Solar Water Heating Notes

8/7/2019 Solar Water Heating Notes

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A

Presentation

onSolar Water Heating Systems

Shankar Singh Dhami

066msree516



OUTLINE

Introduction

Natural Circulation Systems

Forced Circulation Systems

SWHs in low atmospheric temperature regionsBuilding heating:

The f- chart Method for Air and Water Systems,

Fundamentals of Building heating using SWH

Heating System Simulation,

Architectural Considerations and Calculation o

heating Loads



Typical solar water heaters in

Market

Flat plate SWH

Vacuum tube SWH

3





Natural Circulation Systems (Thermo Siphon)

Hot Water Storage with insulation

Heat

Transport System

Cold Water Supply

hot waterdelivery

Electric

auxiliary heater

Thermostat

Drain

(Flush)





Working Principle of SWH

Hot

WaterOutlet

for Use

ColdWater

Supply

Cold waterinlet in

Collector

Hot wateroutlet from

Collector

Storage Tank

Collector H e i g h t

Temperature

1

2

4

3

1

2

3

4

Hot (Less denseexpanded fluid)

Cold (Dense fluid)

Principle of

Thermosyphon flow

7



heattransportsystem

heat

exchanger

cold water supply

connection tohot water tap

control unit

powersupply

Heat Storage with insulation

expansion

vessel

backup heater

thermostatair releasevalve

(blockable)

pressurereleasevalve

(8 atm)

c h e c k

v a l v e

Forced Circulation Systems



Indirect

Heating



Water freezes below 0ocAntifreeze Ethylene Glycol

with in the closed circuit

ETC used



Heat Tube Solar Collector

• Indirect Heating System

• Suitable for freeze prone area without anyalteration

• High efficient system• Can be used for high pressure (forced

convection ) system as well



Flat-plate collectors

Coatings: Normal Matt Black Paint,

Selective coatings: black chrome,

black nickel, and aluminum oxidewith nickel

Insulation: Fiber Glass,

Rockwool, Vacuum

Absorber: Aluminum,

CupperCasing: GI sheet, Aluminum



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 collectors



Open- versus closed-loop SHW systems

Closed 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



Heat for comfort in building can be providedfrom solar energy similar to Solar Heaters

Two most common heat transfer fluids are

water( or water and antifreeze mixtures) and air. The basic components are the collector, storage

unit, distributor and load (i.e., the house orbuilding to be heated)

Active Solar Heating and Passive Solar Heating The term solar house include the integral part of

the building elements that admit, absorb, storeand release solar energy and thus reduce theneeds for auxiliary energy for comfort heating.



If solar energy is available and heat is not needed inthe building, energy gain from the collector is addedto storage.

If solar energy is available and heat is needed in thebuilding, energy gain from the collector is used tosupply the building need.

If solar energy is not available and heat is needed inthe building, and the storage unit has stored energy init, the stored energy is used to supply the buildingneed.

If solar energy is not available, heat is needed in thebuilding, and the storage unit has been depleted,auxiliary energy is used to supply the building need.



Using air, the problem of freezing and boiling in the

collectors are eliminated and corrosion problems are reduced

Design parameters



Design parameters



The f- chart method is used for estimating the annual thermalperformance of active heating systems for buildings(using eitherliquid or air) where the minimum temperature of energy delivery isnear 20oc.

The purpose of the method is to calculate f , the fraction of the hotwater load (total heating load) that is provided by the solar energyfor a given solar heating system (solar fraction). Once f iscalculated, the amount of renewable energy that displaces

conventional energy for water heating can be determined. The method enables the calculation of the monthly amount of

energy delivered by hot water systems with storage, given monthlyvalues of incident solar radiation, ambient temperature and load.



The Primary Design variable is collector area; secondaryvariables are collector type, storage capacity, fluid flow ratesand load and collector heat exchange sizes.

The method is the correlation of the results of many

hundreds of thermal performance simulations of solarheating systems. f is the function of two dimensionless parameters i.e. one is

related to the ratio of collector losses to heating loads, andthe other is related to the ratio of absorbed solar radiation to

heating loads. f charts have been developed for three standard system

configuration, liquid and air systems for space (and hotwater) heating and systems for service hot water only.



Two dimensionless groups X and Y are defined as:

andwhere ,

Ac = collector area,F’R = modified collector heat removal factor,

UL = collector overall loss cœfficient,

Tref = an empirical reference temperature equal to 100°C,

Ta = monthly average ambient temperature,L = monthly total heating load,

τα = collector’s monthly average transmittance - absorptance product,

HT = monthly average daily radiation incident on the collector surface

per unit area, and

N = number of days in the month.



The fraction f of the monthly total load supplied by the solar water

heating system is given as a function of X and Y as:

The surface described by above equation is fairly smooth, so

extrapolation should not be a problem. If the formula predicts avalue of f less than 0, a value of 0 is used; if f is greater than 1, a

value of 1 is used.

For Liquid System

For Air System

F Li id S t F Ai S t



f-Chart Correlation.

For Liquid System For Air System



Standard liquid system configuration Standard air system configuration

Ranges of Design Parameters used in Developing the f- charts fora. Liquid Systems b. Air Systems



The implicit assumptions of f-chart are Systems are well built Flow distribution to collectors is uniform, flow rates are as assumed,

system configurations are close to those for which thecorrelations were developed, and

Control strategies used are nearly those assumed in the f-chart development.

Three steps to check the results from f-chart Comparison with detailed simulation in many locations Laboratory measurements on experimental systems have

also been compared to f-chart Measurements on operating systems in the field also have

been used for comparison.



Active solar heating system challenges to architectural design of buildings. Anybuilding design must be energy conserving, as solar energy and the fuels with

which it competes will be expensive.

Economics studies of active system solar heating indicated that the optimum

fractions of total annual loads ranges from zero to over three-fourths.

For some locations the architect must design into the building collector areas in

the range up to approximately one-half of the floor area of the house (depending

on the collector, the climate and the degree of insulation in the building)

The basic problem faced by the architects and engineers is to integrate the

collectors into the building design in such a way that thermal performance is

satisfactory and the structure is aesthetically satisfying

Collector should be oriented with the designed slope and azimuthal angle. Vertical collectors may be used for high latitudes to answer the problems of

integration of collectors into the buildings and avoiding snow accumulation

Space must be provided in the structure for energy storage units, piping and

ducts, controls, auxiliaries and all associated equipment.



Load calculation is necessary for the service hot water (with or without storage)models. Hot water use estimates are provided for service hot water systems.

No estimate of hot water use is done for aquaculture, industrial or “other”

applications.

The actual load is calculated as the energy required to heat up mains water to the

specified hot water temperature. If Vl is the required amount of water and Th is therequired hot water temperature, both specified by the user, then the energy required

Qload is expressed as:

Where,Cp = heat capacitance of water (4,200 ( J/kg)/ºC),

ρ = density (1 kg/L), and

Tc = cold (mains) water temperature.

Qload is prorated by the number of days the system is used per week.



The flow passagesconduct the working fluid

through it.

In water heating collector,cold water is let in, from

the bottom of collector

and hot water exits from

the top end.







Thank youfor your kind attention!





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Solar Water Heating Notes