ThermaCart

Area: Shared Components · TRL: 3 of 9 (proof of concept on paper) · Prototype budget: about $80 USD · Difficulty: 2 of 5

A sealed, swappable phase-change thermal cartridge in a few standard temperature ranges (cold chain, comfort and hot holding) that stores heat or cold and drops into coolers, cabinets and heat exchangers.

Exploded render · Detail render · Interactive 3D model · Concept blueprint (PDF) · General arrangement (PDF) · Calculations · Review note

Concept rationale

A common cartridge format turns thermal storage into a part you choose instead of a subsystem you design, and lets cartridges move between cold boxes, ovens and heat stores. Phase-change material (PCM) holds heat or cold at a nearly constant temperature, so the grade of the fill, not a thermostat, sets the temperature of the box it sits in. Sealing the PCM in a keyed cartridge keeps a messy or combustible material out of users' hands and stops the wrong temperature going into the wrong box.

The design is open and garage-buildable on purpose. The shell is a stock 6 x 2 in aluminium tube with screwed, O-ring-sealed end caps built up from flat plate, so anyone with a drill press and a tap set can make one without milling, and the envelope fits a GN 1/3 gastronorm slot that commercial kitchens already use. Publishing the envelope, key table and fill recipes lets other open projects, including ColdPod and ZeerBox in this portfolio, design around the same cartridge.

Burning platform

Around 14 % of the world's food, worth about $400 billion a year, is lost between harvest and retail (FAO and UNEP, 2022), much of it spoilage where cooling is missing or unreliable. Cold chains also fail by overcooling: freezing exposure is common in vaccine storage and transport studies, largely because water ice sits at 0 °C or below (Hanson et al., 2017).

On the hot side, about 2.1 billion people still cook with polluting fuels, and household air pollution caused an estimated 2.9 million deaths in 2021 (WHO, 2025). Demand for cooling is rising fast: UNEP expects installed cooling capacity to triple by 2050, with cooling emissions above 10 % of the global total unless efficiency improves (UNEP). Cheap, portable thermal storage lets heat and cold be made where energy is available and used where it is not.

Where it could be used

By industry

Industry	Use
Food service and catering	H70 cartridges in insulated GN carriers keep delivered meals hot without gas or mains
Fresh produce and dairy logistics	C5 cartridges in coolers on motorbikes, vans and market stalls, recharged overnight in a shared freezer
Off-grid refrigeration	A swappable cold store for solar direct-drive fridges and cool boxes instead of built-in ice banks
Telecom and electronics enclosures	C25 cartridges buffer daily temperature swings in outdoor cabinets and battery boxes
Open hardware and product design	A documented storage block for new cold boxes, warmers and heat stores, so designers size the box, not the storage

By country or region

Country or region	Why it matters there
India	A 2022 national study put post-harvest losses at 6 to 15 % for fruits and 5 to 12 % for vegetables (MoFPI, citing NABCONS)
Nigeria	About 45 % of food spoils for lack of cold storage, and solar cold rooms such as ColdHubs show the demand for shared cooling (IFPRI, 2018)
Sub-Saharan Africa (for example Kenya and Tanzania)	About four in five households lack clean cooking (IEA); keeping cooked food hot saves fuel and smoke
European Union	About 132 kg of food per person was wasted in 2022, 54 % of it in households (Eurostat); kitchens already use GN formats
United States	State food codes based on the FDA Food Code require hot food to be held at 57 °C (135 °F) or above (for example Delaware, § 3-501.16); caterers and school meal programs need hot holding away from mains
United Kingdom	Salt-hydrate heat batteries are already used in homes for hot water (University of Edinburgh), showing that sealed PCM storage already sells in high-income markets

What sparked the idea

The starting point was the gastronorm pan. In November 1964 the Swiss association for institutional catering (SVG), the Swiss Hotel Association and Swiss makers of commercial kitchen equipment agreed on a kitchen norm built on a basic 530 x 325 mm grid (SVG, Merkblatt Gastro-Norm), and in 1993 the container dimensions were published as the European standard EN 631-1 (BSI, BS EN 631-1:1993). Because every pan, oven, trolley and carrier now shares those sizes, kitchens mix equipment from any maker without thinking about it. Thermal storage never got the same treatment: each cold box and food carrier still has its own ice pack or heat pack. ThermaCart asks what a gastronorm for stored heat and cold would look like, so it takes the GN 1/3 footprint as its envelope and adds the missing layer, a small set of keyed temperature grades.

Problem

Thermal storage is designed from scratch in every cold box, cooker and heat store, and phase-change materials are messy, corrosive or unsafe when handled loose. Water ice is too cold for vaccines and some produce, salt hydrates corrode aluminium, and paraffins burn and can permeate plastics.

Full problem statement: docs/01-problem.md

Concept

A sealed aluminium cartridge, 323 x 152 x 64 mm, holds 1.10 to 1.18 kg of PCM in one of three grades: C5 (5 °C, cold chain), C25 (25 °C, comfort) and H70 (70 °C, hot holding of cooked food). It fits a GN 1/3 slot, carries by a folding bail, shows its state through a sight window and locks into an adapter frame whose key accepts only its grade. The shell carries a matte black finish that roughly doubles its heat exchange with the air. The TRL 3 calculations (TCT-CAL-001) give 65, 61 and 73 Wh of usable storage, 2.9 to 3.0 kg filled and about $76 in parts per cartridge. One H70 cartridge keeps a GN carrier at 63 °C or more for about 13 h. Two C5 cartridges store enough cold for a 25 L cooler for about 9.7 h at 32 °C, but on paper they cannot draw heat out of the cooler air fast enough to hold 2 to 8 °C for more than about 2.4 h; users are told to lay produce directly on the cartridges, which the calculation does not credit. The cold-chain hold time (R10) and the cost target (R16) are not met; see the review note. The design choices were decided by Amish on 2026-09-25 (TCT-DDR-001, TCT-DDR-002).

Full design precis: docs/02-concept.md · Requirements: docs/03-requirements.md

Key components

1. Cartridge shell: finned 6063 aluminium rectangular tube, matte black finish
2. PCM fill: C5, C25 or H70 paraffin grade (water W0 as an option for produce only)
3. End caps with FKM O-ring seals
4. Fill port plug and seal
5. Folding bail handle on a thermal break, and keyed nose
6. Grade label and melt indicator (sight window)
7. Cabinet adapter frame with keyed stop
8. Cap screws and consumables

The priced bill of materials is in bom/bom.csv: about $76 per cartridge and $16 per frame (indicative), $92 for the first C5 build against the $80 budget; supplier quotes come before any budget change. The parametric model is cad/src/model.py, with STEP and STL exports in cad/step/ and cad/stl/.

Safety

Paraffin fills are combustible: never charge a cartridge on an open flame, set an oven to 85 °C and never let an H70 cartridge exceed 90 °C, and keep cartridges away from fire. Keep C5 below 45 °C and C25 below 65 °C. H70 cartridges reach 70 to 85 °C and can burn skin; carry them only by the bail, prefer pad charging, and after an oven use oven gloves or wait 12 min, as the label says. Fill hot to the specified level and never seal a partly filled cartridge. C5 cartridges come out of a freezer below 0 °C; ThermaCart is not qualified for vaccines or medicines. Deburr all cut aluminium.

Repository layout

Folder	Contents
docs/	Problem, concept, requirements, calculations and design decisions
cad/src/	build123d Python source, the source of truth for all geometry
cad/step/, cad/stl/	Exported models for FreeCAD, other CAD tools and printing
cad/drawings/	2D sketches and dimensioned drawings
bom/	Bill of materials
electronics/	KiCad schematics and PCB layouts
firmware/	Microcontroller code
media/	Renders, perspectives and photos
build-log/	Dated prototyping notes

Documentation

Controlled documents follow the portfolio documentation standard. Each carries a document ID (TCT-PRC-001 for the precis), a version and a revision history. Branded PDFs are built with python .kit/render.py and attached to GitHub Releases when a document is tagged, for example TCT-PRC-001/v1.0.

Credits

Designed by Amish Chadha. See CONTRIBUTORS.md for roles. To cite this design, use CITATION.cff (GitHub shows it as "Cite this repository").

AI assistance (Claude) was used to accelerate concept renders, prototype documentation and first-pass sizing calculations. Design direction and all decisions are Amish Chadha's, recorded in this repository's decision records (docs/decisions/).

Licenses

• Hardware (CAD, drawings, BOM, electronics): CERN-OHL-S v2
• Software (firmware, scripts, notebooks): MIT

A project of the Design Molecule lab.