Thermochemical sorption: a solution for temperature-controlled transport

Thermochemical storage based on a solid-gas chemical sorption process is used in temperature-controlled transport isothermal containers. The principle relies on the coupling, via a gaseous phase, of a liquid/gas phase change of a natural fluid (ammonia) and a reversible reaction between this fluid and a reactive solid. This allows for a deferred and regulated production of cold, providing autonomy to the container (roll) during transport. This article presents the qualification of such a temperature-controlled refrigerated container that allows for the transport of products between 0 °C and +4 °C. Depending on the external temperature profile, the container’s autonomy can vary from 16 to 72 hours.

Introduction

Thermal energy is used in all sectors of economic life and represents the largest share of final energy consumption. Thermal storage helps to address the temporal mismatch between energy demand and resource availability, improves energy efficiency, and reduces greenhouse gas emissions.

It also provides autonomy for temperature-controlled transport and reduces the logistical costs required to maintain the cold chain. Thermal storage can be considered using phase change materials (PCM) or based on the heat of sorption of a thermochemical material (MTC).

This latter concept is very promising in its potential: high-capacity heat or cold storage for long durations with almost no loss.

Thermochemical Storage

Thermochemical storage exploits the heat of sorption implemented in reversible physico-chemical processes of exothermic absorption and endothermic desorption of a gas G on a sorbent medium S: (SG) + heat D S+G. Depending on the nature of the sorbent medium (liquid or solid), we can exploit:

• the heat of dilution of the gas in a liquid solution (binary NH3/H2O or saline LiBr/H2O),
• the heat of crystallization/dissolution of a salt in a saturated solution (hydrates),
• the heat of adsorption of a gas (H2O, NH3, MeOH) on the surface of a microporous adsorbent material, such as activated carbon, zeolite, or silica gel,
• the heat of reaction between a gas (H2O, NH3, CO2, H2) and a solid reactant (hydrates, hydroxides, ammoniates, carbonates, or metal hydrides).

The implementation of thermochemical storage thus involves managing two coupled reversible physico-chemical processes via the gaseous phase: the absorption/desorption process of the gas by the sorbent medium and an evaporation/condensation of this gas. These processes occur in two reservoirs (a condenser/evaporator and a reactor), each equipped with heat exchangers and connected to each other via a valve.

During the storage phase, the supply of high-temperature heat allows for the endothermic desorption of the gas from the sorbent. The released gas is then liquefied in a condenser and stored at ambient temperature (Fig. 1a).

When the sorbent is sufficiently depleted of gas, the valve is closed, separating the two components (Fig. 1b). Thermal energy is then stored in the form of chemical potential in the separated components, indefinitely over time and without energy loss.

Figure 1: Thermochemical Storage Phase

The unloading phase then consists of reacting the two separated components again: a heat supply to the evaporator (cold production) allows for the evaporation of gas, which is reabsorbed exothermically by the sorbent. Depending on the operating conditions (evaporation temperature, ambient temperature), the nature of the sorbent material (solid, liquid, system variance), and the working fluid (H2O, NH3, CO2, H2,…) used, three applications are possible during the unloading phase (Fig. 2):

• cold production at the evaporator (Fig. 2a),
• recovery of heat at Th previously stored in the reactor also at Th (Fig. 2b),
• heat production in the reactor at a temperature higher than that of the stored heat through a heat supply to the evaporator (Fig. 2c).

In this sense, these thermal storage processes are referred to as heat pumps.

Figure 2: Thermochemical Unloading Phase

Thermochemical storage presents very significant capacities compared to conventional thermal accumulators. These storage capacities largely depend on the nature of the physical or chemical bonds between the sorbent and the gas. Furthermore, the existence of a wide variety of sorbent/work fluid pairs allows for coverage of a very broad temperature range (from -50 °C to 1300 °C).

In cold storage mode (Fig. 3), thermochemical sorption processes allow for deferred refrigeration production from available heat between 60 °C and 150 °C with a coefficient of performance ranging from 0.3 to 0.6. The use of ammonia or alcohols allows for negative cold production down to -30 °C with storage capacities of 30 to 150 kWh.m^⁻³. The use of water as a working fluid allows for achieving much larger storage capacities of 50 to 300 kWh.m^⁻³ but at positive temperatures suitable for solar cooling applications.

Figure 3: Cold Storage Capacity for Different Types of Gas Sorption Processes

Application in Temperature-Controlled Transport

The solid-gas thermochemical sorption process can be effectively utilized to produce deferred cold, thus providing interesting autonomy to a temperature-controlled transport container (roll). The principle relies on the liquid/gas phase change of a natural fluid (ammonia) and a reversible reaction between this fluid and a reactive solid.

Isothermal Roll

The isothermal roll equipped with the sorption system is a container made of a rotomolded insulating polyethylene shell into which polyurethane foam is injected. The overall thermal loss coefficient is 0.31 W/m^⁻².K^⁻¹. The roll, with external dimensions of 1200 x 800 x 2035 mm, offers a useful volume of dimensions 985 x 600 x 1310 mm.

Solid-Gas Thermochemical Sorption Unit

This roll is equipped with a solid-gas thermochemical sorption unit, comprising a thermochemical reactor and an evaporator connected to each other via an electrovalve, both ventilated silently (Figure 4). This device allows for the production of deferred and regulated cold for a homogeneous internal temperature. Cold production is triggered when the user decides, providing autonomy to the container during transport. Ammonia, salts, and graphite are perfectly confined in a factory-sealed hermetic circuit, making the system inaccessible and secure, requiring no refilling. The roll is equipped with an onboard traceability system that records the internal temperature of the box and detects door openings. The system offers autonomy of 16 to 72 hours without being plugged into the mains (production cycle). It can be recharged on a 230 V / 50-60 Hz mains in 6 hours and 30 minutes (regeneration cycle). Figure 5 presents the operating principle of the thermochemical system.

The onboard energy corresponds to 7.8 kg of ammonia cycled between the evaporator and the thermochemical reactor, equating to an energy of 2.8 kWh. The average power over an 8-hour cycle is 350 W. The onboard energy varies depending on the number of reactors integrated into the box. The number of thermochemical reactors is proportional to the desired amount of energy. The range extends from 0.5 kWh to 8.4 kWh.

The theoretical COP of the thermochemical reaction is 0.5 because the regeneration enthalpy is 2 times greater than the evaporation enthalpy of ammonia in cold production. The actual COP varies between 0.25 and 0.5. Once the reactor is regenerated, the potential energy is stored “for free” until use, but this does not factor into the COP calculation.

Figure 4: Roll Equipped with the Solid-Gas Sorption System

Figure 5: Operating Principle of the Thermochemical System

Thermal Performance Qualification Tests

The roll equipped with the solid-gas sorption system is developed to preserve fresh products. We focus on the qualification of the roll for this application of transporting agri-food products between 0 and +4 °C under various external temperature profiles.

Testing Protocol

The transport roll is tested with 28 cartons measuring 370 x 260 x 160 mm, of which 6 are loaded with 12 kg blocks of tylose, resulting in a total mass of 72 kg. Nine probes (thermo-buttons) are placed on product units, and the test load is stabilized at +2 °C for 48 hours. To cool the useful space of the container to +2 °C, the thermochemical module is activated one hour before loading. The loading is done at an ambient temperature of +22 °C, positioning the instrumented products at the high and low corners and in the middle of the faces of the container. In all tests, the setpoint is adjusted to +2 °C, corresponding to the center of the required conservation range. The tests are conducted in thermostatic chambers.

Tests Under Constant Profiles of +20 °C and +40 °C

The roll is tested under constant external temperatures of +20 °C and +40 °C.

At +20 °C (Figure 6), the internal temperatures recorded by all probes remained between 0 °C and +4 °C for at least 48 hours, except for the probe placed in the high corner, which reached the +4 °C limit after 35 hours.

At +40 °C (Figure 7), the internal temperatures recorded by all probes remained between 0 °C and +4 °C for at least 24 hours, except for the probe placed in the high corner, which exceeded the +4 °C limit after 16 hours.

The duration corresponds to the net autonomy after cooling the box from +21 °C to 0 °C, as well as the ammonia at its evaporation temperature (around -15 °C). The cooling energy represents about 1/3 to 1/2 of the total energy. Autonomy depends on the temperatures of the box and the ammonia at the start of production and the desired setpoint temperature.

Figure 6: Roll Autonomy Between 0 °C and +4 °C at +20 °C

Figure 7: Roll Autonomy Between 0 °C and +4 °C at +40 °C

Testing Under Standard Profiles ST-48-b and ST-48-d

The same roll is tested under the standard profiles ST-48-b and ST-24-d of the NF S 99-700 standard.

Under the ST-48-b profile (Figure 8), the internal temperatures recorded by all probes remained between 0 °C and +4 °C for at least 48 hours, except for the probe placed in the high corner, which exceeded the +4 °C limit after 45 hours.

Under the ST-48-d profile (Figure 9), the internal temperatures recorded by all probes remained between 0 °C and +4 °C for at least 48 hours. The curves show that the container can offer superior autonomy.

Figure 8: Roll Autonomy Between 0 °C and +4 °C Under the ST-48-b Profile

Figure 9: Roll Autonomy Between 0 °C and +4 °C Under the ST-48-d Profile

Testing Under a Temperate Profile

The same roll is tested under a variable temperate profile consisting of alternating segments at +20 °C and +10 °C. The recorded internal and external temperatures are presented in Figure 10. The internal temperatures recorded by all probes remained between 0 °C and +4 °C for at least 72 hours. The container can offer superior autonomy under this moderate profile.

Figure 10: Roll Autonomy Between 0 °C and +4 °C Under a Temperate Profile

Conclusion

The thermochemical process based on solid-gas sorption allows for the storage of thermal energy and the production of cold in a deferred manner to be utilized as needed. The controlled temperature use of this system through appropriate regulation in isothermal containers thus secures the cold chain of perishable or thermosensitive products with autonomy ranging from 16 hours to 72 hours depending on external temperature conditions, the size of the reactors, and the insulation of such containers.

These temperature-controlled containers combine the advantages of solutions equipped with eutectic plates or PCM and those of dynamic solutions equipped with refrigeration units, offering autonomy and temperature regulation. They can be easily transportable or integrated into standard vehicles. The use of a natural fluid helps reduce greenhouse gas emissions and environmental impact.

Authors: Abbes KACIMI, Director of Cold Chain Expertise, Sofrigam / Francis KINDBEITER, R&D Director, Coldway Technologies / Driss STITOU, Head of the TES Research Team at CNRS-PROMES Laboratory