Developing a Solar-Powered Milk Refrigeration System Using LabVIEW and NI Single-Board RIO

Challenge: Develop a milk refrigeration system for rural areas in India where electricity supply is not guaranteed.

Solution: A refrigeration system that uses solar energy in combination with available grid electricity by converting and storing thermal energy and releasing it when freezing the milk. 

 
Figure 1. Solar photovoltaic array powers the refrigeration system in the blue roof building

 
Figure 2. Simple operation interface designed for dairy farmers

 
Figure 3. The LabVIEW system periodically publishes operating data to the Twitter page

 
Figure 4. Dynamic load balancing using LabVIEW Real-Time and FPGA

programming Author: Sorin Grama - Promethean Power Systems, USATo

address the lack of cold chain infrastructure for transporting fresh food in India, Promethean Power Systems recently developed a refrigeration system that successfully addresses a number of unique challenges facing the Indian dairy industry. The source of milk supply in India is scattered across numerous small farms in the vast rural areas. The current milk collection process is very inefficient, mainly relying on twice-daily hot milk collection, resulting in high transportation costs and frequent milk spoilage—up to 30% in the hot season. If dairy companies can quickly cool raw milk at rural collection centers, transportation costs can be reduced by half, milk spoilage problems can be avoided, and dairy farmers can earn more income. Dedicated milk refrigeration equipment is already available on the market, but rural areas have unstable power supply. If this solution is adopted, it will need to use diesel generators to power these equipment, which will undoubtedly increase investment and operating costs and is not a good solution.

Based on these investigations, we set out to design a milk refrigeration system that is more suitable for remote rural areas. Because we had expertise and technology in leading solar installations and solar was widely considered a viable and economical energy source for sunny regions such as India, we designed the system based on solar energy. However, because the milk cooling system was particularly important and had to operate 24/7/365, we combined solar energy with available grid power to create a more perfect system that could operate even during long periods of cloudy weather or grid outages (Figure 1). A key part of the system

design was the control system that managed the distribution of power from the two sources (solar and grid) to the loads. The load is typically a cold water tank that uses a refrigeration compressor to convert electrical energy into refrigeration work and store thermal energy. This cold water is then used to cool the milk when it is collected in the morning and evening. In addition, a small battery load system was used to ensure that the control system and refrigeration pumps operated when there was no solar or grid power.

We realized that we needed to design an embedded control system to run the complex algorithms while providing a simple operator interface for the dairy farmers. Therefore, we decided to use the NI  Single-Board RIO platform and LabVIEW Real-Time Module as our development system.

Behind the simple operator interface (Figure 2) is a complex system that combines LabVIEW software and NI hardware to control the operation of the milk refrigeration system and collect valuable data for subsequent engineering analysis and design improvements. The system inputs include temperature, current, and flow sensors , and the outputs are digital control signals, most of which are generated by programming the built-in FPGA hardware of the reconfigurable I/O (RIO) platform.

The control software includes multiple different and independent proportional-integral-derivative (PID) control loops running in parallel to control the system temperature to maintain the critical temperature point of the system. In addition, the embedded software collects and stores data for subsequent analysis. A feature of this system is that it uses mobile phone communication - usually the only communication method in remote areas - to publish summary data of operation in conjunction with a simple text communication protocol and a dedicated Twitter account (Figure 3).

An important aspect of the system is the dynamic load balancing algorithm. The algorithm regulates the system to operate in a solar-powered, grid-powered, or solar-powered and grid-powered mode (Figure 4). Dynamic load balancing is achieved by using the FPGA platform to program itself and use PWM signals to control the power flow to the refrigeration compressor.

The system becomes more complex due to the addition of batteries. The battery plays a dual role as a load and a source of power in the system. A control algorithm was designed to determine when the battery needs to be charged and when the battery is the source of power for the system. Careful design of the battery control ensures that the battery pack is always charged efficiently.

Conclusion

Using the LabVIEW Real-Time Module and the NI Single-Board RIO platform, we designed and built a field test prototype system that runs complex control algorithms and collects valuable engineering data while being easy to operate and debug. The system is now operating in a remote area in southern India, cooling milk every day. As we move toward mass production and commercialization, the data collected from this prototype will be used to improve the system and simplify the design.