Grantee Research Project Results
2018 Progress Report: STAR: Solar Thermal Absorptive RefrigerationEPA Grant Number: SV838800
Title: STAR: Solar Thermal Absorptive Refrigeration
Investigators: Choi, Jun-Ki , Ciric, Amy
Current Investigators: Choi, Jun-Ki , Ciric, Amy , Romo, Joshua , Karki, Bipin , Slenska, Tara , Worsham, Matthew , Willard, Katie , Quinn, Natalie
Institution: University of Dayton
EPA Project Officer: Callan, Richard
Project Period: February 1, 2017 through January 31, 2019 (Extended to January 31, 2020)
Project Period Covered by this Report: February 1, 2018 through January 31,2019
Project Amount: $74,975
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet - Phase 2 (2016) Recipients Lists
Research Category: P3 Awards , Sustainable and Healthy Communities , P3 Challenge Area - Air Quality
Timeliness of routine vaccinations requires an appropriate supply of vaccines, but many vaccines are temperature sensitive. Some, such as the polio vaccine, are heat sensitive and must be kept cool, while others, including the DTaP, Hepatitis A and Hepatitis B vaccines, are freeze sensitive and must be stored above 0⁰C. Consequently, timely delivery of routine immunizations requires a robust cold chain that can safely store vaccines at temperatures between 2⁰C and 8⁰C. This is difficult in remote, low income areas where the electric grid is unreliable. Although bottled gas and solar powered refrigerators can be used in these settings, they face challenges: bottled gas refrigerators are dependent upon fuel deliveries, and conventional solar powered refrigerators are powered by photovoltaic cells, which are expensive and often co-opted for other uses.
We are developing an adsorptive refrigerator that does not require electricity, uses ethanol and activated carbon as the refrigerant pair to maintain temperatures between 2⁰C and 8⁰C inside the refrigerator compartment. This alternative technology, Solar Thermal Adsorptive Refrigeration (STAR), uses solar energy and the tendency of ethanol to adsorb onto the surface of activated carbon to drive a refrigeration process. A key philosophy in the development of this project has been a focus on "appropriate technology" design principles, using locally available materials and expertise to implement sustainable, culturally acceptable solutions to engineering problems.
The primary goal of the first period of this project was to build prototypes that would demonstrate the potential for adsorptive refrigeration, using ethanol and activated carbon as the adsorption pair. This prototype was also used to complete a preliminary lifecycle analysis to determine the environmental impact of this innovative technology.
We have developed two scaled-up prototypes (horizontal and vertical systems). For both systems, the governing principles are same. The refrigerator consists of a vacuum chamber, a freezer compartment, and an adsorption bed of activated carbon that operates on a two-phase cycle. In the cooling phase, liquid ethanol in the vacuum chamber evaporates, drawing heat from the freezer compartment and refrigerating its contents. The ethanol vapors flow to the adsorption bed and adsorb onto the surface of the activated carbon. In the regeneration phase, the adsorption bed is heated in a solar oven, causing the ethanol to desorb. The ethanol vapors flow, cooling and condensing as they move through the line, closing the cycle. Unlike conventional solar powered refrigerators, this refrigerator does not require a photovoltaic cell to operate. One of the significant research outcomes in this period of research is that we have achieved the refrigeration temperature 2⁰C which is a more than cold enough temperature for storing vaccine. Other major research progress milestones and achievements to date are: 1) Identifying the effects of temperature, vacuum pressure and desorption time during the desorption process; 2) Determining lower- and upper-limit temperature for desorption of ethanol from activated carbon during desorption; 3) Detecting possible decomposition of ethanol during desorption process at high desorption temperature; and 4) Analyzing life cycle environmental impacts of the STAR system and comparing the impacts with conventional refrigerators.
Results: Our experiments showed that the activated carbon inside the adsorbent bed (glass jar) of the horizontal STAR system was not heated enough for proper desorption. Therefore, a test rig was designed for temperature profiling which helped to understand the rate of heat transfer from outer layer to inner layer of the adsorbent bed. Instead of the glass jar used in the horizontal system, a copper tube was designed for the vertical system to improve the heat transfer rate of the adsorbent bed. The copper tube has a cap at each end, with one having a ¼-inch threaded connection. The copper tube was assembled with a soldering joint. Later, a temperature probe was integrated into the copper tube to record the temperature of the activated carbon inside the tube. These experiments were carried out for a short time duration of 10 to 15 minutes. The two heat lamps were turned on and temperature of the outer layer, the inner layer, and activated carbon was recorded using an Omega HH 147U data logger thermometer into the computer to analyze heat transfer rate.
During desorption, the adsorbed ethanol on the activated carbon is regenerated by the application of thermal heat. Desorption of ethanol from adsorbed activated carbon makes the process cyclic. Desorption temperature and time duration of desorption are crucial to desorb ethanol to establish the adsorption-desorption cycle. A bench-scale desorption test rig was designed to perform experiments to determine the lower- and upper-bound desorption temperatures of ethanol from activated carbon, as well as time duration for desorption. The glass chamber and copper adsorbent bed were filled with ethanol and a varying amount of activated carbon, respectively. The mass of ethanol was constant for all experiments. Dehydrated and undenatured Spectrum 200 proof (99.5% pure) ethanol was used for experimental analysis. Chem glass 29/42 was used as evaporator and nylon tubing of diameter 10 mm was used as the piping connection. A Pittsburgh 2.5 cubic feet per minute (cfm) vacuum pump was used to pull the vacuum on the system through the valve between ethanol chamber and pressure gauge. After pulling a vacuum, the adsorption cycle was carried out until all the ethanol was adsorbed to activated carbon. Heat lamps were turned on during desorption process; however, the uncontrolled temperature on the bench scale desorption and initial experiments on the vertical system made it difficult to understand effects of particular temperature on desorption process.
To solve the issue of the uncontrolled temperature on the bench scale desorption process, an Arduino-based temperature control system was built to control the temperature of the activated carbon during desorption. The system regulates the temperature of the adsorption bed by adjusting the power to the heat lamp and was assembled using an Arduino UNO board, thermocouple Type – K, thermocouple amplifier, An A.C.light dimmer module controller board, breadboard, and wires with sockets. The code was written in C/C++ format using open-source Arduino software. The real-time temperature is displayed on the serial monitor on the Arduino screen.
The results from experiments to date were incorporated into a vertical prototype. The glass jar from the horizontal system was replaced with a copper tube, and power to the heat lamps was controlled by the Arduino system. The vertical system setup consists of following parts: glass tube, yeti cup, fabric, nylon tube, two vacuum pressure gauges, heat lamp, vacuum pump, vacuum trap, three ball type valves, elbows, straight and T-connectors, temperature probe and plywood board to support the system. A Chem glass tube was used as the evaporator, which contains measured liquid ethanol under vacuum. Dow Corning high vacuum grease was used to make the connection vacuum-proof. The cap has two extruded tube-like openings; both openings were vacuum greased, and nylon tubes with 12 mm diameter were inserted into the opening and tightened with 12 mm ABA Sweden hose clamps. One of the openings was closed by a 12mm nylon cap and another opening was connected to the tube using a 12mm to 10mm connector.
For the remainder of the grant period, work will address the decomposition of ethanol during the desorption stage of the refrigeration cycle. Our first set of experiments will assess whether high- temperature treatment in an inert environment can remove the groups of compounds that are catalyzing the ethanol decomposition. We are currently working on preparing the submission of two journal papers on 1) Life Cycle Analysis of STAR and 2) Technical aspects of the STAR system.