A solid thermal-storage steam system charges heat at night using electricity, then releases that stored energy in the daytime to generate steam. It uses high-temperature solid media as the heat store and, with a PLC + HMI control package, delivers on-demand, pressure-stable steam while enabling peak-shaving, cost reduction, and low-carbon, safe operation. Suitable for both retrofits and new heat sources. Drawing on past projects, Henan Rentai outlines how the control system of a solid thermal-storage steam generator is designed and operates.
Control Layers and Architecture
The control system—the “brain” of a solid thermal-storage steam generator—coordinates everything from electric charging to steam delivery. It is typically organized into three layers:
1) Charge Control
Manages the process of injecting heat into the solid medium via electric heaters. Based on scheduled charging power/time (e.g., during off-peak tariffs), the controller starts heaters automatically. Using PID or staged switching, the system raises the store temperature uniformly to an upper target while enforcing multi-level temperature limits to avoid overheating.
2) Storage Monitoring
During the hold period, the controller continuously monitors the storage module, including internal temperature distribution and heat loss. Although there are few active actuators in this phase, the system performs energy balance and estimates the state of charge (SOC). Sensor data are processed and presented locally on the HMI and remotely for operators.
3) Discharge / Steam Control
Governs the conversion of stored heat into steam. Based on downstream demand, the controller modulates feedwater pumps and/or control valves to route water into the storage heat-exchange unit. A closed loop on outlet steam pressure (primary control) coordinates feedwater flow and valve position to keep pressure and flow stable. Steam temperature is also supervised; the system trims the release rate to meet temperature/dryness targets.
A modular architecture ties these layers together: field sensors/actuators at the bottom, a PLC or embedded controller as the core, and HMI plus remote SCADA/IoT services on top. Tight signal coupling between the layers enables fully automatic operation with clear manual intervention paths.
Core Measurements and Signal Acquisition
A solid-storage steam generator relies on a rich set of inputs—its “senses”—to make correct decisions:
Temperature points: Multiple thermocouples/RTDs embedded in the storage medium (center/periphery) provide the heating profile and spatial temperature map for SOC estimation. Additional temperature probes on the steam side supervise outlet temperature to prevent overheat and stabilize quality.
Pressure: Transmitters at the steam outlet (and, where applicable, within the pressure boundary) feed the discharge pressure PID. Over-pressure in any phase is captured for protection.
Flow & Level: Feedwater lines use flowmeters for real-time water mass flow. Designs with drums/tanks will also carry level instrumentation for dry-fire prevention. In once-through evaporators, feed pump discharge pressure/flow still require monitoring and control to maintain steam quality.
Electrical parameters: Smart power meters (often integrated into power controllers) read heater voltage/current, apparent/real power, PF, and harmonics. Cabinet environmental signals (temperature, fan status) help ensure reliable operation.
All signals are conditioned and brought into PLC I/O. The controller logs, trends, and forwards them to the HMI/remote platform to support live operations and historical analytics.
Actuators and Control Loops
The controller’s “decisions” are executed through these loops and devices:
Electric-heater power control: Heating rods are grouped into circuits. Control is via SSRs or SCR power controllers. Basic systems use staged on/off of groups; advanced systems use phase-angle or zero-cross triggering for smooth, stepless power control and tighter temperature uniformity.
Feedwater control: Variable-speed pumps and/or control valves regulate feedwater flow. Closed loops on steam outlet pressure (and sometimes flow) adjust pump speed or valve position. Preheaters, if present, are interlocked and controlled as part of this loop. In once-through designs, feedwater control directly governs production rate and dryness.
Steam outlet & relief control: Motorized control valves meter the outlet; safety valves and emergency vent/relief valves handle abnormal pressure. Under normal conditions, outlet valve motion is coordinated with feedwater to achieve fine pressure/flow control; under abnormal conditions, relief logic opens rapidly to depressurize.
Auxiliary controls: Fans for forced cooling/equalization, manifold switching valves for multi-module storage arrays, cabinet interlocks, and contactor/relay coils are all managed by the PLC. In multi-module designs, the controller selects which module(s) to engage (series/parallel) to meet different load points with optimal efficiency.
Example action: If outlet pressure drops below setpoint, the PLC increases feedwater and/or heater power (within limits) to raise evaporation rate; if pressure rises, it trims feedwater and/or heater power.
Safety Interlocks and Protection
Given the combination of high temperature, high pressure, and high electrical power, a multi-layer safety chain is mandatory:
Over-pressure interlock: Exceeding the upper pressure limit triggers immediate protective actions—cut heater power, close feedwater, and open relief paths until pressure returns safe. Mechanical safety valves provide last-resort protection even if the controller fails.
Over-temperature interlock: Protects both the storage medium and the steam side. If storage temperature exceeds material limits, heating stops and alarms. If steam outlet temperature is too high, the controller reduces heater power and/or increases feedwater for quench. Independent thermal cut-outs embedded in the storage block provide redundant hard trips.
Low-level / no-flow protection: To prevent dry-fire, missing feedwater flow or a level below the minimum safe point immediately trips heating and alarms. Dual pumps and redundant level devices are often used for reliability.
Electrical protection: Over-current/short-circuit trips in the power path; cabinet E-STOP removes main power. Door interlocks and maintenance modes prevent unsafe energization.
Fault diagnostics and safe shutdown: Sensor faults (e.g., open thermocouple), actuator failures (e.g., stuck valve), fan failures, etc., generate alarms. If safety is at risk, the PLC executes a controlled shutdown to a safe state.
These layers—from hardware safeguards to software interlocks—create a robust safety envelope so the unit remains protected even under abnormal conditions.
Putting It Together
Through a clear layered design, comprehensive sensing, precise actuation, and rigorous interlocks, the control system automates the entire charge–store–discharge lifecycle. It truly acts as the unit’s “brain,” coordinating components to convert energy stored in “hot bricks” into stable, high-quality steam—efficiently and safely.
In upcoming deep-dive notes, we’ll cover specific strategies for steam quality control, SOC estimation, and electric power modulation to further open up this “brain.”
Henan Rentai Electric Equipment Co., Ltd. specializes in industrial automation control solutions, offering PLC+HMI, DCS, and remote monitoring services end-to-end.
Contact: +86-371-56520104 / +86-176-3856-3962 · info@hnrentai.com