Electricity-Free Food Storage Design

Introduction

The global perspective problem we have chosen to study is the Peru Food Storage problem centered around finding reliable long-term food storage solutions without the use of electricity. The problem specifically analyzes heat transfer through a stone silo called a qolca, constructed in the Andes mountains for use by a single person. The stakeholders for this problem include rural communities, farmers, and the greater Peruvian community. 

Rural areas and the greater Peruvian community can have limited access to electricity and may not have the resources to fund electrically powered refrigeration. If rural communities had this solution available, its optimization would have a positive impact on food availability, since a greater variety of food would be able to be stored with quality maintained for longer. This would also positively impact the nutrition levels of these rural communities since nutrition tends to decline if storage methods are at under par temperatures. 

Farmers frequently need to store large quantities of grain and other agricultural products in temperature-controlled environments, and this solution would let them do so without the energy demands of conventional storage methods like refrigerated warehouses, etc. This engineering solution would reduce their expenses, which would remove some of the financial burden those who work in the agricultural industry experience.

Overview

This project investigated the thermal and fluid behavior of an electricity-free food storage structure (Qolca) using computational fluid dynamics (CFD) to evaluate its effectiveness in preserving crops in the Peruvian Andes. In Phase I, an analytical heat-transfer model was developed to estimate internal food temperatures, followed by a two-dimensional CFD simulation incorporating conjugate heat transfer and wind-driven convection. Simulation results were validated against the analytical solution to confirm that the model captured the dominant heat-transfer mechanisms and produced physically reasonable temperature distributions.

Building on the validated Phase I model, Phase II focused on design iteration and sociotechnical evaluation. The Qolca geometry, material assumptions, and boundary conditions were modified to improve temperature suitability for a wider range of stored foods and to better reflect real-world construction practices. A second CFD simulation was performed to quantify how these changes affected internal temperature gradients and overall thermal performance. Results from the updated design were compared directly to the original configuration to assess performance improvements, remaining limitations, and trade-offs. This phase emphasized using simulation-driven insights to inform practical design decisions while accounting for stakeholder needs and local constraints.

Methods

Between the first and second iterations of this project, the model was upgraded from a two-dimensional slice to a full three-dimensional simulation. This change should more accurately capture the true heat transfer and airflow around the qolca. Adding the roof geometry allows us to model convective heat transfer over the top of the qolca which was unable to be accounted for in the 2D version of this simulation. In the 3D model, the solar flux is applied only to the roof, rather than across the entire middle section as in the 2D case, which likely reduced artificial heating seen in the earlier results from applying the heat source directly. In addition to the shift from 2D to 3D, we changed how the food was stored within the qolca. Instead of filling the entire interior volume, the quinoa is now placed along a band around the perimeter, which better reflects how a real qolca is used. This change should lead to a more realistic prediction of the internal temperature distribution. A shelf was added halfway up the qolca to create two semi-isolated spaces that would have different temperatures, with a warmer temperature at the upper level. 

Geometry

The geometry of the qolqa was simplified to concentric cylinders that represent the layers of mud and stone that make up the walls of the structure, with a cone on top representing the thatched roof. Instead of assuming that all of the free space inside the qolca was filled with food, we reduced the occupied volume so that only the perimeter was lined with stored goods, simulating the traditional method of using vases placed around the interior. The two similar cylinders along the inner diameter of the qolqa are food. A shelf wrapping around the inner diameter of the qolqa creates a separation between the upper and lower level that creates two sections at differing temperatures. 

Model

A turbulent flow model was selected since the Reynolds number was calculated to be 575714.29. The k-ε Turbulence Model was used to simulate the flow around the qolca due to its simplicity and lower computational cost. The model also pairs well with other physics considerations occurring within the problem, like heat transfer. When used, it provides relatively realistic results for straight forward scenarios occurring within engineering contexts. The heat transfer in solids and fluids model was used to model heat transfer through conduction and convection. 

Boundary Conditions

Mesh

Quality: 232,000 mesh elements (coarser)

Results

Design Reflection

A qolca’s primary purpose is to keep food cool. Thus, the maximum temperature of the food within the qolqa was the primary parameter to decrease. In the first iteration, we measured this maximum to be 297 K. In the second iteration, the maximum food temperature decreased to 283.3 K. The improvement can be attributed to the improved modeling of solar heat flux through the roof and the reduction in the height of the food ring. If the food extended to the roof, the maximum temperature would likely be around the temperature of the inner surface of the roof, ~290 K. This temperature was predicted from the hand calculations done in the first iteration, which gave us confidence in our results.

In reality, the situation being simulated is not static. During the night, the food would become significantly colder as the air temperature drops and the sun goes away. It is then heated back up during the day. We speculate that the real advantage of a qolqa comes from the mitigation of these transient heating cycles. If we had time for a third iteration, we would conduct a time-dependent study and see how long it takes for the temperature of the food to rise during the day.

Social Reflection

It would have been helpful to have a clearer understanding of the construction and features of a qolca. To gather this information, we could speak with either the original builder or a builder who specializes in brick laying, as well as archaeologists who work in Peru. The specific questions we could ask include: what building materials were most commonly used, what construction practices were typical, and whether the mortar layers created a sufficiently airtight seal to limit direct airflow through the walls. We could also ask how the interior was traditionally insulated, since this was not visible from the reference image we were provided.

Another useful piece of information would have been a more comprehensive understanding of the thermal properties of dried mud, since this is difficult to find online. To address this, we could consult experts in geology, earth sciences, or earth-related materials science to discuss whether a similar material can be used as an approximation of dried mud. 

Finally, a deeper understanding of the Peruvian farmers' interaction with qolcas would be important for understanding the impact of engineering decisions, which would allow for a more user-centered approach to the problem. To learn more about the user interaction at play with qolca, we could talk to agricultural engineers and experts, as well as historians who specialize in Peru. Key questions include how often farmers entered and opened the qolca, what preservation methods were applied to foods before storage, and what types of containers or vessels were used inside.