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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



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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Solar water heater Components