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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: [email protected]
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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