Feasibility overview of a solar water heater made of polyethylene

terephthalate bottles for rural areas in Guatemala

Jorge de la Torre

Institute of Science and Technology for Development, Rafael Landivar University

email: jtorre@edu.gt

ABSTRACT A solar water heater prototype was constructed using polyethylene terephthalate bottles to determine its

feasibility as a component of the average household in rural areas in Guatemala. The cost of the prototype

(with materials acquired in Guatemala) is USD 220 and can easily be reduced to USD 127. Additional cost

reductions can be expected if the water heating system is mass produced. It is clear, regardless, that such a

water heating system needs the support of international programs to become available to low income families

in rural areas. Its performance, on the other hand, was found to be acceptable, it heats up water at ambient

temperature (varying from 22 °C to 26 C°) up to 38°C to 45 °C, respectively.

KEYWORDS

Solar, water heater, energy, sustainability, development

INTRODUCTION

Economical context

With a population of nearly 16 million and a GDP of USD 53.80 billion Guatemala

represents the biggest economy in Central America [1]. Nevertheless, the levels of

inequality of Guatemala are among the highest from Latin American countries: 63.1% of

Guatemalans are poor ( those whose income is less than US $ 4 per day 2012 The other

two large socio economical groups are the vulnerability segment ( those who earn between

USD 5 - 9 per day, and lacks any financial stability) represented by a 27.4% of the

population, and the middle class (whose income is US $ 10- 40 per day); which is a 9% of

the population [2].

Poor, vulnerable and middle class group represent a 99.5% of the population.

Water access and wood consumption

In the rural areas 3 out of 4 people are poor. Nevertheless, 89% of the population in rural

areas have water access [1] (The World Bank, 2013) and nine out of ten people depend on

wood energy 75% collects wood and 25% buys it [3].

It is estimated that Guatemala losses between 50 and 70 thousand hectares per year; 60% of

its primary energy consumption comes from wood. It is clear that, like in any other

developing country, wood represents the only available and affordable source of energy for

most households. In fact, more than two billion people depend on wood for cooking and

heating [4].

Water heating and recycling of polyethylene terephthalate bottles

Despite the popularity of wood as an energy source for cooking and heating, its use might

have harmful effects on health [5] when rudimentary stoves are used as the burning means;

such devices allow the smoke to pollute the household air with ashes. As a health–friendly

alternative to rudimentary stoves, improved stoves (that block smoke leakage inside the

house) have been developed and promoted since the early 1980s [6]. Aside from health

benefits, improved stoves have the advantage of higher efficiency. Thus, its implementation

also helps reduce wood consumption or deforestation.

Another potentially cheap source of energy that can be used in rural areas is solar energy

for water heating.

Certain solar water heaters SWHs made of cheap and recyclable materials (such as

polyethylene terephthalate PET - thermoplastic polymer- bottles ) can heat the water

up to 52 ° C [7]

Hot water in rural households can be used for cooking, sanitizing and for hot

showers

As a complement to improved wood stoves, a solar water contributes keeps the

household from using wood, therefore, reducing health risks and deforestation

The collection of PET bottles for the construction of SWHs can be added to

recycling programs

One the advantages of collecting PET bottles for SWHs is to keep them out of landfills,

protecting the environment; PET bottles take about 450 years to decompose [8]. Even in

developed countries like the U.S, the recycling rate of PET bottles is only 31% (each year

1.5 billion PET bottles and containers are recovered in the U.S for recycling)[9]. In Mexico

(the second largest PET bottle consumer after the U.S), the third largest expense for the

typical household, after tortillas and milk are sodas [10].As for solar water heating, with the

goal of reducing hydrocarbon fuels, Mexico already implemented a program about the

installation of 1.8 million square meters of SWHs by 2012 [11] . Although the SWHs are

not necessarily targeted to rural areas, the Mexican government seeks to reduce the

consumption of natural gas with solar energy.

As for Guatemala, it is estimated that 42 thousand tones of PET bottles are recovered each

year for recycling [12].

With the exception of programs that promote the use of various models of improved stoves

[13], programs supporting the use of other means (besides wood) for water heating in the

countryside, such as SWHs are either scarce or nonexistent. Regardless, there already have

been proposals of cheap solar water heaters for rural areas in Guatemala [14,15].

Solar radiation

All things considered, there’s a clear opportunity to exploit solar energy in Guatemala:

many regions experience at least 6 hours of solar radiation per day at 1kW/m2

(see Figure

1). In fact, the largest solar park in Central America, of 5 MW, located in the south east of

Guatemala, will be inaugurated in May 2015[16].

Figure 1. Solar radiation map of selected Central American countries [17].

BASIC PRINCIPLES OF THERMSIPHON SOLAR WATER HEATERS

One of the simplest designs for a SWH corresponds to the thermosiphon SWH, which is

the case for the solar water heater discussed in this paper.

A general diagram for the thermosiphon SWH is shown in Figure 2. Water is heated

directly by solar radiation by circulating through the collector. The cycle starts as the sun

comes out and heats the water inside the collector; hot water rises -a convection current-

and reaches the storage tank which is located above the collector (the top of the storage

tank is connected to the top of the collector). Meanwhile, water at lower temperature inside

the storage tank, flows to the collector (the bottom of the storage tank is connected to the

bottom of the collector); water at lower temperature has higher density, causing it to sink,

leaving the storage tank with water of lesser density: hot water. Thus, a thermosiphon

circulation is triggered. Note that such a system does not require the use of pumps; only

natural forces are involved.

Figure 2. Schematic diagram for a thermosiphon solar water heating system.

It is widely known that a thermosiphon solar water heater has the following advantages:

Simple

Electricity free

Does not need external pumps

However, the main disadvantages of a TSWH involve lack of plumbing knowledge and

skills. Wrong election of tubing may allow air pockets to block the water flow. Besides, if

the pipes that connect the collector to the storage tank are not steep enough, the

thermosiphon effect might not occur. That without mentioning eventual leaks due to bad

connections, poor insulation of the tank etc. In short, the supervision of a professional is

recommended in the construction and installation of a SWH.

Solar Radiation

Storage tank

Solar collector

Cold Water

Hot Water

Thus, if the plumbing is appropriate, unobstructed water circulation will happen. The

therrmosiphon will depend then, on how well solar energy is absorbed by the water in the

collector, actually at the exact part where radiation is to be absorbed, which is usually

called receiver (usually a black surface) The energy balance for a solar collector receiver –

the main component of the collector- is described by [18],

useful = optical - loss (1)

where :

useful = rate of useful energy from the receiver

optical = rate of optical radiation incident on receiver

loss = rate of thermal energy loss from the receiver

Note that for conventional receivers the incident radiation corresponds to short wavelength,

due to the fact that glass -usually the transparent cover of the receiver, which is

encapsulated - reflects most infrared light. As for the useful energy, it can be expressed as

the rate of heat transfer to the fluid passing through the receiver

useful = cp- (Tout – Tin) (2)

where the following quantities refer to the heat transfer fluid:

= mass flow rate of fluid

cp = specific heat of fluid

Tout = temperature of fluid leaving the absorber

Tin = temperature of fluid entering the absorber

Now, when optical and loss are expressed in terms of physical properties, and loss is

written as a sum of the loss mechanisms [18] (convection, radiation and conduction) we get

from (1) and (2)

useful = cp (Tout – Tin) = τα Ia Aa – Ar [ h´ (Tr – Ta) + εσ (T4

r - T4

a)] (3)

where the factors , both of them less than 1, from the first term are

τ = transmittance of transparent cover

α = absorptance at the surface of receiver

The transmittance τ equals the fraction of solar radiation that passes through the transparent

cover, above the receiver. Note that glass, for example, allows most visible light to pass,

while reflecting most of the infrared part of the spectrum. Plastics, in contrast, usually have

high transmittance in both visible and infrared spectrum. Regarding the absorptance α, it

is the fraction of absorbed radiation, in this case, by the receiver surface (usually of black

color, for optimal absorption).

The two factors above are related to the incoming solar radiation in the first term in (1);

where

Ia = solar irradiance entering the collector aperture

Aa = area of the collector

Actually, the factors of the first term, transmittance and absorptance, represent losses, as

radiation travels from the collector aperture (where glass or any transparent material is

placed as a cover) to the receiver. The processes of these losses depend specifically on the

design of a particular collector and happen due to the physical limitations of materials and

imperfections in geometry.

Belonging to the second term of (1) we find the quantities from loss , where Ar = surface

area of receiver, then the first part of the second term, combines convection and

conduction, so that:

h´ = combined convection and conduction coefficient

Tr = average temperature of receiver

Ta = average temperature of ambient air

It is important to note that the main source of heat loss for a solar collector is the convective

one [19] ,therefore, modern solar collectors use vacuum glass tubes with pipes inside.

Since conduction loss is commonly small compared to convection loss, it is usually

combined with the convection loss factors for general approaches.

The second part in the second term represents the loss of energy by radiation, where

ε = thermal emittance of the receiver surface

σ = the Stefan-Boltzmann constant

In general, radiation and conduction losses are comparable for collectors when the

operating temperatures are nearly ambient. Thermal emmittance and absorptance are

parameters that the manufacturer can easily control when appropriate coatings are chosen

[18].

THE PET BOTTLE SWH

The PET bottle solar collector was invented in 2002 by a Brazilian retired mechanical

engineer José Alano. For the invention, Alano was granted the “2004 Ecology Prize for

NGOs” in Brazil [20].

By 2008, six thousand units of Alano’s ecological SWH had already been built. The largest

version -that uses 1,800 PET bottles- was inaugurated that same year in the Brazilian town

of Palmas [21].

Alano’s SWH uses PET bottles to substitute glass (as in conventional SWHs) as the means

to create the greenhouse effect inside the collector – the phenomenon that causes the water

inside the pipes to heat up. However, as mentioned earlier, plastics do not reflect most

infrared radiation. Measurements in the range of 360 to 1040 nm wavelength (which

corresponds to significant portion of the solar radiation spectrum at sea level) have shown

that colorless PET have a high 88% - 90% light transmission [22]. Such property may

reduce the greenhouse effect inside the bottles, and ultimately, the efficiency of the system.

Regardless, Alano’s SWH heats up the water up to 38 °C in the winter (ambient

temperatures of 13°C to 16°C and in summer up to 50 °C (ambient temperatures of 22 °C

to 25 °C [20]. A remarkable efficiency despite the PET bottles optical limitations. On

another hand, the durability of PET bottles allows them to be used in the open air for up to

10 years without important changes in strength and lighting properties [22].

The Prototype

Ever since the PET bottle SWH from José Alano was popularized, many versions of his

invention were created. The prototype discussed in this paper was designed based on those

versions. For instance: instead of using black painted tetrapack cartons as heating surfaces

inside the bottles, the bottom of the bottles was painted black with plastic cement and -as a

receiver- it (see Figure 3) no extra insulation was placed behind , CPVC was used instead

of PVC (see Figure 3), the CPVC pipes were painted black with black plastic cement, 48

bottles were used, instead of 100 [20] and the storage tank was made of an outside tank that

contained a bucket inside (between the two containers crumpled newspaper balls were

placed as insulator, see Figure 4).

Figure 3. PET bottle SWH collector in construction phase. The pipes are CPVC; the bottom

of the PET bottles painted with black plastic cement plays the role of the receiver.

Figure 4. The storage tank is made of two containers within a layer of crumpled newspaper

balls as insulator. A five US gallon bucket served as the inner container.

Below is the assembled working prototype of the PET bottle SWH (see Figure 5), facing

south, at an angle of 14 degrees plus 10 degrees [7], which is the approximate latitude of

the city of Guatemala, where the prototype was set up. Guatemala has a tropical climate.

The month of October marks the end of the rainy season, which is followed by a dry season

along with cold weather. The temperature measures were taken in October, during partly

cloudy days. Water was heated from ambient temperatures of 23 °C and 26 °C up to 41

°C and 48 °C, respectively. The collector area is 1.89 m2, which is nearly the same as the

area of Alano’s SWH, 1.80 m2 ; the performance of the SWH system discussed in this

paper is comparable to the one made by Alano. However, as mentioned earlier, this SWH

design is different from Alano’s, it uses 48 PET bottles instead of 100, it does not use

tetrapack cartons either or other recyclable materials such as styrofoam.

Figure 5. Prototype of a PET bottle SWH. The area of the collector is 1.22 m x 1.55 m,

totaling an area of 1.89 m2. Ambient temperature water (for measurements) is kept at the

bucket in the front.

Materials and costs

The table below (Table 1) shows the cost of the materials used in the construction of the

prototype of the PET bottle SWH system. All materials were acquired in Guatemala, and

are available in popular hardware stores.

Table 1. Costs of the components of the PET bottle SWH system. The exchange rate of Guatemalan

Quetzales GTD to USD is calculated at 1 USD = 8 GTQ.

PET BOTTLE SOLAR COLLECTOR SYSTEM

Components Quantitiy Cost/unit USD Cost/unit GTQ Total Cost

USD

Solar Collector

Pain Thiner (US GAL) 1 5.6 45 5.6

Tangit Glue (CPVC) 2 3.5 28.12 7.0

"Union T" CPVC 0.5 inch 29 0.7 5.8 21.0

Elbow CPVC 0.5 inch 4 0.7 5.2 2.6

Iron base to support the collector 1 40.6 325 40.6

Black Plastic Cement 1/4 US GAL 1 8.2 65.25 8.2

PET Bottles 60 0.0 0 0.0

CPVC Pipe 0.5 inch / 1 m 22 2.0 15.83 43.5

Total 128.6

The tank and its components

Plastic Tank -outside tank 1 5.6 45 5.6

Plasitc Bucket - inside tank(US 5 GAL) 1 0.0 0 0.0

Newspaper 1 0.0 0 0.0

Male Adapter 1 inch CPVC 3 1.0 7.99 3.0

Check valve 0.5 inch 1 9.4 74.99 9.4

Faucet 2 1.9 14.99 3.7

Rubber rings 3 0.5 3.99 1.5

Female CPVC 0.5 adapter 3 0.6 4.99 1.9

Iron base to support tank 1 40.6 325 40.6

Total 65.7

Installation

Hose 0.5 inch 10 meter long 1 6.9 54.99 6.9

Female adapter for faucet 1 4.1 32.99 4.1

Female adaper for 0.5 inch hose 1 1.9 14.99 1.9

Hose 5/8 inch (1 meter long) 3 3.1 25 9.4

Clamp 5 0.8 6.18 3.9

Teflon tape 3/4 inch 2 0.5 3.99 1.0

Total 27.1

Total PET Bottle Solar Collector System 221.4

As seen in Table 1, the total cost of the SWH system is USD 221.4. The most expensive

components are the base used to support the collector, USD 40.6; and the base that supports

the tank USD 40.6. If say, these two components are replaced by much cheaper materials

(say bricks, blocks or a board) , the cost of the system could be reduced to USD 140. Now,

and additional reduction, up to USD 127, could be achieved by replacing the CPVC with

PVC, since the SWH heats the water up to 45 °C, and the melting point of PVC is 60 °C

[23] (MIT Sea Grant College Program, 2008). Note that recyclable materials like the PET

bottles, the newspapers and the five US gallon bucket were priced at USD 0. Also, this

model of PET bottle SWH uses a lesser variety of recyclable materials as the one found in

literature [20] however, it uses less 52% less PET bottles.

FISCAL INCENTIVES AND SPONSORSHIP

The fiscal incentives in Guatemala for the promotion of renewables consist only of tax

credits for investment, there is no capital subsidy, grant or rebate for the implementation of

green technologies [24]. Therefore, the eventual adoption of the PET Bottle SWH in rural

areas for low income households will most likely require the support of international

programs with the alliance of local NGOs [25].

Another possibility is the creation of a disruptive business model such as the one from

Quetsol [26] which applies crowdfunding to bring solar electricity to low income

houesholds. The sponsors, individuals or companies, may benefit through PR, that is, brand

marketing.

CONCLUSIONS AND RECOMMENDATIONS

The PET SWH heats up the water from ambient temperatures of 23 °C and 26 °C up to

41 °C and 48 °C, respectively. The efficiency is comparable to the one found in literature.

It is clear, however, that this design of PET SWH will need the elaboration of a thorough

physical model to determine its maximum potential, that is, its maximum efficiency by

using low cost and easy to find materials.

Although the current cost of the prototype of USD 221 can easily be reduced to USD 127,

by replacing the bases of the collector and the tank with cheaper materials, its cost makes it

available only for the middle class ( 9% of Guatemala’s population) as a DIY project, that

requires basic plumbing knowledge and skills. In such case, further studies are needed to

determine how much electricity is saved when a standard (yet to be determined) PET SWH

is used to find out the payback period.

Due to its cost and the lack of government subsidy for green technologies in households,

the PET solar water heater is not feasible to the low income family in rural areas. Its

affordability for that market, depends, then, on the support international programs, such as

UNDP, or on the creation of a disruptive business model such as Quetsol.

The main advantage of the PET SWH over other low cost systems is the way it addresses

environmental protection and health issues: it uses recycled materials, while reducing wood

(thus, the risk of ashes inside poor households) and electricity consumption. Its main

attractiveness lies then, on the potential to combine important aspects of environmental

protection and health issues.

NOMENCLATURE

Aa area of the collector [m2]

Ar area of the receiver [m2]

cp pressure specific heat [J/(kg/K)

optical rate of optical radiation incident on

receiver [W] h´ combined convection and conduction

coefficient [W/(m2K)]

Ia solar irradiance entering the collector

aperture [W/m2]

mass flow rate of fluid [kg/s]

loss rate of thermal energy loss from the

receiver [W]

useful rate of useful thermal energy from the

receiver [W]

Tin temperature of fluid entering the absorber

[K]

Ta average temperature of ambient air [K]

Tout temperature of fluid leaving the absorber

[K] Tr average temperature of receiver [K]

Greek letters

α absorptance at the surface of receiver

ε

thermal emittance of the receiver surface

σ the Stefan-Boltzmann constant

[W/m2/K

4]

τ transmittance of transparent cover

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