Multi-Disciplinary Senior Design ConferenceKate Gleason College of Engineering

Rochester Institute of TechnologyRochester, New York 14623

Project Number: 08404

SECOND GENERATION SOLAR PASTEURIZER

(DRAFT B)Benjamin Johns Brian T. Moses

SebyKottackal Gregory Tauer Adam Yeager

Copyright © 2008 Rochester Institute of Technology

Proceedings of the Multi-Disciplinary Senior Design Conference Page 2

ABSTRACT

Solar powered pasteurization has the potential to dramatically reduce the prevalence of water born diseases in developing countries. This paper details the development of a flow-through flat plate solar pasteurizer with up-stream temperature regulation. This project focused on the development of a pasteurizer that is inexpensive enough for ownership by third world residents, as well as reliable and effective enough to be of practical value. The resulting prototype was found to produce as many as 100 liters of safely treated water during spring testing in Rochester, NY.

NOMENCLATURE

EPA: U.S. Environmental Protection AgencyWHO: World Health OrganizationHX: Heat Exchanger

Q: HeatM: MassCp: Specific heatT: TemperatureEhx: Heat exchanger efficiencyH: Convection coefficientA: Areaq’’: Heat per meter squaredU: Thermal resistance

INTRODUCTION

The goal of this project was to continue research into solar water pasteurization, and develop a prototype unit capable of pasteurizing 100 liters of water daily, while remaining well suited for purchase and operation in the environment unsafe drinking water is often found.

According to the World Health Organization, 1.1 billion people lack access to an “improved” drinking water supply, of which 1.6 million, 90% of them children, die yearly from diseases directly associated with a lack of safe drinking water [7]. Additionally, both chronic and acute infections resulting from unsafe drinking water are thought to cost the world more than 320 million productive working days every year, due in part to an estimated 133 million people suffering from high intensity intestinal helminthes, or parasitic worm, infections [7]. It is estimated that, depending on the region of the world, economic benefits may range from $3 to $34 for each dollar invested towards improving safe drinking water access. (WHO) [7].

The WHO defines service levels with respect to domestic water quantity on a per capita scale. “No Access” is defined as less than 5 liters of water per capita per day, or 25 liters for a family of 5. At the “No Access” service level, hygiene will be seriously impaired and even basic consumption may be compromised. “Basic Access”, defined as 20 liters per capita per day, or 100 liters for a family of five, provides a substantial improvement over “No Access”. Although laundry will likely need to be processed off plot, basic hygiene tasks should be possible and basic consumption will likely be met [7].

Water treatment at the household level, particularly solar powered treatment, is heavily promoted by the WHO as a low cost and effective method of reducing microbiological contamination in drinking water [4]. Since it doesn’t require heating water to boiling, pasteurization is one of the most promising approaches for “energy-efficient, cost-effective, robust, and reliable” method for purifying water using solar-thermal power [5]. An especially attractive characteristic of pasteurization is its indifference to the density of solid particles in the water, orlevel of turbidity. It is expected that, as impoverished parts of the world continue to develop, the use of highly turbid water for personal consumption will increase [3]. Its large indifference to turbidity gives pasteurization a huge advantage over other, often more volumetrically cost efficient, methods such as ultraviolet disinfection and filtration; both of which are negatively influenced by turbidity[3].

BACKGROUND

Pasteurization is a function of temperature and time. It is performed by raising water to an elevated temperature, then holding that water at the elevated temperature for the amount of time required achieving microbiological decontamination. Various pathogens die at different temperatures, so it is common to set a pasteurization goal designed to kill a hardy target [3]. One such target has been proposed for the destruction of Enteroviruses, one of the traditionally difficult to kill pathogens [6]. This target is represented by the temperature-time curve in Figure 1.

Figure 1: Pasteurization Curve for Enterovirus [6]

Project P08404

For the scope of this project, pasteurization will be defined as having occurred if the treated water crosses this curve for Enterovirus at any point.

This project builds off of the “First Generation Solar Pasteurizer” RIT senior design project, number P07401, completed in spring 2007. This first generation project focused heavily on the concept of an integral heat exchanger, following the work of Dr. Robert J. Stevens in “An Investigation of a Solar Pasteurizer with an Integral Heat Exchanger”. While the work performed by the 2007 team showed great promise for the application of solar pasteurization, they encountered a number of serious issues in their design [1]. Of most concern was their designs inability to guarantee the safe pasteurization of output water under all conditions. Other issues cited by the first generation team were the inefficient cycling behavior of their valve and air becoming trapped near the thermostat valve [1]. The 2007 team cited a number of potential causes and potential solutions to these issues, all of which were investigated by this second generation project.

PROCESS

Needs Overview:

This project’s requirements were separated between the needs of the end user, residents of a developing country without access to safe drinking water, and the EPA, this project’s sponsor. The end user would be mostly concerned with water output, ease of use, safety, and cost. The EPA requires a device that is sustainable, and recyclable. Based on research from the World Health Organization a family of five would require 100L/day [7]. The customer need of safety comes in two parts.

1. The water must achieve the safety zone for pasteurization, and

2. The highest exterior temperature must be low enough as to not pose a burn hazard.

Since some of the expected users live on less than a dollar a day, the resulting device must be inexpensive.

Engineering Specification Overview:

Engineering specifications were created after fully understanding the user needs. These specifications include

Achieve pasteurization safety zone, Resulting coliform density, and Manufactured cost.

Treated-water coliform density is measured using the multiple-tube fermentation technique. The United States standard is set at 5 coliforms per 100mL, with an ideal value of 0 coliforms per 100ml, these values correspond to the marginal and ideal value, respectively [9]. A cost specification was set at 100 dollars per unit, a value thought reasonable based on prior work [1][3].

Concept Selection and Design

Many concepts where generated and later rejected for reasons ranging from cost to manufacturability. Many combinations of individual subsystem concepts were examined. The following are the basic concepts that were considered for use as part of the solar water pasteurizer system.

The two principle system types that were considered are batch and flow-through. A batch pasteurization process is where water is kept inside a container and heated up to an acceptable temperature, then held for the corresponding time needed to pasteurize. The water is then sent to the output container while still hot. This process was rejected because of its inefficient nature. In a flow-through systems water flows through the device while being heated, is held at temperature for an appropriate time through length of the tubing, before exiting the system. A flow-through style system’s greatest strength may be its ability to have a heat exchanger, making the overall system more efficient by reducing the quantity of waste heat lost through output water.

Collector

Evacuated tubes and parabolic collectors were among the collector concepts considered. Evacuated tubes are composed of nested glass tubing, with a vacuum drawn between them. The inner tube would be coated with a selective surface, and the water would be routed through the inner tube. Parabolic collectors are a very simple design composed of a large parabolic mirror that focuses incident solar energy onto a tube held at the mirror’s focal point, thus heating the tube and its contents. These collection methods are highly efficient, reliable and are capable of heating a substance to well above boiling point of water. Unfortunately, they are the highest cost option and can break easily. Fabrication would be especially difficult for evacuated tubes because of the need to seal and pull a vacuum within the tubes. These collection methods would also require tracking the sun to work well. Despite their high efficiencies, the evacuated tubes and parabolic mirror collector concepts were removed from consideration because of their high unit cost and high fragileness.

Proceedings of the Multi-Disciplinary Senior Design Conference Page 4

Although the flat plate collector concept was less thermally efficient than others considered, it was chosen for the second-generation design because of its low cost and high reliability. Within the scope of flat plate collectors are a variety of flow tube configurations. Although many were considered, the following are tube configurations, serpentine and coil, were identified as best fits for our needs.

A serpentine path on flat plate collector concept combination retains the idea of flat plate collection with use of glazing as used in the first generation team’s design. The principle difference between collection techniques is that a serpentine path directs water flow through a path of tubing across the plate to prevent leaks and contribute to easier manufacturing. The flow tubes are attached to the collection plate by thermally conductive glue. The plate surface is coated with a selective surface paint designed for solar heating applications.

A coiled flow tube design was considered as an extremely low cost flow path alternative. This design would integrate the flow pattern and the collection into one flat spiral of black plastic tubing. The coil would serve as a flat plate collector, contained underneath a glazing, and the water would be routed through the coils, heating up as it reached the center. The main problem identified with this concept is that air would collect at the top of the arcs of tubing while the water heated up, reducing efficiency and restricting flow.

The final flat plate collector concept selected for this project incorporated the low cost of a flat plate collector with a serpentine water path. The tubing used for the serpentine path was angled at 12° to allow any released air to escape upwards. The tubing was attached to the plate by wire tie downs every three inches with a silicone free heat sink compound to conduct the heat from the plate into the tube.

Heat Exchanger

The previous design incorporated a flat plate collector with integrated heat exchanger, which was highly efficient, but did have a few critical limitations.

One of the integrated HX’s biggest limitations is its overall cost, as it requires three large metal plates held together with carefully placed screws and spacers, as well as welded flanges. Another limitation of an integrated HX is its comparatively large thermal mass, which results in a longer startup time. The first generation integrated heat exchanger design suffered from leaks as well as bulging of from water pressure, both of which were major drawbacks.

Two alternative HX designs considered by the second-generation team were a tube-in-tube and a coil-in-shell. A straight tube in tube heat exchanger consists of a small diameter metal tube containing hot water running inside of a larger diameter rubber tube with cold water. Hot and cold water are flowed through these tubes in opposite directions. Although inexpensive, this HX had a comparatively low thermal efficiency for the low laminar flow rates involved. The coil-in-shell concept is composed of two tubes, one within the other, with a coiled “hot” tube in between the walls of the larger tubes. The inside of the inner tube as well as the outside of the outer tube is insulated while cool water will flow over the hot coiled tubes to become preheated. This design was eventually rejected because of its potentially high cost and difficulty to manufacture.

Valve

It was decided early in the concept selection phase of this project that a method of upstream temperature sensing was needed to allow an automotive thermostat time to actuate and completely close a valve so as to prevent any contaminated water from finding its way out of the system. This upstream sensing was achieved with the following valve housing design placed approximately two thirds up from the bottom of the solar collector, or heating surface.

Figure 2: Final Design of Valve

Water from the heating surface will flow from points 1 to 2 (figure 2). This flow over the thermostat will help the valve react more quickly to temperature changes. Sensing temperature in this way will introduce a lag in water temperature to output, preventing any unpasteurized water from flowing out of the pasteurizer. Water will flow in a loop from 2 through a gas purge system and back into 3 (figure 2). If

Project P08404

pasteurization temperature has been reached, the valve will be open and water can flow out through 5 into a hot water reservoir. If temperature has not been reached, water will flows back through the system from 4 to 1, powered by a heat convection loop.

This complexity is required to compensate for the slow reaction time of the automotive thermostat. The valve operates when a temperature-calibrated wax goes through a phase change from solid to liquid, during which it expands, pushing a plunger, which is used to actuate a valve.

Hot Water Reservoir

Pasteurization is a function of temperature and time. Water must be held for approximately five minutes at the design temperature of 71C in a reservoir to guarantee pasteurization. The required reservoir size was calculated from the anticipated flow rate and the thermostat’s rated temperature. The reservoir is constructed out of 2” pipe size CPVC tubing, sealed with CPVC caps at both ends. A threaded fitting will be used for water inlet and outlet.

Construction Details

The pasteurizer can be broken down into three main components, a flat plate collector, a heat exchanger, and a valve system.

The collector plate is supported by a plywood shell and insulated with fiberglass to lower thermal losses. To increase absorption of solar irradiance, the aluminum was coated with two thin coats of a high absorption and low emissivity coating as per the instructions provided by the manufacturer. A 30”x50” 1/8” aluminum plate is supported by plywood rails. 5/16” aluminum tubing is routed up the aluminum plate. This tube it is attached to the plate by wire and a thermal paste that encourages heat transfer. Water flows through this tubing up the collector, rising in temperature as it travels. A 1/8” sheet of glass is supported 1.5” above the plate with plywood rails to create a greenhouse effect and to minimize thermal losses off the plate.

The HX is a counter flow, tube in a tube heat exchanger. The same 5/16” aluminum tubing used on the collector runs inside a ½” I.D. Santoprene outer tube. The aluminum tubing carries hot water out of the hot water reservoir to the output bucket. Unheated input water surrounds the aluminum pipe and flows in the opposite direction to the collector, contained by the Santoprene tube.

A valve was constructed from aluminum. This valve assembly is divided into two parts, a collector side and

an output side. The heat sensitive end of an automotive thermostat is positioned on the collector side of the valve assembly. The collector side is sealed from the output side through the use of an O-ring. When water on the collector side of the valve reaches 71 C, the design temperature for pasteurization, the thermostat valve’s piston actuates a shuttle, allowing water to pass into the hot water reservoir from the output side of the valve.

Cost Model

1. Formulation

The Cost Model was developed as a single control volume containing a collector plate, an ideal counter-flow heat exchanger, and an ideal valve. Warm-Up is not included. The “valve” controlled flow rate such that

Equation 1

This constraint mandates that the pasteurizer heat the water to pasteurization temperature. It also prevents water internal to the pasteurizer from exceeding the temperature of pasteurization.

The heat exchanger is modeled as an ideal, counter-flow-type heat exchanger. The final temperature of water at the exit of the heat exchanger is determined as such;

T HX=T Amb+(T Past−T Amb)ε HX mEquation 2

Where

ε HX=(U AHX

mCp)

(1−U AHX

mCp)

Equation 3

The collector is modeled in two parts: one being the solar radiation heating the collector plate and the other being the heat conducted into the water from contact with the plate. The underlying principle is that any energy radiated to the collector and not lost to the surroundings must be transferred to the water, while at the same time the plate must be hot enough and have enough contact with the water such that the necessary rate of heat transfer to water necessary to keep part 1 in equilibrium is the actual rate of heat transfer, based upon the temperature of the plate. Thus;

Proceedings of the Multi-Disciplinary Senior Design Conference Page 6

qwater=qnet' ' −UGlazing−U box=

T plate−T water

1U bond

+ 1U tubing

+ 1hwater

Equations 4a and 4b

Where Ubond =k w

t for the mass of conductive

material[10]

2. Recursive Solution to the Cost Model

Although equation 3 is dependent on m and equation 4 on plate temperature, the solution can be calculated through the use of recursion, where both a flow rate and plate temperature are assumed and the resulting mismatch between the assumed rate and calculated rate is used to determine the correct rate. A similar recursion is used to find the correct plate temperature. For the purpose of the cost model, 3 iterations following the initial assumed solution are used.

3. Cost Analysis

In the Cost Model, many of the parameters are linked to the dimensions of the heat exchanger and collector. By developing a workbook in which these parameters could be calculated via the input of the general dimensions. By actively linking the dimensions to the BOM, a cost estimate was constructed simultaneously with the performance estimate granted by the recursive solution. By using the Solver feature in Microsoft Excel, many different design iterations can be tested in a short period of time, and those that minimize the total cost selected.

4. Final Dimensions

After performing the cost analysis with a forced performance level: daily average flow rate of 3 ml/s in January while using weather data from San Juan, Puerto Rico, the most cost efficient design had approximately 1 square meter of collector area and a heat exchanger efficiency of 60%. The justification for this likely has its basis in the calculation of heat exchanger effectiveness: e increases as (X / 1+X), where X represents improved heat exchanger design parameters (surface area and convective coefficient).

Figure 3: Heat Exchanger EfficiencyDiminishing returns clearly applies to heat exchanger effectiveness, while collector area provides a linear improvement to energy input. Were this pasteurizer designed for a colder or less sunny climate, the importance of the heat exchanger would increase, one could expect optimal heat exchanger effectiveness of 80% or greater. RESULTS AND DISCUSSION

Subsystem Performance Validation:

Figure 4: Valve schematic, showing seals

Valve assembly was tested while isolated from system. Valve temperature reaction was tested, as well as leak testing to assure the seal between pasteurized and unpasteurized water was functional. Valve exhibited proper opening behavior when hot water was introduced to the temperature sensing passage. Flow rate through open valve was acceptable; did not impose significant flow restriction. The valve closed within the time allotted for upstream temperature regulation when cold water was introduced. The spring force on the valve shuttle was sufficient to prevent leaking from the convective loop passage into the output passage. Similarly, the o-ring seal on the automotive thermostat was sufficient to

Project P08404

Inside Collector

OutsideCollector

Water from

lower collector

Water to upper

collector

Water from upper convective loop

Water to hot reservoir

Valve shuttle and o-ring seals effective

prevent leaking of collector water into convective loop water.

Figure 5: Heat Exchanger Behavior for 4/22/08

Heat Exchanger efficiency was calculated during test days while installed in the system. The ½”ID tubing surrounding 5/16” OD Aluminum consistently yielded heat exchanger efficiencies above 70%. In the graph above, first output occurred at 10:52 AM. Cycling behavior of the valve can be seen until 11:30. Average heat exchanger efficiency on this day was 79%.

Results in Rochester

The testing performed in Rochester, NY confirmed that the prototype pasteurizer treated the water to the desired, safe, extent.

Typical Performance

Testing was performed on 6 different days. On all of these days, data was collected to verify that the working flow rate and temperature pairs in the pasteurizer were appropriate to meet the safety zone of pasteurization as seen in Figure 1.

Samples of water was taken on the hottest and coldest days and tested for coliform organism density.

Figure 6: Coliform organism density test.

From Figure 6, on the best day, coliform reduction was well into the zone considered safe by the EPA; 5 per 100ml [9]. On the worst day, the estimated resulting coliform density was 8 per 100ml, however the uncertainty around this estimate does contain the 5 per 100ml specification. Unfortunately, due to time and weather constraints, it was not possible to repeat this test on a similarly cold day.

Translation Model

1. Formulation

The Translation Model is built upon the cost model, taking the single control volume and splitting it into many. These control volumes are arranged in a linear fashion, with the state of the valve able to return water back to the collector or to the reservoir, but not both. This model is also time dependent, computing performance at each step from the initial conditions of the previous.

While the primary calculations are similar to the Cost Model, the implementation is different, with the solution calculated once per time step. Because the translational model has initial conditions from the prior step (1 second earlier), only the change of the system from that state must be calculated. One term of approximation has shown accurate, as temperatures in the system tend to vary less than .1% per time step.

Note that (1) no longer holds in the Translational model. The valve is now assumed to control flow rate via an integration of the temperature of water downstream of the valve to simulate time lag. The limits of this calculation change based on flow rate.

Further detail as well as MATLAB code for the Translational Model can be found in [11]

2. Validation

In order to prove that the that the translation model is accurate, it is held to a certain level of accuracy when attempting to simulate our testing days. This is performed using hour-average data collected throughout the day. This low-resolution data better simulates published, archived weather data.

This was the first testing day during which the solar pasteurizer was operated under passive, specified operating conditions. When the simulation is run using 15 minute averages, the predicated output level throughout the day matches the actual output well.

Proceedings of the Multi-Disciplinary Senior Design Conference Page 8

April 24, 2007

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

9 10 11 12 13 14 15 16

Time of Day (24HR)

Output (Liters)Temp (C)

0.0100.0200.0300.0400.0500.0600.0700.0800.0900.01000.0

Irradience w/m^2

Temperature Measured Output Calculated Output Solar

Figure 6: Model Behavior Using 15-Minute Intervals

However, when the hours average data is used, uncertainties rise. Shown below is a plot of the same test day with 1 hour average data. The fine details of the weather are smoothed away, most specifically the sharp drops in solar irradiance that occurred at 9:30 and 4:30. Lacking these periods of cloudy weather, then simulates over predicts the performance of the pasteurizer.

April 24, 2007

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

8 9 10 11 12 13 14 15

Time of Day (24HR)

Output (Liters)Temp (C)

0.0100.0200.0300.0400.0500.0600.0700.0800.0900.01000.0

Irradience w/m^2

Temperature Measured Output Calculated Output Solar

Figure 7: Model Behavior Using 1-Hour Intervals

3. Translated Performance

The model, run over one year of data from san Juan, Puerto Rico, returns the following results;

The yearly average is 96 L / day and the worst month, December, had an average output of 80 Liters per day.

CONCLUSIONS AND RECOMMENDATIONS

The final prototype successfully improved on the first generation project’s success in demonstrating the practical use of solar pasteurization to treat water. The team was able to thoroughly test the prototype under a wide variety of weather conditions, strongly validating

that water output by the pasteurizer is always safely pasteurized.

Although largely successful, a number of issues arose in this project’s prototype design, and a number of opportunities for improvement have been identified.The unit size and maximum flow rate were selected to hit the target 100 liters of daily output in the worst-case expected season, winter. Due to this, any surplus output will be lost during the summer, since no mechanism exists to increase water flow rate during this more favorable season. This limitation could be overcome by decreasing resistance to water flow and increasing hot-water time in system, to assure adequate time for pasteurization given higher flow rates.

REFERENCES

[1] Aiken, Elaine, et al. “Development of a Solar Water Pasteurizer With Integral Heat Exchanger (SPIHX).” Proceedings of the Multi-Disciplinary Engineering Design Conference (2007): 1-10.

[2] Backer, Howard D. “Effect of Heat on the Sterilization of Artificially Contaminated Water.” Journal of Travel Medicine 3 (1996): 1-4.

[3] Burch, Jay, and Karen Thomas. An Overview of Water Disinfection in Developing Countries and the Potential for Solar Thermal Water Pasteurization. Golden, CO: National Renewable Energy Laboratory, 1998.

[4] Combating Waterborn Disease at the Household Level. Geneva, Switzerland: World Health Organization, 2007.

[5] Duff, William S, and David A Hodgeson. “A simple high efficiency solar water purification system.” Solar Energy 79 (Dec. 2007): 25-32.

[6] Feachem, R G, D J Garelick, and D D Mara. Sanitation and Disease: Health Aspects of Excreta and Wastewater Management. NY: John Wiley & Sons., 1983.

[7] “Health Through Safe Drinking Water.” Water Sanitation and Health. 2008. World Health Organization. 26 Apr. 2008 <http://www.who.int/ water_sanitation_health/ mdg1/ en/ index.html>.

[8] National Primary Drinking Water Standards. 2003. Office of Water. Environmental Protection Agency. 2 May 2008 <http://www.epa.gov/safewater/consumer/pdf/ mcl.pdf>.

[9] Stevens, Robert James. An Investigation of a Solar Pasteurizer with an Integral Heat Exchanger. Raleigh, NC: North Carolina State University, 1998.

[10] Hsieh, Jui. Solar Energy engineering. NJ: Prentice Hall, 1986.

[11] “P08404 Second Generation Solar Pasteurizer“ <http://edge.rit.edu/content/P08404/public/Home>

ACKNOWLEDGMENTS

Project P08404

Jan 1 to March 318,234 Liters in 90 DaysAverage of 91.5 Liters per day

July 1 to September 309,725 Liters in 92 DaysAverage of 105.7 Liters per day

Apr 1 To June 319,219 Liters in 91 DaysAverage of 101.3 Liters per day

October 1 to December 317,962 Liters in 92 DaysAverage of 86.5 Liters per day

The second generation solar pasteurizer team would like to thank our guide, Dr. Robert Stevens, for his expert help and encouragement. The team also thanks all members of the first generation solar pasteurizer team for the initial work that helped seed this project. Additionally, the team thanks Dr. Jeffery Lodge for

his help with understanding and validating pasteurization and Dr. Brian Thorn for his consultation on sustainability and knowledge of the project customer. Finally, the team thanks Steve Kosciol and Charles Thomas for their assistance with the fabrication of the pasteurizer prototype.