Project Number: 17484

Transcription

1 Multidisciplinary Senior Design Conference Kate Gleason College of Engineering Rochester Institute of Technology Rochester, New York Project Number: SOLAR THERMAL WATER HEATER Mike Pastore Industrial Engineering Alex Slabyk Industrial Engineering Sarah Kirk Mechanical Engineering Environmental Science Deven Greenwalt Mechanical Engineering Evan Baileys Mechanical Engineering ABSTRACT The purpose of this project is to model and understand the energy mitigation that results from using solar thermal technologies to heat water. This type of information can be useful in places with cold winters, where energy costs can skyrocket. By using the sun to heat water, the electric,or gas, heating required can be reduced, and thus saves money and the environment. A flat-plate solar thermal water heater was mounted at a 43 angle and tested during the spring months in Rochester, NY. Flow rates between 0.2 and 1.0 gpm were tested in order to collect data on the collector s efficiency and efficacy. Under certain test conditions the water exiting the collector exceeded temperatures of 170 F. Higher temperatures could likely be achieved in summer months with greater solar insolation. During testing under good conditions, the collector was able to generate 1.4 kw of heat energy. Resulting in a monetary offset of 17 cents per hour in electric water heating savings. Upon completion of testing, the single collector will be a used for heating a hot tub near Philadelphia, PA. The outlet temperature of the flat-plate, combined with the low-grade heat requirement of a hot tub, will be perfect compliments to one another. Copyright 2017 Rochester Institute of Technology

2 Proceedings of the Multidisciplinary Senior Design Conference Page 2 BACKGROUND Solar thermal water heating has several different forms: flat plate collectors, evacuated tubes, and solar concentrators. All three of these technologies harness the power of the sun by capturing incoming solar radiation, known as irradiance, and transferring it to a fluid. Flat plate collectors are box-like structures that capture solar irradiance between a dark absorber and surface glass plate. Inner tubes thermally connected to the absorbing layer allow for the transfer of the collected heat to the liquid within the tubes [1]. Evacuated tube collectors operate on a similar principle to flat plate, except they use long black glass tubes as the absorber that contain a copper tube. The copper tube then has an alcohol based solution within them that changes phase from liquid to vapor when heated. The solution is heated by the captured solar irradiance. Then the vaporized solution rises and passes through tubes in a heat exchanger. The liquid to be heated which is often a glycol solution passes over the other side of the heat exchanging tubes and is heated [2]. Another technology is solar concentrators which use long cylindrical mirrors with an absorbing tube running along the central axis. The tube contains the heat transfer fluid and is heated through the collected and concentrated sunlight [3]. From here, the team looked at costs of all the systems and how efficient each system would be. Solar concentrators are one of the most expensive systems, and are typically not as efficient as other systems in more marginal conditions. Two systems were compared, a TitanPower-ALDH29 flat plate collector and an Apricus ETC- 30 evacuated tube collector, which the team had chosen as two possible systems to purchase due to having similar costs and form factors. A model was developed to calculate the efficiency and thus useful energy gain that both collectors would be able to provide throughout a typical meteorological year in the Rochester area. The result was that the flat plate collector was predicted to be able to provide 8.2 MMBTU hrs per year, more than the evacuated tube option of 6.8 MMBTU hrs per year.. For this reason, the team decided to purchase the flat plate system, because the efficiency was high overall, and the price was low. In addition to the energy gain model, each collector has a performance slope associated with it that can be used as a predictor when choosing a collector for a particular climate. The performance slope characterizes the efficiency of each collector as it relates the inlet temperature (T in ), ambient air temperature (T amb ), and incident solar flux (I c ). The slopes of the chart represent the rate of heat lost from each collector. Given our application, climate, and assuming that the inlet temperature of the working fluid (T in ) is kept low by removing useful heat from our system to the application, our team expects to be operating in the circled region of figure 1. This means that the TitanPower flat plate collector is predicted to have better performance than the evacuated tube collector. Figure 1 - Performance chart of flat plate and evacuated tube solar thermal collectors. The initial purpose of this project was to implement one of these solar thermal water heating technologies on campus at Rochester Institute of Technology (RIT); specifically at the Gene Polisseni Center. Currently at the Polisseni Center they use a heat exchanger to heat the water from the main RIT hot water loop. The water is needed to be at 140 F for showers, concession stands, etc. This scope changed, however, and a new customer was found. Tom Kirk was in need of a solar collector for his hot tub. Currently Tom uses electricity in order to heat his hot tub, but with the use of solar thermal heating, he wished to supplement this heating as much as possible. For this redesign the bottom platform was used for mounting and storage and the flat plate would be attached to an inclined roof platform. The latitude where the solar collector was originally going to be used is 43, in Rochester, NY. For that reason the angle of inclination of the plate is 43. Choosing the angle of inclination to be equal to the latitude of the location where the collector is to be used is a general solar collector rule of thumb that takes into account the average location of the sun year round [4]. For maximum efficiency, the tilt angle would have

3 Proceedings of the Multidisciplinary Senior Design Conference Page 3 to be changed throughout the day and year so that the collector could always be perpendicular to the sun's location in the sky. CUSTOMER REQUIREMENTS Initially this project was created for a customer at RIT interested in supplementing the domestic hot water loops of academic buildings at RIT using arrays of solar thermal collectors. This goal changed early on due to a change of customer, which soon after changed a third time to its final customer. The final customer only required the use of one collector to provide supplemental heating for a hot tub. Throughout the evolution of the project, several requirements remained unchanged; including providing a working model to the customer, collecting performance and efficiency data, and providing water at or below 130 F to avoid the risk of burns. For the hot tub application, the collector must be mounted to an angled stand allowing it to be placed on the ground, as it is not certain where exactly the hot tub will be installed relative to a building. The recommended solar collector system must be able to produce 2.0 kwhr of equivalent heating on a yearly basis to make it economically viable for the customer. Energy consumption from the system (pump, control/alert systems etc.) must consume as little 12V DC power as possible in order to be feasible, so the system can run for extended periods from a portable battery power source or a small photovoltaic solar panel. The plumbing requirements include: ½ NPT connections throughout to allow for easy integration into the customer s existing systems, a low collector outlet port for draining the system, and a manual shut-off valve(s) to shut down the system in emergency, if necessary. Ideally the system would operate quietly, use a propylene glycol solution to prevent freezing on cold nights and in the winter (using a heat exchanger to transfer the collected energy to a water supply), and have an integrated control system to prevent it acting as a thermal radiator when it is not receiving enough solar energy to heat the water. Some of these requirements may not be able to be met due to time and budget constraints associated with the multiple customer changes in the first (design) semester. ENGINEERING REQUIREMENTS The engineering requirements were derived from customer requirements, budget, portability, and equipment constraints. Once the collector was selected, a stand was designed to hold the collector, but also be able to fit through the doors & elevator of the Senior Design Center and the machine shop such that the system could be transported outside for testing. The resulting constraints from this requirement included a stand footprint of 30 ft 2 or less, an overall height of 10 ft or less, and as lightweight as possible to allow for easy transport. Additionally, the stand must be robust enough to support the 126 pound weight of the collector while also being on wheels to allow for ease of movement. The collector specifications stated the ideal flow rate was between 0.2 and 0.5 gpm. This information along with the height of the system and the need for portability was used to select a DC pump. The selected pump brought in a constraint of a maximum operating temperature 130 F to protect its internal components. A 12V battery was selected to power the system to reduce the risk of electric shock during testing and demonstration at Imagine RIT. The battery is required to be able to run the system for the duration (6 hours) of the exhibit day. Another engineering requirement is the ability to compare the factory declared efficiency of the solar collector (82%) to the calculated system efficiency of the working model. This resulted in the requirement for several sensors and meters to be selected and installed on the system to capture the necessary parameters including flow rate, fluid temperatures, and incoming solar radiation. METHODS The stand for the collector was designed to be mobile, therefore wheels were attached to the bottom of the stand. The original plan for the stand was to create two platforms, a bottom platform for reservoir, battery, plumbing, etc. and a top platform for the flat plate collector. Our initial quote from our flat plate supplier quoted an independent collector stand. However, the parts received were for a roof mounting flat plate. As a result, a system redesign was done (Figure 2) in order to accommodate this mishap.

4 Proceedings of the Multidisciplinary Senior Design Conference Page 4 Figure 2: Updated, constructed, and installed stand and plumbing. The reservoir (blue chest) can be seen on the left hand side with the inlet, outlet, and bypass hoses. Mounted on the center support of the base are the two pumps mounted side by side for easy interchanging. To confirm meeting the engineering and customer requirements several measurements of the system needed to be taken. The height and the footprint of the system was designed and built to the specifications and checked through measurements with a tape measure and interference testing in taking the system outside from the senior design center. To measure the weight of the system, a scaled pallet jack was used. A flow meter was installed in the pump outflow line to monitor and adjust the flow rate as needed. Several temperature probes were installed in line to keep tabs on the input and output of the system. A temperature sensor with an associated alarm was installed as an overtemperature alert for the operator if the reservoir of system exceeded the design safety temperature of 130 F. The original diaphragm pump was rated to a flow rate of gpm. In order to slow the flow rate to the recommended gpm for the collector, a bypass valve assembly was necessary as the outflow of a diaphragm pump can only be limited slightly before it is unable to operate. Ultimately a smaller centrifugal pump was acquired that had a temperature rating of 212 F and could meet our flow requirements without the need for the bypass valve assembly by limiting the discharge which is possible on this type of pump. The schematic for the piping layout can be seen below in figure 3. This smaller pump would be used in order for the team to see the maximum temperature the collector could reach. The flow rate through the collector was then measured with an analog flow meter in the system between the pump and collector. For the plumbing, the inlet and the outlet for the water was pumped to and from the same reservoir, of about five gallons. The team wanted to measure the time it took for the inlet temperature to reach 130 F using a standard volume of water in our reservoir. The team also wanted to test the most efficient ways to heat water in our system. Specifically, is it more efficient to run our system at a lower flow rate (~ GPM), thus allowing the water in the collector to gather more energy per pass. Or, is it more efficient to run the system at a higher flow rate (~ GPM), collecting less energy per pass, but gathering more heated water in the same time frame.

5 Proceedings of the Multidisciplinary Senior Design Conference Page 5 Figure 3 - The piping schematic for the collector system including the flow meter and other sensors. In the physical system two pumps were installed next to each other such that they could be interchanged for testing through the use of unions in the plumbing. Because getting the temperature of the water too high can be damaging to both people and the diaphragm pump, a temperature alarm was set up. Once the centrifugal pump was acquired, it became the main system test pump, and therefore the maximum temperature could be elevated. However, the team chose to keep the maximum temperature at 130 F, for safety reasons. The alarm would measure the temperature of the reservoir, and was set up with two warning lights, and an audible alarm. When the system is below 120 F, a green indicator light will be illuminated. When the system temperature is at, or above, 120 F, the green light will turn off and a red warning indicator light will illuminate. Finally, when the temperature reaches 130 F, an audible alarm will sound. At this point a response protocol will be followed that includes switching the inlet to drawing water from an ice bath reservoir to cool the system. Additionally if needed the plate will be fully or partially covered in a thermal reflective blanket to stop or decrease the exposed area of the plate and therefore its heat collection. In order to power the system a 12V DC battery was initially acquired. Eventually the team acquired a small photovoltaic (PV) solar panel that is capable of powering the centrifugal pump based on the available solar radiation. As an added bonus, using the PV panel puts a passive control system onto the system-- when the sun is bright, the pump will run. When the sun is dimmer, the pump runs slower, and when it is overcast and the plate is not collecting much or any heat the pump will not run. For demonstration purposes on dim or overcast days and if the pump is not running fast enough for the collector s conditions the pump will be switched to power from the DC battery. A pyranometer mounted parallel to the collector's surface was used to measure incident solar flux. A pyranometer measures solar flux by sensing direct normal irradiance (direct rays from the sun), and diffuse horizontal irradiance (sun rays that have bounced off clouds and the ground). In order to calculate system efficiency (η), equation 1, seen below, was used. η = ΔTemp σ c p ρ Area Solar Flux 100% Equation (1) The change in temperature ( Temp [K]), flow rate (σ [m 3 /s]), specific heat of water (c p [J/gK]), density of water (ρ [g/m 3 ]), collection area [m 2 ] and incident solar flux [W/m 2 ] are used in equation (1) to calculate the actual efficiency of the collector. In order to capture the full extent of weather on system efficiency, the team tested during several different weather conditions: sunny, partly cloudy, and overcast. The reason the team did not test in the rain was to mitigate risk of damage to the equipment. In a system installed outdoors all equipment would be sealed and otherwise protected from rain and other weather conditions.

6 Proceedings of the Multidisciplinary Senior Design Conference Page 6 RESULTS AND DISCUSSION Testing was performed in a variety of cloud cover conditions and ambient temperatures. The cloud cover was found to have a significant impact on the efficiency and productivity of the solar collector; while ambient air temperature was found to have little effect. During cool overcast conditions it was found that the system was not effective for heating water due to insufficient solar irradiance. The effectiveness of the solar collector in overcast conditions can be found in figure 4. Figure 4 - Sample data recorded on a relatively overcast day, with solar flux less than 1000 [W/m 2 ] Additional tests were performed on partly cloudy and on sunny days. Depending on the cloud cover the water was successfully able to be heated to the 130 F target and safety temperature. Some testing was carefully performed above this target temperature in order to increase understanding of the system behavior and efficiency information of the collector. Full sun conditions were able to quickly heat the five gallon testing reservoir as displayed in figure 5. Figure 5 - Sample data taken on a relatively sunny day, with solar flux over 1000[W/m 2 ] (After 60 minutes).

7 Proceedings of the Multidisciplinary Senior Design Conference Page 7 For one set of testing the team wanted to see what the upper limit of our heating was. For this trial, the system was set up and allowed to run until the outlet temperature stagnated. However, during testing, solar flux dropped and the system was no longer able to continue heating the water. During this trial we were able to achieve a maximum water temperature of ~170 o F. Also, before the solar flux drop, the system achieved its maximum efficiency across all trials, approximately 70%. These results can be seen below in figure 6. Figure 6 - Test data showing maximum temperature achieved, and maximum efficiency. The weight of the assembled system was found to be 346 lbs including the battery and plumbing. This weight is well above our specified maximum weight of 200 lbs; however the higher weight was deemed acceptable due to the design changes made to our build after the original weight requirement was set. The original requirement was put in place to ensure the team could easily manipulate the system and carry it if quick movement was required. With the addition of four rotating and locking wheels, installed on the corners of frame, the lifting requirement no longer was a constraint and thus violating the 200 lbs specification was allowable. A rudimentary heat transfer analysis was performed on the hot tub that the collector will be used with. Analysis assumptions include: no heat transfer through the sides and bottom of hot tub, only heat transfer through 3 in. polyurethane foam cover, no convection/radiation, ambient temperature in the summer, fall, winter, spring; 70 F, 55 F, 30 F, 50 F respectively, constant collector inlet temperature of 100 F, efficiency determined from SRCC efficiency curve [1]. The analysis concluded that the hot tub would lose about 325 W in the summer, 488 W in the fall, 758 W in the winter, and 542 W in the spring. Based off of these same conditions and assumptions, during high solar flux conditions (262.5/ft 2 ) the collector would be able to produce 377 W in the summer, 278 W in the fall, 116 W in the winter, and 246 W in the spring. During medium solar flux conditions (196 W/ft 2 ) the collector would be able to produce 233 W in the summer, 136 W in the fall, 0 W in the winter, and 103 W in the spring. During low solar flux conditions (130 W/ft 2 ) the collector would be able to produce 90 W in the summer, 0 W in the fall, winter, and spring. 0 W collections can occur due to the efficiency of the collector being less than zero, which in practicality would result in the collector not running. The analysis concluded that the solar collector would be able to provide enough energy to maintain the temperature of the hot tub during periods of high solar flux through all seasons, and partly during periods of medium to low flux through the seasons of fall to spring. To conduct flow rate testing, approximately five gallons of water was heated from 70 o F to 130 o F, using various flow rates. In each trial, the time to heat the water was recorded. The results from this testing can be seen below in figure 7. Figure 7 - Results of flow rate testing.

8 Proceedings of the Multidisciplinary Senior Design Conference Page 8 While this data seems to imply that the flow rate recommended by the manufacturer, ~0.5GPM, is the most effective, this data does have some limitations. The most obvious being the sample size. Due to the small amount of data collected, more testing should be done in order to verify these results. Also, on the day this testing was conducted, solar flux data was unavailable. As a result we assumed that the solar flux was relatively constant throughout the during of this testing. However, without solar flux data, we are unable to definitively say if the difference in heating times is due to the flow rate changes, or differences in solar flux. CONCLUSIONS AND RECOMMENDATIONS As a result of our analysis, modeling, and data collection the team was able to draw several conclusions from our test system. Firstly, we were able to prove that a single flat plate solar collector is an effective way of heating water to high temperatures. As a result, the system could be used to reduce heating costs in many different applications, including providing supplemental hot water to a hot tub. There are several possible improvements the team would recommend for the system in the future. Firstly, the team would recommend fully automating the data collection process. Currently, temperature and flow rate data is collected by hand and entered into a spreadsheet. If this data could be collected automatically by a computer, more data points could be taken, and more accurate conclusions could be drawn. Second, the team would recommend making the angle of the solar collector adjustable in order to collect the most solar irradiance during any season, in any location. Currently the system is optimized for 43 latitude but will be installed in 40 latitude thus making the optimal mounting angle slightly less than its current design. Thirdly, the team would recommend creating an automatic solar tracking system in order to optimize its solar collection ability. Currently, the system must be moved by hand in order to track the movement of the sun across the sky. In the future, a system that could continuously track the sun, would allow for better solar collection throughout a given day. However, these tracking systems are significantly, and potentially prohibitively, expensive. REFERENCES [1] Certified Solar Collector [PDF]. (2013, August 27). Cocoa: Solar Rating & Certification Corporation. [2] Evacuated Tubes. (n.d.). Retrieved March 28, 2017, from [3] How Does a Solar Concentrator Solar Dish Work? (n.d.). Retrieved March 28, 2017, from [4] How does the tilt angle and/or orientation of the PV panel affect system performance? Photovoltaic Lighting Lighting Answers NLPIP. (2006, July). Retrieved April 20, 2017, from ACKNOWLEDGMENTS Special thanks to Multidisciplinary Senior Design Office, Richard Stein, Dr. Tom Kirk, Dr. Robert Stevens, Dr. Brian Thorn, and Charlie Tabb.