i1

B.Eng. Mechanical Engineering

Natural Water Heating on Roofs

A.R. Townley

May 2009

Supervisor: Mr J.G. Heppell

Thesis submitted to the University of Sheffield in partial fulfilment

of the requirements for the degree of Bachelor of Engineering

Department Of Mechanical Engineering.

ii ii

Summary

This project was carried out with the primary aim of creating a sustainable, affordable

solution for naturally heating water in the village of Pabal, India. Wealthier residents use

systems comprising of blackened pipes on their roofs for water heating, but such systems

are too expensive for the majority. Cost and availability of materials are major restrictions

in this project, so the importance of a cheap, rugged solution cannot be emphasised

enough.

In order to reach a final design, it was necessary to be well familiarised with the village of

Pabal, have a sound knowledge of different natural water heating systems and make use of

a structured design procedure. As the project developed it became apparent that the sun

would be the most prevalent heating resource, so it was therefore also important to become

competent in understanding the way the sun behaves to ensure that the design would utilise

this natural resource as best as possible.

A product design specification was carried out and six concepts were detailed, these were

then further examined by means of a decision matrix concept evaluation, which confirmed

the most suitable design idea. The batch heater was taken forwards for further

development and a suitable final design was completed. Although the final design idea

was satisfactory upon completion, utilising a broad range of understanding and technical

input throughout the development process, confirmation of whether or not it actually holds

any sustainable value for the people of Pabal will only be evident upon its installation. It

would therefore be advised that further work investigating both the performance and

economic viability of the batch heater would be of value.

iii iii

Nomenclature

A Area

B Breadth of strut

Cp Specific heat capacity at constant pressure

d Depth of strut

E Young’s Modulus

I Second moment of area

k Thermal conductivity

L Length of strut

m Mass of fluid being heated

s Thickness of insulating wall

dT Difference in temperature

∆T Change in temperature

T Temperature of a radiating body

Te Temperature of air surrounding a radiating body

𝑄 Energy

𝑄 Heat

Wy Euler buckling load

𝜀 Emissivity

𝜎 Stephan-Boltzmann constant, 56.7×10-9

Wm-2

K-4

iv iv

Contents

Summary .................................................................................................................. ii Nomenclature.......................................................................................................... iii Contents ................................................................................................................... iv Acknowledgement .................................................................................................... v

1. INTRODUCTION ................................................................................................ 1

1.1 Pabal ................................................................................................................. 1 1.1.1 Overview .................................................................................................. 1 1.1.2 Available Technologies ............................................................................ 2 1.1.3 Public Amenities ...................................................................................... 3

1.2 Natural Water Heaters ...................................................................................... 3

1.2.1 Systems and Applications in More Economically Developed Countries . 4 1.2.2 Designing for Less Economically Developed Countries .......................... 6

2. UNDERSTANDING THE SUN .......................................................................... 7

2.1 The Sun in the Sky ........................................................................................... 7 2.2 Solar Irradiation ................................................................................................ 8 2.3 Pabal and The Solar Track................................................................................ 8

3. METHODOLOGY ............................................................................................. 11

3.1 Primary Design Stages ................................................................................... 12 3.1.1 Flat Plate Collector ................................................................................. 14

3.1.2 The Evacuated Heat Pipe ........................................................................ 15 3.1.3 Concentrated Solar Power ...................................................................... 16 3.1.4 Solar Batch Heater .................................................................................. 18

3.1.5 Absorption Heat Pump ........................................................................... 18 3.2 Concept Evaluation ........................................................................................ 19

3.3 Detailed Design .............................................................................................. 21

3.3.1 Optimising Design .................................................................................. 21

3.3.2 Checking for Errors ................................................................................ 25 3.3.3 Preliminary Parts List and Production Details ....................................... 30

3.3.4 Final Detailed Design ............................................................................. 32

4. CONCLUSION .................................................................................................. 36

4.1 Further Work .................................................................................................. 37

5. REFERENCES ................................................................................................... 38

6. APPENDICES ........................................................................................................ 39 6.1 Appendix A .................................................................................................... 39 6.2 Appendix B ..................................................................................................... 40

6.3 Appendix C ..................................................................................................... 41

6.4 Appendix D .................................................................................................... 42

6.5 Appendix E ..................................................................................................... 43 6.6 Appendix F ..................................................................................................... 44

v v

Acknowledgement

I would like to take this opportunity to thank all those who have supported and

encouraged me throughout this project and my academic career. The course has

proved to be a struggle at times and without these people, I would not have reached

the final year.

In particular I would like to thank Mr G Heppell for his help and guidance through

this project, Ms E Rodriguez-Falcon for her unwavering encouragement, support

and patience both in this project and throughout my time here at the University of

Sheffield and Robin Mills for his kind help with natural water heating fundamentals

and his ongoing academic support.

Finally I would like to thank my parents, family and friends for the continuous

motivation they offer me.

1 1

1. INTRODUCTION

Natural water heating solutions exist on varying scales with design ideas as

simple as a series of blackened pipes, right through to the most complex solar

furnaces, which concentrate the suns radiation and are capable of temperatures

that will melt steel plate.

In this project, possible sustainable solutions for naturally heating water are to

be investigated and developed. The system is to be designed so that it will be

suitable for installation in the village of Pabal in Maharashtra, India. The

Natural Water Heater (NWH) will be required to integrate with a suitable water

supply as well as a means of storing the water so that hot water can be used a

period of time after heating.

The primary objectives are as follows:

To fully understand the environment in which the natural water heater

will be placed, the role it must perform and any restrictions in place.

This must include a comprehensive understanding of the way the sun

behaves.

To generate a list of design ideas and develop these ideas into valid

concepts.

To finalise a sustainable concept, based upon the results of a sound

concept evaluation.

The main factor that will influence the feasibility of implementing any NWH

design is the limited resources in Pabal. Particular advanced materials and

manufacturing processes may prove to be inaccessible, placing constraints on

possible concepts.

1.1 Pabal

1.1.1 Overview

Pabal is a rural village, with a population of around 350 people in the centre of

the village and around 9000 people in the surrounding areas[1]. It is located in

2 2

the state of Maharashtra, which is in the west of India, with Pabal being located

slightly to the east of the nearest major city, Mumbai.

It is a relatively poor village and its major economic focus is agriculture with

peanuts, lentils, onions and potatoes being the main types of crops grown on

the surrounding farmland.

Figure 1.1. A view of typical rooftops in Pabal[2].

Currently some of the more wealthy members of the Pabal community use

natural water heating solutions for washing but the problem for the vast

majority of residents is that such systems are currently cost prohibitive[2].

Those that cannot afford such systems tend to use natural gas or kerosene

heaters for their cooking needs at around a cost of Rs300 (around 40p) per

month[3]. It is worth noting that these heaters are not used to heat water for

washing.

1.1.2 Available Technologies

The main technological resource in Pabal is Vigyan Ashram, a school founded

in 1983 by scientist turned teacher, Late Dr S.S. Kalbag[4]. Vigyan Ashram

teaches subjects concerned with sustainable engineering and concentrates on

3 3

rural development. Relating to this project the most useful facility within

Vigyan Ashram is the FabLab, a well-equipped workshop with skilled machine

operators.

Further from Pabal is the University of Pune, whose focus is more academic

and is located a few hours travel south west of Pabal. Its presence is still

useful, with potential for a valuable working relationship for the development

of natural water heating technology suitable to be implemented in communities

in the surrounding areas.

It is necessary to be mindful of these facilities at all stages in the design

process, and although there are some fairly advanced machines in the FabLab,

for example a laser cutter and welding equipment, there will always be benefits

associated with simple, rugged design for Less Economically Developed

Countries (LEDCs) such as India.

1.1.3 Public Amenities

In recent years a dam has been built in the Pabal area so that water is more

readily available throughout the year; prior to this most families had either their

own well or access to a well[1]. There is no organised sewage infrastructure

and it may be that in many of the dwellings a humble hole in the ground is

used. Following correspondences with an EngIndia representative, it can be

reasonably assumed that the types of dwellings focused upon in this project

will not already have any form of plumbing installed[3].

1.2 Natural Water Heaters

NWHs can essentially be described as heat exchangers, which utilises a natural

source of energy to heat the water or other working fluid. In this project it is

water that is required to be heated from its ambient temperature to a

temperature that will result in the water being hot enough to wash in after one

night in storage.

4 4

Given worldwide issues with emissions and their effects on global climate

change there is mounting pressure at many levels to be conscious of sustainable

energy sources. Since the Kyoto Protocol was introduced in 1992 both local

and national governments of those countries who have signed into the

agreement are obliged to meet certain targets devoted to reducing their

emission of greenhouse gases[5].

1.2.1 Systems and Applications in More Economically Developed Countries

As a direct result of such protocols, money is being invested in alternative

energy sources all over the world. Many of the systems being developed

involve heating fluids naturally. A fine example of natural water heating on a

large scale is the PS10 Solar Tower in Seville, Spain. This is shown in figure

1.2.

Figure 1.2. The PS10 Solar Tower[6].

The solar tower uses a Concentrated Solar Power (CSP) system with mirrors

that track the suns movement, to focus the radiation onto the solar receiver at

the top of the tower. Water is heated to power a steam turbine which in turn

drives the alternator. This is a fine example of the power of natural water

5 5

heating and in this instance the suns power. A CSP system of this scale is

incredibly expensive however the cost of such technologies can be expected to

decrease with time, in the same way as wind power.

Despite the eventual reduction in the cost of such systems they are still

unsuitable for Pabal because their focus is on providing for a large area. A

good example of a type of NWH being installed to work in a single residence is

the ground source heat pump.

Figure 1.3. A typical ground source heat pump layout [7].

A heat pump uses a refrigeration cycle with a working fluid to utilise heat in

the ground at a relatively low temperature. In a vapour compression cycle the

refrigeration cycle is effectively used as a lever for increasing the useful heat

out as a result of the mechanical work in. It is worth noting that it is necessary

to have that mechanical input and therefore necessary to have a readily

available supply of electricity. It is only the coefficient of performance that

makes it possible to offset the cost of such a system over a period of time.

Ground source heat pumps are normally installed in new builds with an “eco-

friendly” focus, or a prudent owner wishing to save on energy bills over the

coming years. An initial outlay of between £6000 and £12000 is normal for a

8-12kW ground source heat pump[8].

6 6

1.2.2 Designing for Less Economically Developed Countries

Returning to the project brief, the primary aim is to design a solution to be

implemented in rural Pabal. The design must be simple or at least possible to

manufacture locally. This is not to say that the technology employed must also

be basic. Various concepts and design ideas, of varying suitability will be

explored and developed.

There are a number of restrictions that come about when designing for places

such as Pabal and these restrictions prevent the simple installation of existing

systems. The main limiting factor is cost, most natural water heating devices

used in more economically developed countries seek to save money over a long

payback period, with little limit on initial costing. This is not the case in Pabal

and even if paying for a system makes economic sense, it may still be

impossible due to a lack of capital to fund the system. The second limiting

factor is the availability of materials and manufacturing facilities, in order to

encourage development in the local community it is important for manufacture

to take place in the Pabal area. If the resources available are not given

consideration, the design will most likely fail in production.

7 7

2. UNDERSTANDING THE SUN

In order to take advantage of the sun as a resource, it is necessary to fully

understand how much energy is available, how much of this energy can be

harnessed and how the sun behaves in the sky. Much of the literature review

was devoted to both understanding basic celestial definitions and gathering data

specific to the area of interest, Pabal.

2.1 The Sun in the Sky

The sun rises in the east and sets in the west, making an arc as it travels across

moving in two planes. It makes two angles, one with the horizon and one with

the due north. The angle with the horizon is known as the Altitude where 0

would see the sun on the horizon. The angle the sun makes north/south is

known as the Azimuth and by convention 0 is due north. This coordinate

system, which is sometimes referred to as the alt-az system, is illustrated in

figure 2.1[9].

Figure 2.1. A sketch illustrating the two main celestial coordinates.

As discussed, each day the sun will follow a path, which will be referred to as

the solar track. This solar track varies each day and with latitude so as a result

solar devices that follow the sun are normally expensive due to the need for

sophisticated control systems.

8 8

2.2 Solar Irradiation

The suns intensity varies at different latitudes, so once the solar device has

been positioned correctly by taking into account the solar track for the area of

interest the useful irradiation available needs to be known so that calculations

for the device can be carried out and its efficiency validated. Such data is

available from weather records and such like and is normally illustrated on a

map such as the one in figure 2.2.

Figure 2.2. A solar irradiation map of the world [10].

The typical unit for solar irradiation is kWh/m2/day and careful examination of

figure 2.2 gives a value of around 5kWh/m2/day for Pabal. This is an average

value for the year and the length of day and night has been taken as equal at

43200seconds for calculations

2.3 Pabal and The Solar Track

Values for the alt-az coordinate system can be calculated manually, but when

the required values are over a year, this would prove tedious and time

consuming. A number of free programs are available to perform these

calculations on your behalf, provided that you know the latitude of your area of

interest, Pabal is at approximately 18N. On this occasion the SunPosition tool

by Christopher Gronbeck of Sustainable By Design was used [12] giving a

discrete number of solutions for the year at an interval of every six hours.

Altitude varies little month to month, so it was decided that the first day of each

9 9

month would be used and three plots were done at 0600, 1200 and 1800 hrs

shown in figure 2.3.

Contrary to convention this tool takes due south as 0 datum for azimuth but

this makes no difference provided it is noted. It can be observed on the 1200

plot that the altitude appears a smaller angle in June and July, this is however

not the case because the azimuth shifts north, with Pabal being south of the

tropic of Cancer. It is also evident that the sun remains high in the sky

throughout the year with a mean altitude of 71.5˚, this is discussed further in

section 3.3.1.

10 10

Figure 2.3. Plots showing Altitude and Azimuth for 18N.

-20

0

20

40

60

80

100

120

1 2 3 4 5 6 7 8 9 10 11 12

An

gle

(d

eg

rees)

Month

0600

Altitude

Azimuth

0

20

40

60

80

100

120

140

160

180

200

1 2 3 4 5 6 7 8 9 10 11 12

An

gle

(d

eg

rees)

Month

1200

Azimuth

Altitude

-120

-100

-80

-60

-40

-20

0

20

1 2 3 4 5 6 7 8 9 10 11 12

An

gle

(d

eg

rees)

Month

1800

Altitude

Azimuth

11 11

3. METHODOLOGY

In order to develop the design of the NWH it was necessary to follow a suitable

design process so that the problem could be fully understood and correctly

addressed. The strategy followed in design is much the same in all industries

and it can be loosely broken down into the gathering of relevant information in

order to define the problem and understand surrounding relevant issues, the

development of several possible solutions, the evaluation of these concepts to

allow the most suitable concept to be prevalent, finally followed by refining the

chosen solution. This logical record, which is gradually being compiled is not

only an aid to the designer or engineer but can also be an important document

in a culture of litigation and may serve to convict or exonerate the investigated

party in the event of failure resulting in loss to persons or property[12]. The

documented work is therefore not only useful to the designer during the design

process, but may serve to protect him or her in the future.

Figure 3.1. A Design Process Schematic [13].

12 12

Figure 3.1 is a schematic showing the basic design process, it is worth noting

the presence of feedback loops due to the importance of constantly reviewing

and modify existing ideas and information, with further knowledge being

gained as the design process evolves.

3.1 Primary Design Stages

In the first stage of the design process the problem must be identified so that

the specification can be elaborated. A mind map investigating natural water

heating was carried out revealing details about Pabal’s location, different types

of NWHs and the thermodynamic processes involved (Appendix A). Once the

initial understanding is in place, the Product Design Specification (PDS) can be

carried out. The PDS should be well documented and under constant review,

once again incorporating the important iterative feedback loops present

throughout the whole design process.

A product design specification (figure 3.2) was completed and key points were

identified from the initial mind mapping exercise, allowing each criterion to be

weighted both by importance and relevance. Once the key specifications are

evident, a secondary literature review can be carried out allowing concepts to

be developed. Concept development should be a free-thinking time in the

design process, with as many theoretical processes and practical applications as

possible investigated. Six concepts are detailed.

13 13

Figure 3.2. Excel worksheet Product Design Specification.

No. Description Importance Relevance

1 PERFORMANCE - Comfortable shower temp. can be 4 4

taken at around 40degrees C. Water temp must be

around this value agfter storage.

2 ENVIRONMENT - Maharashtra experiences summer 4 5

temps around 40degrees C, winter around

10 degrees C. May be placed on rooftops. Will

intergate with water collection device and storage.

3 LIFE IN SERVICE 5 4

4 MAINTENANCE - Target maintenance free. 4 4

5 PRODUCTION COST - Prototype budget £250. Must 5 5

include machine times etc in final production cost.

6 COMPETITION 1 1

7 SHIPPING - Materials to be sourced locally, to be 3 3

produced/fabricated locally. Eliminate many shipping

costs.

8 PACKING - If the product is to be made in large 3 4

quantities then thought must be given to fitting

the water heater on pallets etc.

9 QUANTITY - Initially one prototype 1 1

10 MANUFACTURING FACILITY - Fab Lab, carpenter, 5 4

blacksmith.

11 SIZE - Footprint not so important. Needs to have a 3 2

large surface area - heat exchanger.

12 WEIGHT - Must be able to be handled by 2 men. 2 2

13 AESTHETICS, APPEARANCE & FINISH - finish must be 2 2

weather proof. Must not look too intrusive in situ.

14 MATERIALS - Materials with relevant thermodynamic 4 4

properties to be chosen. Availability of materials.

15 PRODUCT LIFE SPAN - Determined by resistance to 4 4

weather, no moving parts little wear, water corrosion.

16 STANDARDS & SPECIFICATION - Made to a spec 2 2

determined by the designer.

17 ERGNOMICS - Very little human contact once set up. 2 2

18 CUSTOMER - May require education on advantages 2 1

of the system. Consultations where possible.

19 QUALITY & RELIABIITY - High quality required. 2 2

Must be reliable in day to day service.

20 SHELF LIFE 2 1

21 PROCESSES - Will be made in house in Pabal. How 4 3

this will be controlled must be specified.

22 TIME SCALES - Only limit academic year. 2 1

23 TESTING - Prototype may undergo be tested. Must 4 4

meet pre-designated criteria.

24 SAFETY - once installed it is unlikely to prestent 3 3

risks, but consideration is still required.

25 COMPANY CONSTRAINTS 0 0

26 MARKET CONSTRAINTS 1 1

27 PATENTS, LITERATURE & PRODUCT DATA - several 3 2

existing designs. Will base loosely around those

being careful of patent infringement.

28 POLITICAL & SOCIAL IMPLICATIONS - The 4 2

installation strategy will be handled by EWB who

already have links established with the village.

29 LEGAL 3 2

30 INSTALLATION - Must interface with collection and 4 4

storage solution. Must fit on Roofs.

31 DOCUMENTATION - Clear, complete user manual for 4 4

installation to be provided. Will explain any user

input required.

32 DISPOSAL - Ideally environmentally friendly 3 2

materials where possible. Life span will be long

though.

14 14

In order to have a water heating system it must not be forgotten that a readily

available supply of water is essential. Although this obvious parameter can be

taken for granted in most environments, outside of the monsoon season there is

relatively little water available, even with the recent dam arrangement a water

infrastructure is not in place for all dwellings. An investigative project

studying possible rain water harvesting solutions is being carried out parallel

with this one, however at present no results have been disclosed. It is

necessary to assume that there is a supply of water and that it can be moved to

the NWH for heating, whether this is by means of a pump or a gravity fed

system. Due to the lack of electricity in the area a gravity fed system would be

desirable, with a header tank above the NWH. The head of water will also be

used to provide pressure at the hot water tap.

3.1.1 Flat Plate Collector

The flat plate collector (FPC) is amongst the most common solar water heaters

and is widely used for heating water for a multitude of purposes as well as air

space heating systems. There are two main types of FPC, unglazed collectors,

which are popular for heating swimming pools and the like. For better

performance the collector is glazed which will help reduce heat emitted by the

black body[14].

Essentially the FPC consists of a box with blackened tubes or troughs running

through, with an inlet and an outlet for the working fluid. The box is insulated

underneath the tube and depending on the type of FPC, glazed over the top, as

illustrated in figure 3.3.

15 15

Figure 3.3. A sketch of a Glaze Flat Plate Collector.

FPCs, like most NWHs, are normally operated with a second circuit that will

contain the fluid required for output from the system; this would require either

a pump or a thermal siphon. Although the FPC is inherently quite low cost and

could be expected to perform more than adequately in the Indian conditions, its

necessity to be part of a more bulky twin loop system, may hinder the overall

suitability of this type of water heater in the Pabal environment.

3.1.2 The Evacuated Heat Pipe

The evacuated heat pipe (EHP) is a more specialist heating system using a heat

pipe, which is contained in an evacuated tube tilted at an angle to encourage

convection. The fluid heated by the sun remains in the heat pipe with the

hottest fluid rising into the bulb at the top of the heat pipe.

Figure 3.4. The basics of the Evacuated Heat Pipe [15].

16 16

Normally a number of EHPs are used in one system arranged in a similar

manner to a conventional cross flow heat exchanger. This can be very

efficient but the problem with the EHP for use in Pabal is the manufacturing

process required and the costs associated with it. The glass itself is expensive

to produce and evacuating the tube once it has been fabricated is again,

difficult and expensive.

3.1.3 Concentrated Solar Power

There are two types of Concentrated Solar Power (CSP) systems, the first of

which is the Dish system, which concentrates solar radiation to a focal point in

order to increase the intensity of the radiation. Frensel lenses can also be used

but these lenses are very expensive to produce in the large sizes necessary; for

this reason curved mirrors are much more common. In order to focus the

radiation correctly the lens or mirror must be perpendicular to the angle of

incidence of the sun, it is this requirement that increases the complexity of a

CSP system due to the need for a sun tracking device.

Figure 3.5. A dish CSP system in Spain powering sterling engines [16].

Although such arrays of dish CSP systems are far beyond what will be possible

in Pabal, ideas for creating reflective dishes by spinning plaster of Paris whilst

it sets to create a parabolic shape were investigated (figure 3.6). Such a

manufacturing technique makes the dish CSP system a very cheap and

interesting solution.

17 17

Figure 3.6. Plaster of Paris spun in a drum at angular velocity, .

The second variation on CSP is the Trough style CSP system, which uses a

pipe running along the length of the trough, placed at the focal point to absorb

the solar radiation reflected. Once again the curved mirrors are costly, but it

would be possible to have many passes of pipe between the ridges in a

corrugated metal sheet coated in reflective foil (figure 2.7). The problem with

both the spun plaster of paris dish and the corrugated sheet ideas is that

although these reflectors are very easy to produce, a sun tracking device is still

required, making their feasibility limited.

Figure 3.7. A sketch of the corrugated sheet trough CSP idea.

A CSP system of either type would need to operate on two loops, with a

working fluid being heated in the first loop, with a heat exchanger in the water

storage tank. Ideally thermal convection would provide the flow of the

working fluid, though further testing would be required to confirm the

efficiency of such a system.

18 18

3.1.4 Solar Batch Heater

Arguably the simplest of all solar water heaters, the batch water heater is both a

water heating and hot water storage device. Convention is for a blackened tank

mounted in a box that will serve to reflect radiation onto the underside of the

tank during the day and insulate the tank overnight whilst it is in storage

mode[17].

Figure 3.8. A sketch of a typical batch heater.

In some respects the water in the batch heater will tend to heat just by existing

in a hot environment, conversely it will also tend to emit radiation to the

surrounding area so unless this is accounted for in design it could prove

unsustainable. The simplicity and low cost nature of the batch heater mean that

it could be suitable for application in Pabal.

3.1.5 Absorption Heat Pump

By far the most complex type of NWH design idea investigated is the

absorption heat pump. This system replaces the conventional pump in a vapour

compression refrigeration cycle with an absorber and a generator to create

changes in the density of the working fluid to create a flow. In theory solar

energy can be used for the heat in to the generator, but the problem is that the

absorption cycle is one fifth the efficiency of a compressor. Studies are

currently being undertaken by Camfridge Ltd [18] to develop an absorption

19 19

cycle refrigerator so the possibility of using this technology in a heat pump

exists.

Figure 3.9. A schematic of an absorption refrigeration cycle [19].

The required mechanical work input into the system is so low that if designed

correctly it can be neglected completely and a gravity fed circulation system

can be used. Once installed such a system could be incredibly cost effective

provided the coefficient of performance offered is adequate but the limiting

factor is the level of investment required for development. In addition local

manufacture would prove difficult with limited access to the necessary working

fluids and materials for construction.

3.2 Concept Evaluation

A structured concept evaluation is a necessity when selecting the best

preliminary layouts of the final design or concept, a key step in the flow chart

in figure 3.1. A short literature review of common concept evaluation

techniques was carried out and several methods were considered. Many of the

common techniques require a group of designers to carry out, for example a

vote method. Other methods included making several prototypes of different

concepts then selecting the best performing or most suitable concept. Time and

money limitations mean this sort of evaluation is also not possible. It was

therefore decided that a decision matrix would be most suitable.

20 20

In order to carry out the evaluation a discreet set of technical and economic

criteria are required. In order to select these criteria the PDS was examined and

those rated three or higher for importance were included in the decision matrix.

This also serves to evaluate the various concepts against the appropriate

technical and economic criteria which is an integral part of selection.

Criteria Flat Plate

Collector

Evacuated

Heat Pipe

Dish

CSP

Trough

CSP

Absorption

Heat Pump

Batch

Heater

Performance -1 2 2 1 2 1

Maintenance 1 1 1 2 0 2

Cost 2 -2 0 2 1 2

Shipping* 1 1 0 1 1 1

Packing* 1 1 0 1 1 1

Manufacturing

facility

2 0 1 2 1 2

Size* 1 1 0 1 1 1

Materials 1 -1 1 1 0 2

Product life

span

0 1 1 1 2 1

Processes 0 0 1 1 0 1

Legal* 0 1 1 1 1 1

Installation 1 1 0 2 1 2

Total 7.5 4 7.5 14 11 15

Figure 3.10. The Concept Evaluation Decision Matrix.

*These criteria were rated 3 in the PDS, therefore they have been weighted

half in the concept evaluation. This is accounted for in the total.

Scoring system: -2 - very poor

-1 - poor

0 - acceptable, average

1 - good, above average

2 - very good

At this stage it is evident that the solar batch heater (2.1.4) emerges as the

strongest concept based upon the criteria detailed with a total of 15 points. The

trough CSP system rated second with 14 points.

21 21

3.3 Detailed Design

The two highest scoring designs were further analysed bearing in mind the

most relevant elements of the PDS, which were considered to be; production

cost, available materials, manufacturing facilities, working lifespan and

required maintenance. When referring back to the concept evaluation (figure

3.10), it can be seen that both concepts scored maximum points in these

categories. It was therefore necessary to further evaluate the details of the

concepts in order to bring forward a prevalent design.

As discussed in chapter 3.1.3, the Trough CSP would be a twin loop system,

utilising a thermal siphon to drive the flow of the working fluid but it would not

be unlikely for a supplementary pump to be required for best efficiency. Due

to the lack of a reliable electricity supply in the Pabal area and the requirement

to make the NWH self sufficient in its operation such a pump is undesirable.

The Batch Heater eliminates the need for other components in the system due

to its integrated design, the main tank being both the heater and the storage

unit. This makes the whole concept inherently simple, so primarily for this

reason, the Batch Heater was chosen over the Trough CSP system and was

taken forwards to the optimising stage in the design process. Fewer

components ultimately lead to a system that will require less maintenance

therefore ultimately better suited to an environment where cheap, trouble-free

running are the priorities.

3.3.1 Optimising Design

It can be seen in chapter 3.1.4. that convention with batch heater design is to

mount the in a box so the efficiency can be maximised during the day but also

to insulate the stored water at night. It is desirable to have a simple but elegant

solution that would fulfil these requirements rather than a cumbersome

removable lid.

22 22

After a mind mapping session was carried out, a possible solution emerged; this

was to use the mirrors for insulation when the tank is in storage mode. This

could be achieved by a gul-wing style arrangement allowing the mirrors to fold

neatly up around the tank. In order to investigate the plausibility of such a

system and to reveal any initial conflicts with the geometry a small model near

to scale was assembled (figure 3.11). This was important because it gave a

tangible prototype that could be reviewed in the same way a person using the

NWH may review the design upon first encountering it. Checking for errors at

each step makes the design process more efficient by reducing the chance of

finding omissions further along in the final design stages, and once again refers

to the importance of iteration in the design process.

Figure 3.11. The initial Batch Heater model.

Straight away a number of problems can be identified. With a similar idea to

spinning the CSP dish to create the convex shape, an idea was to use Euler

buckling theory applying a tension across the reflective gul-wings in order to

create the desired parabola. It can clearly be seen, that with this arrangement,

the tie bars will foul the drum, in this instance fouling on the pins holding the

drum in place. In addition to this problem, using a triangular base at each end

offers a solid mounting solution as far as the frame itself is concerned;

however, the interface between the drum and the frame is not of an acceptable

style. It would be difficult to design well and hard to manufacture.

23 23

Bearing in mind the initial concept, a refinement of the design idea was carried

out. The drum is to be supported by a cradle, rather than a pin joint and instead

of a buckling arrangement for the reflectors, the shape can be achieved using a

series of ribs held together with longitudinal struts. This technique is well

proven in the building of boat hulls and can also be seen in the construction of

curved bicycle ramps, both are applications that require a rugged, low

maintenance solution. The ribs could be laser cut at the FabLab, further

encouraging the use of this design. Integrating a hinge into the rib is

straightforward, with a hole drilled through which a spindle can pass. A brass

bush would easily press in if wear at this point were considered an issue after

use. The rib design also gives a cavity, which will be filled with straw. Straw

has excellent insulation properties and is cheap and readily available.

Figure 3.12. A sketch of the improved “ribbed gul-wing” design.

Batch heaters and other NWHs are normally angled according to latitude so

that the NWH is perpendicular to the suns angle of incidence for as much of the

day as is possible. Convention is to have the NWH at the same angle from the

horizontal as the locations latitude, for example Sheffield is at approximately

53N so a good approximation would be to angle the NWH at 53 to the

horizontal, facing south.

24 24

Figure 3.13. Excel spreadsheet showing the suns altitude on the 1st of each

month as well as the mean angle for the year.

It can be seen in figure 3.13 that this theory transfers well for this scenario as

Pabal is at around 18N. The suns mean altitude is 71.5 so it is clear that

angling the batch heater at 18 is appropriate (90-71.5=18.5).

Figure 3.14. A free body diagram of moments acting on the frame.

Batch heaters normally lie north south but as figure 3.14 illustrates, inclining

the batch heater north south creates problems with the frame design, especially

if an adjustable frame is to be used. These problems come about because as

soon as an angle is introduced into the system, so is a moment. If the batch

heater were to be inclined in this direction then further cross bracing would be

required to support the frame and prevent a failure.

Month Altitude

Jan 48.15

Feb 53.69

March 63.44

April 75.18

May 86.14

June 93.04

July 94.32

Aug 89.3

Sept 79.97

Oct 68.15

Nov 57.21

Dec 49.45

Mean 71.5

25 25

3.3.2 Checking for Errors

A typical oil drum is 44 imperial gallons, which equates to 0.2m3. Given that

water has a density of 1000kg/m3[20] and a specific heat of 4.19kJ/(kgK)[20]

the energy in joules required to raise the 0.2m3 of water by temperature T is

given by:

𝑄 = 𝑚𝐶𝑝∆𝑇 3.1[21]

The heat lost 𝑄 through a wall of thickness s, area A is given by:

𝑄 = 𝑘𝐴𝑑𝑇

𝑠 3.2[20]

Where k is the thermal conductivity of the insulating material, in this case

straw for which k=0.09W/(mK)[22] and dT is the difference in temperature

across the wall. In a cylinder, area changes with radius, making for a more

complicated scenario, however due to the nature of the design environment the

flat plate equation 3.3 was deemed suitable.

A comfortable shower temperature is around 45-50C so ideally, after storage

overnight the water temperature should be in this region. Night-time

temperatures in Pabal can be as low as 2C but the average low is around 6C

[23]. Let it be assumed that the water can be heated to 60C and that the

outside temperature at night is 5C, let it also be assumed that the temperature

at the wall equals these temperatures and that the “night” is 12hours long. The

heat lost by conduction over this period and the subsequent drop in temperature

is given below.

𝑄 = 𝑘𝐴𝑑𝑇

𝑠

𝑄 = 0.09 × 2.56 ×55

0.04

𝑄 = 316.8𝑊

∴

𝑄 = 316.8 × 43200 = 13.69𝑀𝐽

∴

∆𝑇 =𝑄

𝑚𝐶𝑝=13.69 × 106

200 × 4190= 16.3℃

26 26

In this near worst-case scenario with an ambient night time temperature of 5C,

the analysis shows that the water will be at around 44C in the morning if

heated to 60C the previous day.

Now let it be assumed that the ambient water temperature from the supply is

10C, therefore it will be necessary to increase the temperature by 50C. As

discussed in section 2, it will also be assumed that the “daytime” is 12hours.

𝑄 = 𝑚𝐶𝑝∆𝑇

𝑄 = 200 × 4190 × 50 = 41.9𝑀𝐽

∴

𝑄 =41.9 × 106

43200= 970𝑊

With a solar irradiation of around 5kWh/m2/day in the Pabal area

approximately 417W is available for each square meter, the batch heater has an

area of approximately 1m2. This is around half the 970W required which has

been shown above. In order to maintain the compact nature of the design,

increasing area is somewhat difficult, referring back to equation 3.2 it can be

observed that by reducing the mass of the fluid, the energy required reduces

proportionally. For this reason it was decided to half the tank size.

It is also necessary to note that a black body will tend to emit heat to the

surroundings, as well as having good absorption properties. The emissivity of

a body that is not perfectly black, i.e. grey, is a factor introduced to correct the

amount of heat emitted by said body.

𝑄𝑒𝑚 = 𝜎𝐴𝜀(𝑇4 − 𝑇𝑒4) 3.3

Equation 3.4 shows the heat emitted, 𝑄𝑒𝑚 , in watts by a grey body where is

the Stephan-Boltzmann constant, 56.710-9

Wm-2

K-4

, A is the radiating area,

is the emissivity of the body and T is the temperature (in Kelvin) of the body

and Te is the temperature of the surroundings. The temperature in Pabal is

normally between 30 and 46C, so let Te=303K, T=333K.

𝑄 𝑒𝑚 = 𝜎𝐴𝜀 𝑇4 − 𝑇𝑒4

𝑄 𝑒𝑚 = 56.7 × 10−9 × 1.88 × 0.95 × (3334 − 3034)

𝑄 𝑒𝑚 = 391𝑊

27 27

This is figure is significant, which means that there will be considerable heat

loss over a whole day if this problem is not addressed. Although in practice the

value of 𝑄 𝑒𝑚 will not be as great as the above due to a constant heat in from the

sun, it will still be imperative to have a cover over the batch heater to maintain

efficiency. A simple, close fitting, transparent “shower cap” type arrangement

would serve to reduce the problematic heat loss. It can be seen from equation

3.4 that as the difference between the temperature radiated to, Te and the tanks

temperature, T approaches zero, the heat emitted, 𝑄 𝑒𝑚 , also approaches zero. It

is therefore desirable to increase the temperature of the air around the batch

heater as close to the temperature inside the tank as possible. If the tank is

covered so that there is little or no flow of air, the sun will tend to heat the

enclosed space, vastly improving the overall efficiency of the batch heater.

Figure 3.15. A sketch of the further improved design, optimised for heating.

The batch heater will be aligned east west with the flat face of the of the drum

making an angle of 18 with the horizontal and facing south. Trough CSP

systems tend to be arranged in this manner because it allows better tracking of

the sun as the azimuth changes through the day. Figure 3.16 shows the suns

differing altitudes for the first day of the month for January to June. From July

to December the altitude angle decreases again, so plotting these angles on the

diagram is unnecessary. It is clear from figure 3.16 that 18 is the optimum

angle to set the batch heater at for year round operation.

28 28

Figure 3.16. The Suns’ six major altitudes from January to June.

Furthermore figure 3.16 illustrates the role of the mirrors in the batch heater

and any potential problems with either shadowing or the suns radiation

reflecting straight back, missing the tank. A sketch examining the mirrors is

shown in figure 3.17, this example is for the month of June and for the

purposes of illustration, the radiation has been broken into lines so that the

angle at which it will reflect can be inspected. When the line meets the curve it

will be reflected symmetrically from a tangent to the curve at that point, in

figure 3.17 it can be observed that for this altitude angle, a satisfactory amount

of radiation is reflected onto the underside of the tank, with minimal waste

reflection and shadowing only coming about as a result of the tanks presence.

29 29

Figure 3.17. Areas of shadow and the mirrors reflection directions.

The east west arrangement also eliminates the problems of moments within

frame design that are illustrated in figure 3.14, the legs were analysed to be

sure that they were capable of withstanding static loading. For simplicity, the

two legs are treated as struts, each one supporting W/2N where

W=100x9.81=981N. The Youngs Modulus (E) for wood, is between 8 and

13GPa [20] and the second moment of area, I is calculated using equation 3.4.

3.4[20]

Which gives:

The Euler Buckling load for a strut loaded along its centre pin jointed at both

ends is given by equation 3.5, where L is the length of the strut, in this case the

height.

3.5[20]

Which gives:

I Bd3

12

I 0.29 0.043

12

I 1.546106m4

Wy 2EI

L2

Wy 2 10109 1.546106

0.332

Wy 1.4MN

30 30

Although this is far beyond the load that the drum will ever exert upon the legs,

it is worth having the sturdy arrangement for lateral stability and longevity. It

is also worth nothing that this is a fairly bad approximation for the maximum

buckling load because the legs are almost as wide as they are tall making them

relatively unsuitable for strut equations, however, in this instance it is clear that

they will perform adequately from practical experience with similar scenarios.

3.3.3 Preliminary Parts List and Production Details

The batch water heater will be assembled from the following parts:

1x 44 imperial gallon oil drum, cut in half, painted black

2mm steel plate 580x870mm

2x 40mm thick wooden legs

2x triangular stabilisers

6x ribs

2x mirrors

1x 10mm steel bar

6x thick wooden struts

2x thin wooden struts

2x straw insulated end caps

2x pipe connectors

2x securing catches

1x elastic transparent cover

For any concept designed with the intention of going into production, it is

essential for it to be possible to actually make. Many designs are excellent in

conceptual stages but fail when the components cannot be machined or

clearance for tools in the assembly process has not been accounted for. For the

batch heater, there should be relatively few problems with tight access but the

manufacturing process for each component and the order of assembly must be

considered. Documentation is an important part of any design and clear

instructions for manufacture need to be communicated.

31 31

The process will be as follows:

Mark the drum for cutting. Using an angle grinder, cut down the

centre of the drum. Mark the ends of the drum so that they can be

drilled to accept the inlet and outlet pipes. Thoroughly clean the inside

of the drum. De-bur any cut edge.

The legs will be made from wooden board (MDF equivalent), mark the

legs ready for cutting and drilling. Cut the legs to shape and drill the

hole in each to accept the spindle.

The ribs will also be made from wooden board (MDF equivalent) and

will incorporate three processes. The board will first be laser cut so

that the outline of the rib is evident. The hole for the spindle will then

be drilled, followed finally by routing the clearance shoulder.

Mark the longitudinal struts to length, and cut.

Assemble the gul wing arrangement by gluing the struts into the

dedicated slots in the ribs. Using the narrow profile planking, lay the

planking around the outer edge of the curve and screw each plank into

position, being sure to butt each plank tight against the previous one.

Use caulk sealant to create a weatherproof seal. Now lay the planking

on the inner edge of the curve in the same manner as the outside edge,

stuff the straw insulation as the planking progresses. Use spray

adhesive to stick the aluminium foil to the inner face to create the

mirror. Attach the clips screwing into ribs for security, not just

planking.

Mark the lower end caps and cut to shape. Mark the upper end cap to

shape and cut. Use a router to profile the shoulder so that the upper

end cap lies flush with the gul wings when in storage mode.

Rest the tank onto the legs, drill down through the drum where it sits

on the legs, then fasten the drum using the coarse threaded screws.

Fasten the pipe fittings into the ends. Offer up the lower end caps to

the end of the drum, once again drill through the drum and then screw

through from the inside using a coarse threaded screw. Use caulk

sealant to watertight the areas around all the screws.

32 32

Cut the top face of the tank to size using a bandsaw, if no bandsaw is

available mark the plate and cut using an angle grinder. De-bur the cut

edge.

Tack the top plate onto the drum, once happy with the position of the

top face, weld along the joint and allow the tank to cool.

Mask the legs and end caps and paint the tank using matt black paint,

leave to dry.

Offer up the gul wings and pass the spindle though the holes in the legs

and the ribs, finally threading the securing nut on the end the spindle.

Fasten the hinges onto the lower end caps.

Fasten the upper end caps to the hinges.

3.3.4 Final Detailed Design

The batch heater components were modelled and then assembled using Solid

Works CAD software. CAD software has many useful features for the

engineer working with a new concept, allowing a full size model to be created

on screen, which can vastly reduce the number of prototypes necessary. Given

both time and powerful computing resources, the latest software packages, such

as Solid Works, can perform finite element and computational fluid dynamics

analysis, examine assemblies for problems in manufacturing and animate

moving components, as well as integrating with CNC machine software for a

smoother transition from the drawing board to the workshop floor.

The batch heater will utilise technologies ranging in complexity. FabLab has a

well equipped workshop capable of cutting complex two dimensional shapes

using a Laser cutter, as well as a range of CNC milling machines. Welding

equipment is available for the drum.

Figure 3.18 and 3.19 illustrate the two working modes of the batch heater and

the components involved. In figure 3.18 the batch heater is operating in

heating mode, with the mirrors lowered to their stops and the end caps folded

down so that there is no undesirable shadowing. Conversely in figure 3.19 the

33 33

batch heater is in storage mode, the end caps are upright, creating a seal with

the mirrors, which are now wrapped around the drum for insulation. Note that

in the illustrations the ribbed framework of the mirrors is exposed, in operation

a thin outer skin, pinned to the ribs would enclose straw to reduce the thermal

conductivity of the cover.

Figure 3.18. The Batch Heater, open in heating mode.

34 34

Figure 3.19. The Batch Heater, closed in storage mode.

There are four main moving parts in the batch heater, the mirrors and the end

caps. These are best illustrated in figure 3.20 and both utilise a hinge system;

the mirrors share a common spindle fixed into a hole through the legs whilst

the end caps use two conventional hinges at each end. Wear has not been

examined at the point where the mirrors meet the spindle, it would be difficult

and time consuming to perform such a task analytically so this would be an

area requiring monitoring in service. If the holes in the wooden ribs were

observed to show any sign of wear that was deemed unacceptable, then as

previously mentioned it would be a straightforward task to press in a bush to

alleviate the problem. This task could once again be undertaken by FabLab.

35 35

Figure 3.20. The underside of the Batch Heater.

36 36

4. CONCLUSION

The objective of this project was to develop a sustainable solution for naturally

heating water in the village of Pabal in India. In order to arrive at such a

solution it has been necessary to recognise and understand different existing

natural water heating solutions as well as becoming familiar with the village of

Pabal and the resources available there. Furthermore, the application of a

structured design process was imperative and the absence of such a process

may have ultimately led to the failure of any designs.

As a design exercise this project has been a success, with a well structured set

of criteria followed allowing a concept that could realistically be manufactured

in the designated environment to be developed. The understanding of the sun

gained has been key to the success of the final design and the importance of the

positioning of any natural water heater must be stressed. Given the limited

resources available it has been necessary to make a compromise between low

technology and durability, and performance. Although the analytical work

done shows the 100 litre batch heater should perform adequately, the true test

will be the installation of a prototype.

Despite the apparent success, thought must be given as to how suitable

implementing the use of natural water heaters of any kind in Pabal actually is.

It has been established that the dwellings that would be potential candidates for

having the batch heater installed currently have no plumbing either for hot or

cold fresh water or arrangements for sewage. There is also a limited electricity

supply with the majority of villagers using natural gas or kerosene as their main

fuel for cooking. As a mechanical engineer it is difficult to put forward a

single solution that will actually bring real benefit to the community. True

progress within the village can only really come about as a result of a larger

scale infrastructure development scheme along with the education and

involvement of local people. The people of Vigyan Ashram are carrying out

such work admirably with their Rural Development Education System.

37 37

“Many accept that science education must be coupled to action in the

laboratory, how many will accept the real life world as the best laboratory to

learn science?”[24] The local attitude towards education is summarised well

here by Dr Kalbag and it is clear that the true test of any developments in Pabal

will be their performance in the field.

4.1 Further Work

This project has focused primarily on the academics of the design process itself

to give rise to a viable natural water heating solution. What cannot be learnt

from this approach is how well the batch heater will actually perform day to

day and the effect of the changing weather conditions and sun position

throughout the year. It would therefore be advised that any further work should

focus on long term field testing of a prototype batch heater. As well as adding

knowledge regarding the water heating performance of such a system, the

quality of the integration with the available water supply and the hot water taps

would also be verified. It would also be of benefit to assess the economic

impact of such a system on local people, if it were to be implemented on a

commercial scale.

As is commented upon in section 4, the positive impact one single device can

have is limited by the fluidity of its integration within the community. For this

reason it would be worthwhile commissioning further work focusing upon a

means of linking the proposed batch heater or similar water heating solution

into a local plumbing infrastructure system. In order for such a project to be a

success, all parties involved in the various design tasks must work closely

together, ideally with the support of either Vigyan Ashram or the University of

Pune to ensure well grounded local integration. Unfortunately limits on time

and money for this particular project, have prevented such an endeavour, at

least for the time being.

38 38

5. REFERENCES

1. [Online] engindia.wikidot.com

2. Natural Water Heating on Roofs Project Brief, engINdia & EWB-UK

Research, 2008

3. Courtesy of Pooja Wagh, endINdia

4. [Online] vigyanashram.com

5 Shogren J, The Benefits and Costs of the Kyoto Protocol, American Enterprise

Institute, 1999

6. [Online] dessi.nu/cleantech/theteam.html

7. [Online] acclimatize.co.uk/heating.html

8. [Online] energysavingtrust.org.uk

9. [Online] astro.columbia.edu/%7Earchung/labs/fall2001/lec01_fall01.html

10. [Online] oksolar.com/abctech/images/world_solar_radiation_large.gif

11. [Online] susdesign.com

12. MEC333 Integrated Design Skills - The Engineering Design Process, The

University of Sheffield, 2008

13. Pahl G and Beitz W, Engineering Design, London Design Council, 1984

14. International Solar Energy Society – UK Section , Flat Plate Collectors and

Solar Water Heating 4th

Ed, ISES, 1975

15. [Online] solarproject.co.uk

16. [Online] cetstation.com

17. Langa S, Sun On Tap – The Best We Know, Rodale’s New Shelter 22,

July/August 1981

18. [Online] camfridge.com

19. Rogers G and Mayhew YEngineering Thermodynamics: Work & Heat

Transfer, Longman Scientific and Technical, 1992

20. J Calvert and R Farrar, An Engineering Data Book, Palgrave, 1999

21. S Beck Et Al, The Little Book of Thermofluids, 3rd

Edition, The University of

Sheffield. 2006

22. [Online] engineeringtoolbox.com/thermal-conductivity-d_429.html

23. [Online] solconnectrealtor.com/marvelrealtors/about-pune.php

24. Kalbag S S, The Challenge of Education, Vidnyan Ashram

39 39

6. APPENDICES

6.1 Appendix A

Appendix A. Initial Natural Water Heating Mind Map

40 40

6.2 Appendix B

Appendix B. The Flat Plate Collector and Concentrated Solar Power

41 41

6.3 Appendix C

Appendix C. Concentrated Solar Power – Dish

42 42

6.4 Appendix D

Appendix D. Concentrated Solar Power – Dish

43 43

6.5 Appendix E

Appendix E. Evacuated Heat Pipe

44 44

6.6 Appendix F

Appendix F. Absorption Heat Pump