Solar Direct Drive (SDD) software development

How S2E Software Systems delivered an embedded software stack and automated test suite for Secop's Solar Direct Drive module during a 6-month project.

About Secop

Secop is a global leader in hermetic compressors and electronic controls for refrigeration systems. With a heritage spanning more than 65 years originating from Danfoss Compressors, Secop designs and manufactures high-efficiency cooling solutions across commercial, mobile, and medical cold chain applications.

Project Goal

Operating medical refrigeration equipment in remote or off-grid areas presents severe technical constraints. In developing regions with limited electrical grid infrastructure, vaccine and medical cold chain storage must rely on solar photovoltaic (PV) power. Traditional solar-powered systems depend on chemical battery banks, which represent the primary failure point due to high ambient temperatures, limited cycle life, and high maintenance overhead.

To solve this, Secop developed the Solar Direct Drive (SDD) Power Management Module. The SDD unit connects solar panels directly to variable-speed DC compressors without intermediate batteries, managing variable solar irradiance and unstable AC grid supplies. Given the mission-critical nature of vaccine storage, the product was designed to meet strict World Health Organization (WHO) Performance, Quality, and Safety (PQS) prequalification standards.

Secop faced tight time-to-market constraints and needed senior embedded engineering capacity to take the SDD controller software from early prototype to production without pulling resources away from their core compressor platforms. They engaged S2E Software Systems for a 6-month project to deliver a turn-key software solution, build an automated validation framework, and cleanly hand over the codebase to Secop’s internal engineering team.

What We Achieved

Over the 6-month engagement, we transitioned an experimental prototype into a production-grade embedded software stack written in C. We architected the system on top of the Zephyr RTOS, utilizing its modular driver model and threading primitives to build a deterministic, maintainable control architecture.

The SDD module functions as an intelligent power manager and controller for the refrigeration compressor drive. Key technical deliverables included:

  • Zephyr RTOS integration: Structured application tasks across preemptive threads, separating power sensing loops, compressor state machines, and communication interfaces.
  • Custom Modbus protocol extensions: Implemented serial Modbus communication between the SDD module and the compressor drive unit. Because standard Zephyr Modbus services lacked support for multi-packet binary transfers at the time, we extended the stack to handle firmware updates, parameter configuration, and diagnostic telemetry compatible with Secop’s Tool4Cool® interface.
  • Maximum Power Point Tracking (MPPT): Implemented the MPPT control loop that dynamically tracks the optimal operating voltage of the solar array under changing irradiance and temperature, adjusting compressor speed to use maximum available solar energy.
  • PV power assessment & false-start prevention: Implemented low-latency panel voltage and current measurement routines to evaluate available solar wattage before engaging the compressor motor. By preventing rapid cycling and false starts during low-light conditions (such as dawn, dusk, or cloud cover), the control software minimizes thermal and mechanical stress on the compressor.
  • Automated hardware-in-the-loop test suite: Developed an automated end-to-end test framework using Robot Framework. The test harness simulated PV voltage ramps, grid brownouts, and edge-case operational faults against a configurable hardware bench, enabling continuous regression testing prior to field deployment.

Business Impact

Our software architecture and validation framework enabled Secop to bring the SDD product to market on schedule within a fixed budget. Following laboratory validation and field testing, the SDD system received official WHO PQS prequalification, certifying it for worldwide deployment in remote vaccine cold chains.

The clean codebase, documentation, and automated test suite allowed Secop’s engineering team to smoothly take over long-term software maintenance following a 1-day training and independently adapt the platform for future controller variants.