This document describes assembly and initial testing of a Peltier based heating and cooling system designed to maintain the temperature of a 14 L prototype bioreactor. Also, described below are the methodologies used to approximate the bioreactor’s thermal energy needs and select a Peltier module.
One heat sink ($35) with a contact plate surface area of at least 40x40mm2
One aluminum water cooling block ($10) with a 40x40mm2 surface area
One 40x40mm Peltier ($60) Thermal Electric Cooler/Heater
One 24V 15A 360W Regulated Power Supply ($30)
One 24V Water Pump ($25)
Plywood or acrylic to secure parts
Sheet metal screws, in this case #8 x 5/8” screws were used
One square foot rubber foam to insulate water block
Two thick nylon washers 5/16 x 9/16
Two machine screws M3-.5 x 12mm
Based on DIY aquarium chillers (e.g. – used to keep shrimp and axolotl happy), a Peltier thermal electric cooler (TEC) system was designed; to provide temperature stability inside BioMONSTAAAR’s prototype bioreactor and allow the reactor temperature to be adjusted. The ability to adjust temperature makes the bioreactor (1) adaptive to the optimal growing temperatures of different microorganisms and (2) allows the exploration of the effects of temperature fluctuations on cultures inside the reactor.
The listed parts, when assembled create a heating and cooling (H&C) system capable of removing or adding heat to BioMONSTAAAR’s 14L prototype bioreactor. The system was modeled and designed to regulate and maintain the internal temperature of the bioreactor from 15 to 35°C (59-95°F ), when the reactor is housed at room temperature (20°C or 68°F). Although internal temperatures above 35°C will likely be achievable.
A Peltier based system has the ability to both heat and cool using the same components, because the direction of heat transfer is analogous to the polarity of the current powering the Peltier. Furthermore, a Peltier based system is much smaller than a compressed refrigerant (e.g. – R-410A) system. Peltier elements have no moving parts; are reliable, and do not require maintenance (more on Peltier TEC). BioMONSTAAAR’s H&C system components are controlled with a custom electronic control board.
Choosing a Peltier Module
Experimentation is necessary to obtain a Peltier TEC that will provide the heating and cooling ability desired. A description of a Pelier based aquarium cooler provided some guidance for our experiments. Two types of experiments were run to determine the rate of heat loss or gain from the bioreactor.
In both experiments, the bioreactor was almost completely filled with distilled water (to the likely maximum operating height), and a digital thermometer was used to measure changes in the water temperature over time. The two types of experiments are as follow:
1) To determine the heating power the H&C system must use to maintain the temperature inside the reactor, record changes in the temperature of the water inside the reactor, over time, when the water temperature is a few degrees above ambient temperature.
2) To determine the cooling power the H&C system must achieve to maintain the temperature, record changes in the temperature of the water inside the reactor, over time, when the water temperature is equal to the ambient temperature and the growth lamps are on.
These experiments provide an idea of how much heat is transferred, to or from the water, in scenarios that will occur when the reactor is operational and the temperature must be held constant.
Figure 1 and 2 – Heat loss from the bioreactor when growth light are off and heat gained when the growth lights are on.
The rate of heat loss, when the water inside the tank is above the ambient temperature and the lights are turned off, is about 3.6W. Prior to these experiments, the rate of heat loss was modeled with equations found in heat transfer text books  and online. Plugging in the conditions of our experiment into the math model predicted a heat loss rate of 4W, which is close to the experimental value of 3.6W! These experiments helped validate the model, which can be useful in early stages of bioreactor design.
BioMONSTAAAR’s prototype bioreactor is a photobioreactor that uses lamps to provide growth light for cultures inside the reactor (e.g. – algae). After setting up four 15” linear lamps, placing the lamps against the glass bioreactor, equidistantly from each other. Starting with water inside the reactor, at room temp, the lamps were turned on, and the increase in temperature inside the reactor was recorded. After data for two experiments were collected, the water inside the reactor was determined to have gained approximately 30W of thermal energy due to the lamps. So, the H&C system must provide approximately 30W of cooling when the lights are on to maintain a constant temperature inside the reactor.
The H&C system is designed not only to maintain temperature but also decrease or increase the temperature to a desired set point in a reasonable amount of time. With a Peltier based system, one should designed for cooling first, because the Peltier has less capability to cool than heat. The design should allow a bioreactor operator to cool the internal contents of the reactor at a reasonable rate. For BioMONSTAAAR’s prototype bioreactor a decrease of 0.1°C/min or 6°C/hr was targeted; based on the maximum amount of water that will be in the reactor 100W off cooling power will be necessary. Thus, the maximum amount of cooling power needed (lamps on) is approximately 130W. Similarly, approximately 104W of heating will be required (lamps off) to increase the temperature at the same rate. There are unavoidable heat losses that reduce the H&C system ability, overdesigning the system is wise. Therefore, a Peltier that can approximately 200W of heat transfer should be adequate for BioMONSTAAAR’s prototype bioreactor.
High power Peltiers are best found through online suppliers. There are several Peltier suppliers but very few of the suppliers had the dimension and power required. After some searching, a 40x40mm Peltier TEC with a max heat transfer of 225W from Custom Thermoelectric was chosen. Peltier TEC modules lose efficiency when run at voltage/current greater than the maximum specified, because the hot side becomes too hot and interferes with the cold side, reducing the Peltier’s ability to transfer heat. It is not detrimental to run the Peltier at the max voltage/current specified, but rather the Peltier won’t be efficient (Thanks to Andy from Custom Thermoelectric for his guidance). Having a good heat sink, will let us get close to max voltage. The Peltier module is best run at 80% or below the maximum voltage/current. Heat transfer does not increase much above 80% power, as seen in the Peltier specifications. The Peltier at 80% power, in ideal conditions, should provide 180W. Ultimately, conditions are not ideal, compared to manufactures specification, and the H&C system must be tested. However, the above methodology provided sound guidance in selecting a Peltier that should provide BioMONSTAAAR’s prototype bioreactor temperature control.
H&C System Assembly
Modify Use of Components Included With the Heat Sink
Modify use of the components that come with the heat sink: back plate, anchoring mount plate, screws, washers, and posts, as shown in the image to secure the heat sink, Peltier, and water block.
Secure Heat Sink Back Plate and Pump Holder
Drill guide-holes into the plywood for the sheet metal screws; in this case #8 x 5/8” screws were used. Then secure the heat sink back plate and pump holder to the plywood with the screws.
Water Block Insulation
To minimize heat loss from the sides of the water block use foam insulation. A square foot of rubber foam from your local hardware store will be sufficient. Cut out rubber foam to match the dimension of the water block. Here, a water-soluble non-toxic adhesive was used to affix the rubber foam to the water block. Apply a thin coat of adhesive on the rubber foam cut-outs and the sides of the water block that will be insulated. Insulate between the water block nozzles as well (not shown). Allow to dry for 2 hours. Leave one of the water block sides free of rubber foam, so that the 40x40mm Peltier can contact the water block.
Spread Thermal Compound and Mount Components
Apply thermal compound/grease, spreading a thin layer on the surfaces that will be in contact. Make sure all surfaces are clean before apply thermal compound and follow thermal compound or heat sink manufacturer’s instructions (example methodologies for applying thermal grease). The surface spread method was used for our assembly as observed in the images.
Match the insulation-free side of the water block with and the cold side of the Peltier when positive current power the Peltier. With thermal grease on both surfaces, place the Peltier on the water block. Next, fit the 3/8 inch ID vinyl tubing to the water block and clamp the tubing with the metal screw clams. Spread thermal compound on the remaining side of the Peltier and the base plate of the heat sink. Place the heat sink on top of the Peltier, carefully resting the base plate on the Peltier. Line up all edges so that the 40x40mm2 surface areas on the Peltier are covered with the base plate of the heat sink and the water block.
Use the mounting plate included with the heat sink, to clamp all components tightly together. The two machine screws – M3-.5 x 12mm – will be needed to secure the mounting plate to the anchoring mount because the machine screws provided with the heat sink will not be long enough. Place a thick nylon washer (5/16 x 9/16) between the mounting plate and anchoring mount to limit the pressure that is applied on the Peltier. When mounting heat sink onto Peltier, tighten screws evenly do not tighten tight one side much more than the other. A circular bubble level can help level components when assembling the heat sink, Peltier, and water block.
Connect all 3/8 vinyl tubing so that water flows from the outlet of the pump through the water block and into BioMONSTAAAR’s acrylic water jacket. Connect the 1/2 inch ID vinyl tubing so that water flows from the outlet of the bioreactors water jacket to the water pumps inlet. The tubing connected to the inlet of the pump must be filled with water for the pump to function properly. Large air bubbles will cause the pump to fail!
Testing the H&C System
The H&C system was used to change the temperature of 37dL (~1 gallon) of water. The fittings were water tight and the pump functioned as expected. Heating and cooling was both tested at approximate 60% (8A/13A) of the Peltier TEC’s operating power; approximately 130W and 120W of heating and cooling were achieved, respectively (spreadsheet with data and analysis). The calculated values will be slight greater and smaller for heating and cooling power because heat was loss occurred at the water air interface, that was not accounted for in calculations. These values are within the expected heating and cooling abilities of the H&C system. It will be interesting to see H&C capability when the Peltier is at 100% operating power and connect to BioMONSTAAARs prototype bioreactor.
Improvements and Next Steps
The plate underneath the water becomes cold when the system is in cooling mode – heat loss from the water block to the back plate is occurring. An additional layer of insulation should be added.
The pump warms up when running, decreasing the H&C system’s cooling capability. The pump should be slowed down to minimize heat generation. The pump is currently powered with the same voltage as the Peltier. Reducing the pump flow rate or using a different pump with a slower flow rate should be tested to determine if the cooling ability improves.
Tube length will be reduced once the water jacket for BioMONSTAAAR’s prototype bioreactor is complete.
The H&C system needs to be tested on BioMONSTAAAR’s prototype bioreactor once it is all assembled
1. Incropera FPD, David B. , editor. Fundamentals of heat and mass transfer. 5th ed. New York: Publisher: J. Wiley; 2002.