1. High Thermal Insulation Envelope System

To withstand the extreme ambient temperature of 43°C, 200 mm Vacuum Insulation Panels (VIPs) are adopted as the core enclosure material for the cold storage. The VIPs feature thermal conductivity ≤ 0.007 W/(m·K), with the overall heat transfer coefficient controlled below 0.035 W/(m²·K).

Each VIP uses fumed silica as the core material wrapped with a high-barrier composite film, maintaining internal vacuum ≤ 10 Pa and a design service life of over 20 years. In terms of thermal insulation performance, a 200 mm VIP outperforms over 400 mm conventional polyurethane panels. The slim high-efficiency insulation drastically reduces wall thickness and improves space utilization of the 800 m³ storage room.

The construction team implemented rigorous treatments for critical structural joints:

Dual-seal panel joint design: inner dedicated butyl tape for VIPs plus on-site polyurethane foam filling on the outer layer to completely eliminate thermal bridges at seams.

Floor insulation consists of double staggered layers of 200 mm XPS boards with compressive strength ≥ 300 kPa, plus an underlying vapor barrier to prevent floor condensation.

Electric sliding insulated door with 200 mm thick leaf, built-in anti-condensation heating wires inside the frame, and triple airbag seals at the door bottom.

Thermal Performance Comparison of Envelope Materials

As shown in the table, the VIP system cuts heat flux down to 1.51 W/m², merely 22% of the value of the 150 mm polyurethane scheme, fundamentally mitigating heat infiltration under high-temperature surroundings.

2. Refrigeration System Configuration

Two Carrier Carlyle Model 05G semi-hermetic reciprocating compressors are deployed with an active-standby redundant setup. Each unit delivers a cooling capacity of 12.8 kW under -10°C evaporating / +40°C condensing conditions, and the combined output fully covers total cooling load with ample safety margin.

Compressor Advantages

• Suction-cooled motor design to accommodate frequent start-stop cycles and variable operating conditions

• 3-stage capacity modulation (33%, 66%, 100%) to match actual indoor heat load

• Built-in oil separator to guarantee reliable oil return

• Standard crankcase heater to avoid refrigerant migration during shutdown

Air-cooled condensers are mounted on the steel platform atop the warehouse, sized and verified for a 43°C design ambient temperature to cap condensing pressure during extreme heat. EC variable-speed fan motors adjust rotation speed automatically based on condensing pressure, balancing stable high-pressure operation and energy savings.

Four ceiling-mounted evaporators are installed, each equipped with dual expansion valves for redundant liquid supply precision. Every evaporator has an independent hot gas bypass defrost circuit, limiting indoor temperature fluctuation within ±1.5°C during defrost cycles.

3. 48-Hour Standby Power Supply System

The backup power system serves as a critical safeguard for irreplaceable germplasm resources. A diesel generator set is configured as the emergency power source with the following load calculation:

• Rated power of single compressor: 9.2 kW

• Condenser fan total power: 2 × 0.75 = 1.5 kW

• Evaporator fan total power: 4 × 0.37 = 1.48 kW

• Lighting & control system: 1.5 kW

• Maximum total load: 13.68 kW

A 30 kW prime power diesel generator is selected with sufficient capacity buffer to withstand motor inrush current. An Automatic Transfer Switch (ATS) starts the generator and completes power switchover within 15 seconds upon mains failure. The 500 L fuel tank supports roughly 55 hours of continuous operation at 75% load factor, exceeding the 48-hour emergency demand with extra allowance for fuel degradation and load fluctuations.

The generator is housed in an independent equipment room fitted with acoustic enclosure and exhaust silencer, restricting noise below 75 dB(A). The unit automatically conducts a 30-minute loaded test run every month to maintain ready status.

4. Precision Temperature & Humidity Control System

The seed cold storage adopts centralized monitoring architecture combining PLC and HMI, with core control logics as follows:

1. Cascade PID temperature control: average temperature measured from six indoor points acts as the primary control variable, while evaporator outlet superheat serves as the secondary variable to suppress liquid supply temperature disturbances.

2. Independent humidification/dehumidification circuits decoupled from temperature control, eliminating temperature swings caused by traditional temperature-for-humidity compromise control.

3. Compressor rotation logic: the two units switch operation automatically every 48 hours to equalize runtime and extend service life.

4. Tiered fault response: automatic isolation of faulty sensors; single-point faults trigger alarms without shutdown; dual-point faults trigger automatic switchover to standby compressor; level-3 faults push remote notifications via SMS and mobile APP.

The remote monitoring platform supports real-time viewing of indoor temperature, humidity, equipment status and alarm logs via web and mobile terminals. Historical data is stored for a minimum of 24 months to meet traceability requirements for germplasm resource management.

5. Key Construction & Commissioning Milestones

• During VIP panel installation, knocking or cutting is strictly prohibited. All panel seams undergo helium mass spectrometer leak detection with vacuum leakage rate ≤ 1×10⁻⁷ Pa·m³/s.

• Refrigerant piping is welded under nitrogen shielding. The system holds pressure for 24 hours without pressure drop and is evacuated to a vacuum below 50 Pa.

• A 72-hour continuous operation test is performed during commissioning, with temporary heaters used to simulate a 43°C external environment. Indoor temperature remains stable between 4.0°C and 5.2°C with fluctuation ≤ 1.2°C.

• Power failure switchover test: the ATS activates the generator in 13 seconds under simulated mains outage. Indoor temperature rises only 0.6°C during blackout and recovers to setpoint within 18 minutes after power restoration.

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