Today's testing focused on characterizing the refrigerator's performance in terms to temperature holding and power consumption. The second figure shows a time history of a sensor placed at the coldest point of the cold well, which varied considerably (-7 to 2 C, after startup). The third figure shows that the temperature of the blood products remained remarkably constant at 5.5 C. Temperature is controlled by a thermostat built into the refrigerator, which will be replaced by a controller that measures and responds to the temperatures of the blood products.
The second figure also reveals the duty cycle of the refrigerator, which is powered on less than 1/3 of the time (when the line descends) and otherwise does not draw power. This is significant because it means for every 1 minute of battery power carried onboard the cart, temperature of blood products can be maintained for 3 minutes.
Testing also included a makeshift battery power system. Tim's colleague in the Bayview Pathology department is excited about the project, and scrounged up an old Uninterruptible Power Supply (UPS, the black box at bottom left). Though the UPS's rated output of 450 Watts is substantially larger than the refrigerator's requirement of 360 Watts, this UPS unit immediately complains that the load is too high, probably because it's battery capacity has deteriorated with age. However, the idea of using an off-the-shelf battery system is definitely superior to a custom solution from both cost and risk time perspectives. There are a number of potentially viable solutions from the household solar industry that will be investigated.