COLD STORAGE DEVICE

Description

[0001] This invention relates to a cold storage device, particularly for storage and / or transportation of vaccines and / or medical products, to methods of operating such devices and particularly to providing an indication of autonomy for such devices.

[0002] To ensure their quality, longevity and effectiveness, vaccines must be stored and transported at an optimum storage temperature, generally ≥+2°C and ≤ +8°C. Consequently, specialised vaccine refrigerators and vaccine transport devices are used to ensure that the cold chain to which the vaccines are exposed during storage and transport conforms to predefined specifications. Particular challenges occur with vaccination programs, for example in developing countries, when ambient temperature can be extreme, vaccination centres can be remote, and a reliable source of mains electricity may not be available. One way these issues is addressed, for example, at remote health centres in developing countries, is to use solar powered SDD (solar direct drive) ice-lined vaccine refrigerators. Such solar powered ice-lined vaccine refrigerators are configured such that their electrical cooling circuits generate an ice-lining which acts as a thermal capacitor; the ice-lining absorbs heat from its surroundings and contributes to maintaining the vaccine storage compartment within a desired temperature range when solar power is not available to power the electrical cooling circuit, for example at night or at periods during the day when insufficient solar radiation is available. Another specific situation in the cold chain arises where outreach vaccination programs are carried out in villages remote from a health centre. Vaccines are generally transported from a health centre having a specialised vaccine refrigerator to an (often more remote) outreach location using passive, ice-lined vaccine transport containers.

[0003] One aim of the present invention is to provide an improved ice-lined cold storage device, particularly an improved ice-lined vaccine storage device.

[0004] The following documents were cited by the European patent office acting as International Searching Authority: WO2021 / 063877 A1 (also published as KR10-2022-0070515 A) which relates to a system for evaluating the insulation properties of a thermally insulated transport unit; WO2017 / 151656 A1 (also published as CN108780013 B) which relates to a system and method of reverse modelling of product temperatures; WO2020 / 020812 A1 which relates to an ice-lined vaccine refrigerator; and WO2015 / 065899 A1 which relates to systems and methods for modelling product temperature from ambient temperature.

[0005] In accordance with one of its aspects, the present invention provides a cold storage device in accordance with claim 1. Other aspects are defined in other independent claims. The dependent claims define preferred or alternative features.

[0006] In one of its aspects, the present invention is based on the realisation that a significant improvement in the cold chain, particularly for vaccines and medical products, can be made by reducing uncertainties regarding the duration for which a cold storage device can maintain its payload within a pre-specified temperature range without use of a cooling circuit to remove energy from the cold storage device.

[0007] Cold storage devices will often declare a holdover time. The holdover time is defined by the World Health Organisation for a vaccine refrigerator as being the time in hours during which all points in the vaccine compartment of a vaccine refrigerator remain below +10°C, at the maximum ambient temperature of the temperature zone for which the appliance is rated after the power supply has been disconnected. The WHO rated temperatures are +43 °C for a hot temperature climate zones, +35°C for temperate climate zones and +27°C for moderate temperature climate zones. For vaccine freezers, the holdover time is defined by the World Health Organisation as the time in hours during which the vaccine compartment remains below -5°C. Although the holdover time provides some indication of performance, it does not, and it not intended to, provide any indication of the duration for which an individual cold storage device in real circumstances will maintain its payload below a pre-specified temperature without use of a cooling circuit to remove energy from the cold storage device. Such an indication would be valuable, for example, to provide an indication of the urgency of dealing with an issue that risks exposing vaccines being stored or transported to an undesired temperature which may render them unusable. Such issues may occur if a cooling circuit of the cooling device cannot be used, for example due to failure of the cooling circuit or failure of a power supply for the cooling circuit. One of the aims of the present invention is thus to provide an indication of the autonomy of an ice-lined cold storage device in a way which balances the often conflicting requirements of a sufficient level of accuracy, which takes into account the most significant real-life factors which affect the autonomy, preferably including opening of a lid of the storage compartment to allow access to the vaccines, and the actual ambient temperature, and which is nevertheless sufficiently simple to allow implementation using simple and reliable components in circumstances where access to an external power source cannot be guaranteed.

[0008] According to one aspect, the present invention provides an ice-lined cold storage device, notably a vaccine storage device, which is provided with an indication of simulated autonomy, in which the ice-line storage device comprises: a storage compartment adapted to store a payload, notably vaccines, at a controlled temperature; an ice-lining configured to absorb heat from the storage compartment; and a moveable lid of the storage compartment; and in which the simulated autonomy is provided using a temperature model which uses data which is specific to the ice-lined cold storage device and by: a) detecting an initial ambient temperature, an initial storage compartment temperature and an initial ice-lining temperature of the ice-lined cold storage device; b) running a first simulation using the temperature model and using the initial ambient temperature, the initial storage compartment temperature and the initial ice-lining temperature to determine a first simulated storage compartment temperature after a first simulation time step; c) running one or more subsequent simulations, each subsequent simulation being run using the temperature model and using data created in a previous simulation, to determine a subsequent simulated storage compartment temperature after a subsequent simulation time step; d) determining the simulated autonomy of the ice-lined refrigerator using at least one of the subsequent simulated storage compartment temperature(s).

[0009] The use of a temperature model allows use of a simplified simulation which reduces complexity of required components and still takes into account the most significant real-life factors which affect the autonomy, notably the factors having the most significant effect upon heat transfers affecting the storage compartment temperature. The use of data which is specific to the ice-lined storage device, for examples the dimension, mass, heat transfer properties and temperature characteristics of individual parts of the ice-lined cold storage device, assists in providing a suitable level of accuracy and, in addition, facilitates adapting the temperature model to difference ice-lines storage devices.

[0010] The simulated autonomy is preferably displayed at the ice-lined storage device, for example on a display, notably a screen, of the ice-lined storage device; this facilitates its use. Additionally or alternatively, the simulated autonomy may be transmitted to a remote monitoring system. The simulated autonomy may be displayed as a numerical indication, for example in hours and minutes. However, it preferred to display the simulated autonomy on a visual scale, for example as a length or a bar; this provides a simpler representation. The simulated autonomy may be associated with an alarm which provides a warning indication, notably in the case of the simulated autonomy being below a pre-set autonomy alarm threshold.

[0011] In preferred embodiments, the cold storage device is a vaccine storage device configured to maintain its cold storage chamber at a temperature between +2°C and +8°C. The simulated autonomy of the cold storage device may be determined in terms of a simulated duration prior to the storage compartment temperature reaching or exceeding a maximum acceptable storage compartment temperature; the maximum acceptable storage compartment temperature, particularly for a vaccine refrigerator, may be +10°C. Thus, in a further aspect, the invention provides a cold storage device as defined in any of the claims, in which the cold storage device is a vaccine storage device configured to maintain its cold storage chamber at a temperature between +2°C and +8°C, and preferably in which the simulated autonomy of the cold storage device is determined in terms of a simulated duration prior to the storage compartment temperature reaching or exceeding a maximum acceptable storage compartment temperature of +10°C.

[0012] As used herein, the term "ice-lined cold storage device" means a device configured to maintain its cold storage compartment within a controlled temperature range which is below the temperature of its surroundings based upon thermal insulation of the cold storage device and the provision of the ice lining which acts as a thermal capacitor to absorb heat from the cold storage compartment. Preferably, the ice lining is provided by one or more ice-packs or by an ice-jacket. The ice-lining is preferably filled with water; this allows the use of readily available materials, facilitates maintenance and replacement and allows the ice-lining to be provided by containers which can be filled locally rather than being transported ready-filled. Alternatively, the ice-lining may be water-based, for example comprising at least 70 wt%, at least 80 wt% or at least 90 wt% water.

[0013] The invention is particularly suitable for solar powered ice-lined cold storage devices. As used herein, the term "solar powered cold storage device" means a cold storage device having an electrically powered cooling circuit comprising a compressor which is powered directly by solar panels, rather than being powered by connection to a mains electricity supply. Such a solar powered, ice-lined cold storage device is preferably configured such that its electrically powered cooling circuit cools integrated ice-packs to form an ice-lining, so that, when the solar power is not available the pre-formed ice-lining absorbs heat from its surroundings and contributes to maintaining the cold storage compartment within its desired temperature range. Preferably, the solar powered cold storage device has a battery-free electrical cooling circuit, that is to say, an electrical cooling circuit which does not include a battery configured to provide power to a compressor or other heat transfer driver of the cooling circuit (as opposed to a battery which provides power to a control circuit for controlling, for example, a compressor). This avoids issues associated with transport, replacement and disposal of such batteries.

[0014] Particularly for such a solar powered, ice-lined cold storage device: a) the volume of the cold storage compartment may be ≥ 15L and / or ≤ 260L; this provides for storage or a suitable quantity of vaccines. It may be ≥ 40L, ≥ 50L or ≥ 55L and / or ≤ 1500, L ≤ 100L, ≤ 90L or ≤ 85L; and / or b) the hold over time of the cold storage device may be ≥ 12 hours when tested with a surrounding temperature of 32°C and / or ≥ 8 hours when tested with a surrounding temperature of 43°C.

[0015] The ice-lining, notably for solar powered ice-lined cold storage devices, preferably extends around at least 50%, at least 60%, at least 70% or at least 80% of periphery of the cold storage compartment; this helps to ensure consistency of the temperature within the cold storage compartment.

[0016] The cold storage device may be provided with an inner liner which preferably forms a continuous barrier between the cold storage compartment and the ice-lining. The inner liner may comprise a thermally insulating layer, for example of foam. A preferred form of inner liner comprises a thermally conducting layer, for example of aluminium, facing the cold storage compartment and a thermally insulating layer, for example of foam facing away from the cold storage compartment. The inner liner contributes to the control of heat flow from the cold storage compartment to the ice-lining to avoid over-chilling the cold storage compartment and provides a physical separation to avoid direct contact between vaccines and the ice-lining.

[0017] The invention is also potentially suitable for passive cold storage devices. As used herein, the term "passive cold storage device" means a cold storage device which does not comprise a refrigeration circuit, for example an electrically powered refrigeration circuit, and relies upon passive means, notably pre-frozen ice-packs and thermal insulation, to maintain its cold storage compartment within a controlled temperature. When in use to store vaccines or other products, for example during transport of vaccines from a health centre to an outreach program, such passive cold storage devices are not connected to an external electrical energy source such as a mains electricity supply or solar panels.

[0018] Particularly where the ice-lined cold storage device is a passive cold storage device, the device may be a transport device configured to store products, particularly vaccines, during transport. The passive cold storage device may have: a cold storage compartment having a volume which is ≥ 0.4 L and / or ≤ 25L, for example between 0.5L and 2L, or between 2L and 10 L; and / or a holdover time at +32°C of at least 16 hours ; and / or a holdover time at +43°C of at least 8 hours; and / or cold life at +43°C (without lid openings, up to +10°C) of at least 16 hours. Such devices are particularly suitable for storing vaccines during transportation.

[0019] The indication of simulated autonomy is preferably provided by components integrated in the ice-lined cold storage device, that is to say as a permanent part of the cold storage device; this ensures its presence and use each time the cold storage device is used and avoids the risk of a non-integrated standalone system being lost or omitted. Alternatively, or additionally, some parts of the simulated autonomy may be determined using remote data and / or remote simulation(s); particularly in this case, data may be transferred between the cold storage device and a remote system, for example a remote monitoring system. Such data transfer is preferably wireless.

[0020] The simulated autonomy is provided by: initiating a simulation, notably comprising detecting an initial ambient temperature, an initial storage compartment temperature and an initial ice-lining temperature of the ice-lined cold storage device; running a first simulation to simulate temperatures and / or energy levels of components of the ice-lined cold storage device at the end of the first simulation time step; and running one of more subsequent simulation to simulate temperatures and / or energy levels of components of the ice-lines cold storage device at the end of each subsequent simulation time step. Surprisingly, it has been found that using relatively long simulation time steps, for example setting one, more or all simulation time steps to be ≥ 10 minutes or ≥ 15 minutes, for example in the range of 10 minutes to 60 minutes, preferably in the range 20 minutes to 40 minutes provides sufficient accuracy for the autonomy simulation. The use of a relatively long simulation time step reduces the total number of subsequent simulations when determining the simulated autonomy; this reduces the amount of power required to provide the autonomy simulation and is particularly beneficial when no mains power is available.

[0021] The autonomy simulation may be determined at pre-set time intervals, notably a pre-set time interval which is between 15 minute and 60 minutes, notably between 20 minutes and 40 minutes. For example, when no external power is available, for example no solar power is available to power a cooling circuit, the simulated autonomy of the ice-lined cooling device may be determined every 20 minutes. Such a periodic updating of the simulated autonomy provides a sufficiently accurate indication of autonomy at any time without requiring too frequent updates which would consume available power. Alternatively or additionally, the autonomy simulation may be determined in response to detection of an event likely to affect the autonomy of the ice-lined cold storage, for example detection of the moveable lid of the storage compartment being opened and / or closed or detection in a change in the ambient temperature.

[0022] By way of simplification, the simulated autonomy may be determined based on an assumption that the ambient temperature will remain constant over the time period for which the autonomy is simulated. Although this assumption introduces a divergence from the real-life conditions to which the ice-lined cold storage device will be exposed, compensation may be provided by subsequent periodic updates of the simulated autonomy using the ambient temperature at the time of the updated simulated autonomy. Alternatively, the simulated autonomy may take into account expected variations in ambient temperature, for example on the basis of historical data collected for the ice-lined cold storage device or anticipated temperature changes.

[0023] Advantageously, determining the simulated storage compartment temperature comprises determining a simulated ice lining energy, notably by simulating one or more energy flows into the ice lining. The ice-lining is generally configured to be the principal energy storage component in the ice-lined cold storage device; consequently, determining simulated autonomy of the ice-lined cold storage device based upon focussing on simulating energy flows and the amount of energy in the ice-lining provides a simple yet reliable determination of the simulated autonomy. During any prolonged periods in which heat is not removed from the cold storage device, the ice-lining remains at a temperature of 0°C for a significant period of time. Indeed, it is during such periods when absorption of energy by the ice-lining is causing the ice to melt that the ice-lining serves its principal purpose. However, at any time when the ice-lining comprises a combination of a solid phase and a liquid phase, the temperature of the ice-lining can not reliably indicate the energy of the ice-lining or the autonomy that the ice-lining will provide. Focussing upon, and keeping track of, the energy of the ice-lining provides a practical and sufficiently accurate indication of autonomy.

[0024] An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, of which: Fig 1 is a schematic perspective view of an ice-lined cold storage device with its lid open; Fig 2 is a simplified representation of one part of the ice-lined storage device; Fig 3 is a representation, of the ice-lined storage device in terms of thermal resistances; Fig 4 is a flow chart for simulating autonomy; Figs 5a, 5b and 5c show, respectively, graphs of storage compartment temperature, ice-lining temperature and compressor power determined during tests of ice-lined cold storage devices at an ambient temperature of 43 °C; Figs 6a, 6b and 6c show, respectively, graphs of storage compartment temperature, ice-lining temperature and compressor power determined during tests of ice-lined cold storage devices at an ambient temperature of 32 °C; and Figs 7a, 7b and 7c show, respectively, graphs of storage compartment temperature, ice-lining temperature and compressor power determined during tests of ice-lined cold storage devices at an ambient temperature of 25 °C

[0025] Fig 1 shows an ice-lined vaccine refrigerator 1 having a body 2 and comprising: a storage compartment 3 adapted to store a payload, notably vaccines, at a controlled temperature; a plurality of ice-packs 4 which, together make up an ice-lining configured to absorb heat from the storage compartment 3; and a moveable lid 5 of the storage compartment 3.

[0026] The ice-lined vaccine refrigerator 1 has a solar powered, battery free electrical cooling circuit comprising a DC compressor which, when in operation, circulates a refrigerant though a cooling circuit. The cooling circuit includes a plate evaporator located immediately behind each of the ice-packs 4 i.e. at the side of the ice-pack facing away from the cold storage compartment 3. The evaporators thus remove energy from the ice-lining 4. An inner liner, comprising a sheet of aluminium facing towards the storage compartment 3 and a layer of insulating foam (EPP) facing towards the ice-lining 4, surrounds and defines the periphery of the cold storage compartment 3. An electrically powered heated is provided in the base of the cold storage compartment 3, powered by the solar electricity supply, to provide heat to the cold storage compartment 3 if required, to avoid the temperature of the cold storage compartment falling below a pre-set minimum temperature of +2°C. A display panel 6 of the refrigerator has a plurality of displays 7 including a bar display indication autonomy. A solar array 8 provides the solar power.

[0027] The three heat transfer modes were initially considered for an autonomy simulation: radiation, convection, and conduction. The ice-lined cold storage device is not intended be used in direct sunlight and the inside temperature differences between adjacent components are small. Consequently, radiation was considered not to contribute significantly to the heat transfer and was thus excluded from the simulation; this provided a useful simplification.

[0028] Convection and conduction were included in the simulation using the following formula to compute the heat flow Q̇ in W as a function of the surface area A in m 2< and the heat flux q in W×m -2< : Q ˙ = A × q For each of these heat transfer modes, the simulation was based on considering heat flux.

[0029] For convection: q = h c × Δ T As convection q in W×m -2< occurs through a fluid in motion, it was assumed that only the outer surface of the ice-lined cold storage device has significant heat transfer though this mode; here the convective heat transfer coefficient h c in W×m -2< ×K -1< is dependent on the ambient air in contact with the outer surface and the heat flow is dependent on the temperature difference ΔT in K between the ambient air and the outer surface of the ice-lined cold storage device.

[0030] For conduction we used the following equation using a Cartesian coordinate system: q = − k × dT dx Conduction is based upon the temperature differential between the two faces of a material as this will create a heat flow; therefore it is dependent on the material's thermal conductivity k in W×m -1< ×K -1< and the temperature gradient between the two surfaces of the material dT dx in K×m -1< .

[0031] The components of the ice-lined cold storage device modelled included: a storage compartment adapted to store a payload of vaccines at a controlled temperature of between +2°C and +8°C with a maximum permitted temperature of the storage compartment being +10 °C.; an ice-lining configured to absorb heat from the storage compartment; a moveable lid of the storage compartment; an electrically powered cooling circuit comprising a DC compressor which, when in operation, circulates a refrigerant through the cooling circuit comprising a valve, a condenser and evaporators which are configured to remove energy from the ice-lining; an electrically powered heated configured, when in operation, to provide energy to the cold storage compartment, and an electronic controller configured to activate and deactivate the cooling system and the heater. The ice-lined cold storage device has an outer shell made of polypropylene (PP), lined with a thick polyurethane (PU) foam insulation layer that is contained by another polypropylene shell. The ice-lining is arranged around the internal periphery of the ice-lined cold storage device with an expanded polypropylene (EPP) layer of foam insulation and an aluminium sheet of a liner arranged to protect the payload and positioned between the ice-lining and the storage compartment; the internal surface of the aluminium liner defined the periphery of the cold storage compartment. The heater is located at the bottom of the cold storage compartment.

[0032] In normal use, prior to loading the payload of vaccines into the cold storage compartment, the electrically powered cooling circuit is used to cool down the ice lining until it is completely frozen. The ice-lining subsequently contributes to the autonomy of the cold storage device, in particular by absorbing energy from the cold storage compartment, notably due to the amount of energy required to provoke a solid to liquid change in phase of the ice-lining.

[0033] A schematic representation of one side wall and of the lid of the cold storage device, and of the thermal resistance modelling used in the simulation is shown in Fig 2. In this thermal resistance modelling, each of the heat transfer mode being modelled is replaced by a resistance and the energy storage is replaced by a capacitor. The equivalent to the "voltage" here is the temperature difference that will vary between each "resistance". This approach allows calculation of heat fluxes and temperatures with one single equation that will be dependent on the variable that it is desired to determine using the selected heat transfer modes. Furthermore, it allows transformation of a 3D problem into a 1D problem; this makes computation faster and uses less computation power.

[0034] A more complete version of this approach, where all the resistances are shown, is shown in Fig. 3.

[0035] The cold storage device is composed of different materials and there are different heat transfer modes that according to Eq. (2) and Eq. (3) have different parameters for different materials; these are also sometimes temperatures dependent such as the thermal conductivity of PP and EPP insulating material. Table 1 summarises the different parameters and values: Table 1. Thermodynamic parametersParamete rValueUnith c 2W×m -2< ×K -1< k PP 0.0114-0.0278W×m -1< ×K -1< This variable is not linear with respect to temperature; it was divided into three sections to better interpolate the valuesk EPP 0.0397-0.0468W×m -1< ×K -1< k Al 239W×m -1< ×K -1< k air 0.02436-0.02757W×m -1< ×K -1< c pwater 4220W×kg -1< ×K -1< h fusion 334000W×kg -1< c pPP 1300W×kg -1< ×K -1< c pair 1005W×kg -1< ×K -1< c pEPP 1300W×kg -1< ×K -1< ρ PP 23kg×m -3< ρ air 1.225kg×m -3< ρ EPP 60kg×m -3<

[0036] A further important parameter in the equations of heat transfer is thicknesses and surface areas of the components of the ice-lined storage device. For the specific example, the left side and the right side of ice-lined cooling device were identical in their configuration and the front side and rear side were identical. The top and bottom had the same surface areas but the difference in their thicknesses was taken into account.

[0037] Using the thermal resistance method, Eq. (1) was written as Q ˙ = U × A × T i − T 0 where A in m 2< represents the area as defined previously and U is the overall heat transfer coefficient. It was then defined that: 1 U × A = 1 h c , i × A + L A k A × A + L B k B × A + 1 h c , 0 × A This equation can be used to solve the complete system and be used to compute the amount of energy in the ice lining at any given moment in time, provided that the initial conditions are known.

[0038] The heater in the example cold storage device was a constant power heater; thus the amount of heat added to the cold storage compartment when the heater is operated was calculated from the heater power and its operation time. This heater is used to avoid the risk of the temperature of the cold storage compartment falling below a pre-set minimum value (generally + 2°C for vaccine payloads).

[0039] The amount of heat removed by the electrically powered cooling circuit depends on external parameters that affect its Coefficient of Performance ("COP") . The COP is defined as: COP = Q ˙ W where Q is the heat removed from the system in watts and W the power consumed by the compressor in watts.

[0040] The effect of opening the lid of the cold storage compartment was simulated by setting the thermal resistance to zero when the lid is open as, in effect, opening the lid removes the thermal insulation at the top of the cold storage compartment.

[0041] Further simplifications were made in the following ways: a) High thermal conductivity coefficient materials will not contribute significantly to determining the heat flux; this is also the case for very thin layers (compared to the large thickness of the PU insulation layer in the side walls, bottom and lid). Therefore, the effect of the aluminium layer of the inner liner was ignored due to its small thickness and high thermal conductivity; furthermore, the layers of PP encapsulating the PU insulation were also assumed to be negligible as they are very thin. b) It was also assumed that the temperature variation across a layer is linear and therefore, the average temperature of a given layer can be found by the average of the two extreme temperatures. c) It was assumed that all the energy stored within the cold storage device is stored in the ice lining, the PU insulation layer, and the air volume of the cold storage compartment as these are the largest volume and mass components of the cold storage device whose change in energy levels has the most significant effect upon the temperature of the cold storage compartment. In particular, in the example ice-lined cold storage device it was assumed (and verified by calculation) that, under normal conditions, the amount of thermal energy of the vaccine payload did not have a significant effect.

[0042] Fig 4 provides an overview of an algorithm used for simulating autonomy of the ice-lined cooling device.

[0043] Validation tests were carried out to compare the simulated autonomy calculated for the example ice-lined cold storage device to the autonomy actually obtained for the ice-lined storage device in a test situation at constant ambient temperatures of 25°C, 32°C, and 43°C. The example ice-lined cold storage device was a solar powered vaccine refrigerator but for the validation tests, a mains powered DC power supply was used. For each validation test, the ice-lined cold storage device was initially placed in a temperature-controlled room at the selected test temperature (25°C, 32°C, and 43°C) with the lid of the cold storage compartment open and the power of its cooling circuit disconnected. The device was left in this condition until the room reached the setpoint temperature and the device showed stable temperatures across thermocouples configured to monitor its temperature. Then, the lid of the cold storage compartment was closed and the power of the cooling circuit was connected. Operation of the cooling circuit subsequently removed heat from the ice-lining and cooled the cold storage compartment. Once the cold storage device had reached its standard "in operation" cooling conditions, the device was allowed to run for a few cycles using its control system to activate and deactivate the cooling circuit in the usual way to ensure that the ice-lining was fully frozen. Subsequent to this, the external power supply was disconnected (to simulate a failure in the electrical cooling circuit or in the external power supply) and the temperature of the cold storage compartment was monitored via thermocouples. The duration between the external power supply being disconnected and the temperature of the cold storage compartment reaching +10°C was recorded and taken to be the experimentally determined autonomy.

[0044] Three cold storage devices were tested at each of the selected test temperatures (25°C, 32°C, and 43°C).

[0045] Detailed results from the validation tests are shown in Figs 5a, 5b and 5c (constant ambient temperature of +43°C), Figs 6a, 6b and 6c (constant ambient temperature of +32°C) and Fig 7a, 7b and 7c (constant ambient temperature of +25°C). Table 2 below sets out the experimentally determined autonomy obtained for each device tested at each ambient temperature, the average experimentally determined autonomy for the three devices at each ambient temperature, the autonomy determined by simulation for a device in the same conditions as the validation test and the difference between the average experimentally determined autonomy determined by the validation tests and the autonomy determined by simulation. Table 2Autonomy determined by validation testAverage autonomy determined by validation testsAutonomy determined by simulationDifferenceAmbient TemperatureDevice 1Device 2Device 343 °C118.5h116.5h119.7h118.2h120h1.5%32 °C170.2h170.5h174.8h171.8h168.8h1.8%25 °C227.8h226.0h233.7h229.1h224.5h2.0%

[0046] The maximum variation in the autonomy determined in the validation tests was: 2.7% (3.2 hours) at 43°C ambient temperature; 2.6 % (4.55 hours) at 32°C ambient temperature; and 3.4% (7.72 hours) at 25°C ambient temperature. The differences between individual devices can be attributed to manufacturing variations between the three cold storage devices. The small difference (maximum 2.0%) between the average autonomy determined by validation tests and the autonomy determined by simulation indicates the usefulness of the simulated autonomy in indicating the autonomy that will actually be achieved. A safety factor could be incorporated into the simulated autonomy to ensure that the simulated autonomy is always slightly less that the experimentally determined autonomy.

[0047] Figs 5a and 5b, which show temperature in °C on the x axis (obtained by measurement during the validation tests) and time in hours on the y axis, and Fig 5c which shows compressor power (obtained by measurement during the validation tests) on the x axis and time in hours on the y axis, present data obtained from the validation tests with an ambient temperature of +43°C. Table 3 below provides a key applicable to Figs 5a, 5b, 6a, 6b, 7a and 7b: Table 3Symbol Data device TC1cTemperature of the cold storage compartment extracted from the cold chain controllerfirst deviceTC2csecond deviceTC3cthird deviceTC1mAverage of the temperature of the cold storage compartment measured with a plurality of thermocouples arranged at different positions within the cold storage compartmentfirst deviceTC2msecond deviceTC3mthird deviceTL1cTemperature of the ice-lining extracted from the cold chain controllerfirst deviceTL2csecond deviceTL3cthird deviceTL1mAverage of the temperature of the ice-lining measured with a plurality of thermocouples arranged at different positions of the ice-liningfirst deviceTL2msecond deviceTL3mthird deviceCP1Compressor power consumptionfirst deviceCP2second deviceCP3third device

[0048] Corresponding data is shown, using the same symbols, in Figs 6a, 6b and 6c for an ambient temperature of 32°C and in Figs 7a, 7b and 7c for an ambient temperature of 25°C.

[0049] Referring to Figs 5a, 5b and 5c which show data for the validation tests at a constant ambient temperature of +43 °C, the average ice-lining temperature (from a plurality of thermocouple), average cold storage compartment temperature (from a plurality of thermocouples) and power consumption of the compressor can be seen. A horizontal line in Figs 5a and 5b indicates the limit of +10°C. All three units behave in a similar fashion during cool down from 0h until approximately 25 hours. At about 25 hours, a jump in the ice-lining temperature can be seen from about -6°C to about 0°C; this jump is thought to be due to supercooling, with different individual sensors arranged at different positions of the ice-ling having the jump at different times since the step from supercooling to freezing is not the same everywhere. The unit is left running until all ice temperatures start decreasing again below 0°C, this can be seen at around 60 hours. After about 60 hours, the cold storage devices enter a phase of stable running with their controllers periodically turning the compressor off and on to maintain the desired temperatures.

[0050] The phenomenon of local supercooling of the ice-lining would be an issue if the simulation of autonomy was based upon measuring the temperature of the ice-lining during this period as it indicates that a simple, single temperature measurement of the ice-lining is not always reliable to determine the energy of the ice-lining.

[0051] On Fig 5a, the start and end of the observed autonomy during the validation tests is indicated in Table 4: Table 4Cold storage device testedStart of autonomy periodEnd of autonomy periodFirst5152Second5354Third5556

[0052] Figs 6a, 6b and 6c show data from the validation tests conducted with a constant ambient temperature of +32°C. The three phases can again be seen with cool down from 0h until approximately 45 hours and a supercooling effect occurring again at approximately 15 hours. On Fig 6a, the start and end of the observed autonomy during the validation tests is indicated in Table 5: Table 5Cold storage device testedStart of autonomy periodEnd of autonomy periodFirst6162Second6364Third6566

[0053] Figs 7a, 7b and 7c show data from the validation tests conducted with a constant ambient temperature of +25°C. The effect of supercooling can again be seen; some parts of the ice stays supercooled and do not jump to the freezing point but simply stays cold until fully frozen. The cool down phase can be seen until approximately 48 hours. On Fig 7a, the start and end of the observed autonomy during the validation tests is indicated in Table 6: Table 6Cold storage device testedStart of autonomy periodEnd of autonomy periodFirst7172Second7374Third7576

[0054] Table 7 provides an indication of data used in the simulation of autonomy and, for each piece of data, an example of the source for that data (which was the source used in the example simulation of autonomy): Table 7.DataSource of datastorage compartment temperatureCold Chain Controller (CCC)Ice-lining temperatureCold Chain Controller (CCC)heater statusCold Chain Controller (CCC)ambient temperatureExternal sensorDoor openingExternal sensorpower consumption of electrically powered cooling circuitampere meter and voltmeter measuring the current and voltage across the compressor The Cold Chain Controller (CCC) is the electronics board present in the ice-lined cold storage device used for the validation tests for which the firmware was modified and a communication connection added to allow the data to be obtained. For the door opening a simple contact switch was added to the system. The storage compartment temperature and the ice-ling temperature are used to initialize the autonomy meter; this allows determination of the initial level of energy of the system. The other four parameters are used to keep track of the relevant heat exchanges. Data regarding the heater is required since the heater will add energy to the system and therefore decrease the autonomy; the ambient temperature is one of the factors which determines the heat flux between the cold storage device and the ambient atmospheres. The door switch is used to indicate if the insulation provided by the moveable lid of the storage compartment is present or not. The power consumption of the electrically powered cooling circuit is used to calculate the heat being removed from the system.

[0055] The simulated autonomy was based on using energy transfer methods and by keeping track of the energy inside the system by accounting for losses through the insulation of the sides and base of the ice-lined cold storage device, losses through the top of the ice-lined cold storage device (with the lid open or closed), energy added by a heater intended to avoid freezing of the vaccines, and removal of energy by the electrically powered cooling circuit. The validation tests demonstrate the validity of the approach, and in particular the validity of using an energy transfer approach for providing a simulated autonomy for an ice-lined storage device. The example simulated autonomies was provided using an Arduino board that was configured to run Python scripts; this prototype was tested and provided a proof of concept. Further refinement, for example to avoid the risk of noisy readings in the temperatures leading to imprecision and to design an appropriate printed circuit board would be desirable for optimisation. The example simulated autonomies were provided using a Coefficient of Performance for the compressor of the ice-lined cold storage devices used in the validation tests which was determined experimentally at different temperatures; adaptation would be required for different compressors and / or different cold storage devices.

[0056] One advantage of the simulated autonomy described herein is the ability for it to be provided for new ice-lined storage devices (and in this case, potentially integrated into a cold chain controller) or provided, for example by retrofitting, to existing new ice-lined storage devices.List of reference numbers:

[0057] 1: ice-lined cold storage device 2: body of storage device 3: storage compartment 4: ice-packs forming the ice-lining 5: lid of storage compartment 6: display panel 7: displays 8: solar array Nomenclature

[0058] cccCold chain controllerPCBPrinted circuit boardCOPCoefficient of performancePPPolypropyleneEPPExpanded polypropylenePUPolyurethaneQ̇Heat flow (W)ρDensity (kg×m -3< )ρ air Air density (kg×m -3< )ρ EPP EPP density (kg×m -3< )ρ PP PP density (kg×m -3< )ASurface area (m 2< )A 1 Area of the left and right (m 2< )A 2 Area of the front and back (m 2< )A 3 Area of the top and bottom (m 2< )A 4 Area of 1 side of the EPP insulation (m 2< )c pair Air heat capacity at constant pressure (J×kg -1< ×K -1< )c pEPP EPP heat capacity at constant pressure (J×kg -1< ×K -1< )c pPP PP heat capacity at constant pressure (J×kg -1< ×K -1< )c pwater Water heat capacity at constant pressure (J×kg -1< ×K -1< )h c Convective heat transfer coefficient (W×m -2< ×K -1< )h fusion Water heat of fusion (J×kg -1< )kThermal conductivity (W×m -1< ×K -1< )k air Air thermal conductivity (W×m -1< ×K -1< )k Al Aluminium thermal conductivity (W×m - 1< ×K -1< )k EPP EPP thermal conductivity (W×m -1< ×K -1< )k PP PP thermal conductivity (W×m -1< ×K -1< )L bottom Thickness bottom (m)L EPP EPP thickness (m)L sides Thickness around the refrigerator (m)L top Thickness lid / top (m)m air Mass of air inside the unit (kg)m EPP EPP mass (kg)m water Mass of water inside the ice lining (kg)qHeat flux (W×m -2< )TTemperature (K)V PP PP volume (m 3< )xThickness (m)

Claims

1. An ice-lined cold storage device (1), notably a vaccine storage device, which is provided with an indication of simulated autonomy, in which the ice-lined storage device comprises: - a storage compartment (3) adapted to store a payload, notably vaccines, at a controlled temperature; - an ice-lining (4) configured to absorb heat from the storage compartment (3); and - a moveable lid (5) of the storage compartment; and in which the simulated autonomy is provided using a temperature model which uses data which is specific to the ice-lined cold storage device (1) and by: a) detecting an initial ambient temperature, an initial storage compartment temperature and an initial ice-lining temperature of the ice-lined cold storage device (1); b) running a first simulation using the temperature model and using at least the initial ambient temperature, the initial storage compartment temperature and the initial ice-lining temperature to determine a first simulated storage compartment temperature after a first simulation time step; c) running one or more subsequent simulations, each subsequent simulation being run using the temperature model and using data created in a previous simulation, to determine a subsequent simulated storage compartment temperature after a subsequent simulation time step; d) determining the simulated autonomy of the ice-lined cold storage device (1) using at least one of the subsequent simulated storage compartment temperature(s).

2. An ice-lined cold storage device (1) in accordance with claim 1, in which determining the simulated storage compartment temperature comprises determining a simulated ice lining energy, notably by simulating one or more energy flows into the ice lining (4).

3. An ice-lined cold storage device (1) in accordance with claim 1 or claim 2, in which the first simulation comprises: - using the initial ice-lining temperature to calculate an initial simulated ice-lining energy, - using the initial storage compartment temperature to calculate an initial simulated storage compartment energy, - using the initial ambient temperature and the initial ice-lining temperature to calculate, for the first simulation time step, a first simulation flow of energy into the ice-lining from the surroundings of the ice-lined storage device; - using the initial ambient temperature and the initial storage compartment temperature to calculate, for the first simulation time step, a first simulation flow of energy into the storage compartment from the surroundings of the ice-lined storage device; - using the initial ice-lining temperature and the initial storage compartment temperature to calculate, for the first simulation time step, a first simulated flow of energy from the storage compartment into the ice-lining; - using the initial simulated ice-lining energy, the first simulated flow of energy from the storage compartment into the ice-lining and the first simulation flow of energy into the ice-lining from the surroundings of the ice-lined storage device to calculate a first simulated ice-lining energy; - using the first simulated ice-lining energy to calculate a first simulated ice-lining temperature; - using the initial storage compartment energy, the first simulated flow of energy from the storage compartment into the ice-lining and the first simulation flow of energy into the storage compartment from the surroundings of the ice-lined storage device to calculate a first simulated storage compartment energy; and - using the first simulated storage compartment energy to calculate a first simulated storage compartment temperature.

4. An ice-lined cold storage device (1) in accordance with claim 3, in which at least one of the subsequent simulations comprises: - using an indication of ambient temperature at the start of the subsequent simulation time step, notably the initial ambient temperature, and the simulated ice-lining temperature from an immediately preceding simulation to calculate, for the simulation time step, a subsequent simulation flow of energy into the ice-lining from the surroundings of the ice-lined storage device; - using an indication of ambient temperature at the start of the subsequent simulation time step, notably the initial ambient temperature, and the simulated storage compartment temperature from an immediately preceding simulation to calculate, for the subsequent simulation time step, a subsequent simulation flow of energy into the storage compartment from the surroundings of the ice-lined storage device; - using the simulated ice-lining temperature from an immediately preceding simulation and the simulated storage compartment temperature from an immediately preceding simulation to calculate, for the simulation time step, a subsequent simulated flow of energy from the storage compartment into the ice-lining; - using the simulated ice-lining energy from an immediately preceding simulation, the subsequent simulated flow of energy from the storage compartment into the ice-lining and the subsequent simulation flow of energy into the ice-lining from the surroundings of the ice-lined storage device to calculate a subsequent simulated ice-lining energy; - using the subsequent simulated ice-lining energy to calculate a subsequent simulated ice-lining temperature; - using the simulated storage compartment energy from an immediately preceding simulation, the subsequent simulated flow of energy from the storage compartment into the ice-lining and the subsequent simulation flow of energy into the storage compartment from the surroundings of the ice-lined storage device to calculate a subsequent simulated storage compartment energy; and - using the subsequent simulated storage compartment energy to calculate a subsequent simulated storage compartment temperature.

5. An ice-lined cold storage device (1) in which the ice-lined storage device comprises a lid position indicator, notably a switch, which provides an indication as to whether the moveable lid (5) of the storage compartment is in an open position or a closed position; and in which the ice-line storage device is selected from: a) an ice-lined storage device in accordance with any preceding claim in which - determining the simulated autonomy of the ice-lined cold storage device (1) comprises adjusting the simulated autonomy to account for energy flows during any time periods during which the lid position indicator indicates the moveable lid (5) as being open; and b) an ice-lined cold storage device (1) in accordance with claim 2 or any of claim 3 to 4 as dependent upon claim 2, in which - determining the simulated storage compartment temperature comprises adjusting the simulated ice-lining energy to account for energy flows during any time periods during which the lid position indicator indicates the moveable lid (5) as being open.

6. An ice-lined cold storage device (1) in accordance with any preceding claim in which the ice-lined cold storage device (1) is a solar powered cold storage device, notably a solar powered cold storage device having a battery-free, electrical cooling circuit configured, when in operation, to remove energy from the ice-lining (4).

7. An ice-lined cold storage device (1) in which the ice-lined cold storage comprises an electrically powered cooling circuit configured, when in operation, to remove energy from the ice-lining (4); and in which the ice-line storage device is selected from: a) an ice-lined storage device in accordance with any preceding claim in which - determining the simulated autonomy of the ice-lined cold storage device (1) comprises adjusting the simulated autonomy to account for energy removed from the ice-lining by the electrically powered cooling circuit; and b) an ice-lined cold storage device (1) in accordance with claim 2 or any of claim 3 to 6 as dependent upon claim 2, in which - determining the simulated storage compartment temperature comprises adjusting the simulated ice-lining energy to account for energy removed from the ice-lining by the electrically powered cooling circuit.

8. An ice-lined cold storage device (1) in accordance with claim 7 in which the electrically powered cooling circuit comprises an electrically powered compressor which, when in operation, circulates a refrigerant through the cooling circuit, in which the adjusting to account for energy removed from the ice-lining by the electrically powered cooling circuit comprises determining the power consumption of the compressor, notably comprising determining the magnitudes of an electrical current and an electrical voltage driving the compressor, and using the power consumption of the compressor and a coefficient of performance of the compressor to calculate the energy removed from the ice-lining by the electrically powered cooling circuit, notably comprising selecting the coefficient of performance of the compressor depending upon a temperature of the electrically powered cooling circuit.

9. An ice-lined cold storage device (1) in which the ice-lined cold storage comprises an electrically powered heated configured, when in operation, to provide energy to the cold storage compartment (3); and in which the ice-line storage device is selected from: a) an ice-lined storage device in accordance with any preceding claim in which - determining the simulated autonomy of the ice-lined cold storage device (1) comprises adjusting the simulated autonomy to account for energy provided to the cold storage compartment (3) by the electrically powered heater and b) an ice-lined cold storage device (1) in accordance with claim 2 or any of claim 3 to 8 as dependent upon claim 2, in which - determining the simulated storage compartment temperature comprises adjusting the simulated ice-lining energy to account for energy provided to the cold storage compartment (3) by the electrically powered heater.

10. An ice-lined cold storage device (1) in accordance with any of the previous claims - in which the ice-lined cold storage comprises an electrically powered cooling circuit configured, when in operation, to remove energy from the ice-lining (4); - in which the electrically powered cooling circuit is configured to operate upon detection of an ice-lining temperature that is greater that a maximum steady state ice-lining temperature; - in which initiation of the process for determining the simulated autonomy is provoked by detection of a change in the ice-lining temperature from a temperature which is less than the maximum steady state ice-lining temperature to a temperature which is greater than the maximum steady state ice-lining temperature.

11. An ice-lined cold storage device (1) in accordance with any of claims 1 to 5, in which the ice-lined cold storage device (1) is a passive cold storage device, notable a passive vaccine storage device, particularly a device configured to store vaccines during transport.

12. An ice-lined cold storage device (1) in accordance with any preceding claim, in which in c) running one or more subsequent simulations, each subsequent simulation being run using the temperature model and using data created in a previous simulation, to determine a subsequent simulated storage compartment temperature after a subsequent simulation time step; the using data created in a previous simulation comprises using data indicative of whether the ice-lining (4) is a) entirely in a solid phase, b) a mixture of solid phase and liquid phase, or c) entirely in a liquid phase, to determine the subsequent simulated storage compartment temperature after the subsequent simulation time step, notably comprising using the data indicative of whether the ice-lining is a) entirely in a solid phase, b) a mixture of solid phase and liquid phase, or c) entirely in a liquid phase to calculate the ice-lining temperature after the subsequent simulation time step and using the ice-lining temperature to calculate the simulated storage compartment temperature.

13. An ice-lined cold storage device (1) in accordance with any preceding claim, in which - the using a temperature model which uses data which is specific to the ice-lined cold storage device (1) comprises providing temperature dependant data, notably values of thermal conductivity, and selecting the data to be used in the temperature model as a function of one or more temperature(s) which have been determined or simulated.

14. An ice-lined cold storage device (1) in accordance with any preceding claim, in which the data which is specific to the ice-lined cold storage device (1) and which is used by the temperature model comprises: - areas of surfaces forming sides, base and top of the cold storage device; - thicknesses and thermal conductivity of insulating material of the cold storage device; - mass and thermal properties of the ice-lining (4); and - mass of air within the storage compartment (3).

15. An ice-lined cold storage device (1) in accordance with any preceding claim, in which the temperature model comprises a model of thermal resistance(s) of components of the ice-lined storage device.

16. A method for determining a simulated autonomy of an ice-lined cold storage device (1), notably an ice-lined cold storage device (1) in accordance with any preceding claim, using a temperature model which uses data which is specific to the ice-lined cold storage device(1), in which the ice-lined cold storage device (1) comprises - a storage compartment (3) adapted to store a payload at a controlled temperature; - an ice-lining (4) configured to absorb heat from the storage compartment; and - a moveable lid (5) of the storage compartment; and in which the method comprises: a) detecting an initial ambient temperature, an initial storage compartment temperature and an initial ice-lining temperature of the ice-lined cold storage device (1); b) running a first simulation using the temperature model and using the initial ambient temperature, the initial storage compartment temperature and the initial ice-lining temperature to determine a first simulated storage compartment temperature after a first simulation time step; c) running one or more subsequent simulations, each subsequent simulation being run using the temperature model and using data created in a previous simulation, to determine a subsequent simulated storage compartment temperature after a subsequent simulation time step; d) determining the simulated autonomy of the ice-lined cold storage device (1) using at least one of the subsequent simulated storage compartment temperature(s).