Sjoerd Nienhuys
Project Engineer, capacity building seismic strengthening housing. At Nederlandse Aardolie Maatschappij NAM, sjoerd@nienhuys.info

Authors' profiles:

Sjoerd Nienhuys has 40 years of development work experience in several countries in Africa, Latin America, Asia and Middle East. The activities have ranged from house design and settlement upgrading to thermal insulation, indoor air quality, ICS, ecosan and renewable energy.

In addition he is an expert in earthquake engineering and retrofitting for low-income housing. His work has included innovative product development, including clean and fuel-efficient stoves (ICS), small-scale rural and urban biogas, solar thermal energy and solar wood dryers, thermal insulation technologies for housing, ecosan-toilets with urine separation.

He has a deep and broad experience in small and micro-enterprise development, employment generation, behaviour change communication, microfinance and credit development. In building energy analyses he uses Netherlands-KOMO certified software programmes.


This article presents a new way of cooking food in regions with fuel shortages. The Heat Retention Box (HRB) works on the principles of thermal insulation. Food is heated up using the traditional methods of cooking before being placed within the HRB.

The insulation provided by the HRB enables the food to continue cooking for several hours. The HRB can hold two pots and have a thermal insulation value of RC = 2.2 m2.K/W. Experimental results show that the HRB is able to keep the food above the slow cooking temperature (>65°C) for at least four hours.

The paper also outlines how the HRB has been promoted to the community through cooking clubs which provide information on the components and how to construct a HRB.


hay box; heat retention box; self-assembly; cooking clubs

How to make a Heat Retention Box 

In the Himalaya regions, the main energy consumptions within the home are for food preparation and space heating. At higher altitudes, space heating becomes an increasing fuel drain on households, using approximately twice the amount of fuel needed for cooking at altitudes over 2500 m.

This is especially the case in traditional housing with poor thermal insulation (Nienhuys 2012b, Nienhuys 2012a, Nienhuys, 2000). Fuel usage can be reduced by thermally insulating the home. In mountainous villages, the cooking stove is often additionally used as a space heater. In poorly insulated houses, the stoves need to burn for substantial periods to maintain a healthy room temperature.

With fuel scarcity and increasing costs, savings can be obtained by thermally insulating houses and by improvements to stove design. Using improved cookstoves (ICS) and changing the way people cook saves considerable amounts of fuel, however many prefer to adhere to the traditional methods of which include long cooking times, meaning that fuel savings may not be as significant.

The process of slow cooking, inside a thermally insulated box is a solution to this problem. This cooking method has been used for hundreds of years in many countries (Countryfarm Lifestyles, 2008-2013) and with improved insulation technologies and materials, this solution is easily accessible.

(Figure 1)
Fig 1 - Stove Research Group - Printed.jpg

Demonstration cooking exercises of local foods (rice and potato-based dishes) were carried out with the use of a pressure cooker and the HRB (Heat Retention Box). The stove research group (Figure 1) determined that the use of the pressure cooker combined with the HRB saved up to 90% of the cooking energy and using the HRB alone saved up to 75% compared with the traditional cooking methods using firewood, electric or gas stove.

Firewood, kerosene, gas or electricity all have different energy values (Biomass Energy Centre, 2013) and various firewood stove designs also have different energy consumption for the same amount of heat produced (Partnership for Clean Indoor Air, 2006). The percentage of energy savings through the HRB will be the same for all types of equipment.

Good thermal insulation is the key factor for cooking once the pot is removed from the heat source. This paper explains how to make a thermal insulation box for cooking. The HRB must:
  • Keep food at slow cooking temperature (>65°C) for over three hours.
  • Have an easy-to-manufacture design using local materials
  • Be easy to operate with regard to placing the pots inside.
  • Be easy to clean.
  • Look nice in the kitchen.

The development of village-based cooking clubs will create a platform for introducing the new cooking method to many households at one time, as well as the opportunity to develop nutritious and timesaving recipes.

Thermal Insulation

The HRB is insulated with layers of highly reflective foil facing towards the heat source (the cooking pot), for best insulation results, a thin air space (15 mm) should be maintained between layers of foil. The insulation values quoted are based on polyester two-sided reflective foils.

Once the contents of the pot begin to boil, it is placed inside the HRB. The air around the pot will heat up and the heat radiated by the pot is reflected by the foil and contained.
The thermal insulation value of the foil varies considerably with differences in air space between it and the sides of the box.

Good quality reflective foil with a 15 mm air space on both sides has a value of is RC = 0.4 m2.K/W. Using two sheets of foil also affects the thermal insulation value. Two quality reflective foils with 15 mm air gaps either side increases the value to RC = 2.2 m2.K/W. These values (Nienhuys, 2012d) have been calculated on the basis of the Stefan-Boltzmann constant and the technology is now used in insulation construction (The Engineering Toolbox, 2008).

This high insulation value of 2.2 m2.K/W enables the cooking pot to stay above the 65°C (slow cooking temperature) for several hours; meaning the food inside the cooking pot continues to cook while no additional heat is produced. This saves cooking fuel and controlling time.

Testing and Demonstration

The testing for the HRB took place over one week. Rice and a side dish was cooked for 16 trainees, half the meals were cooked using one of the test HRBs while the other half were cooked using traditional methods.

The following HRB designs were tested:
  • Carton HRB made with three layers of PE backed reflective foil.
  • Hardboard HRB with three layers of PE backed reflective foil.
  • Cardboard HRB filled with small pieces of EPS and PE foam around the cooking pot.

The three HRB designs were tested by measuring the amount of energy required to boil 0.5kg of dry rice with one litre of water. These were then compared with the amount of energy required to cook using the traditional methods of cooking on an open pot.

The traditional method of cooking was kept consistent for all five trials, the rice was allowed to boil vigorously for three to four minutes before the gas was lowered and the dish was simmered for 27 minutes. It is important to note that more water was used in the traditional method due to the effect of water evaporation when cooking.

The (2) hardboard HRB gave the best performance. The (1) carton boxes would not be a neat commercial product, and the (3) HRB with loose EPS was not practical to work with.

Participants took part in a taste session to compare the different cooking methods. Participants carried this out blind as some felt that the rice would taste differently when not cooked in the traditional way. In order for the HRB to be successful, this new cooking method would have to produce tasty food.

Table 1 The time taken to cook plain rice using the traditional method


Table 2 The time taken to cook plain rice using the HRB


The traditional method took approximately four minutes to reach boiling point and the rice was allowed to boil for three minutes before being the temperature was reduced to simmering. The water and rice were allowed to boil for five minutes before being placed in the HRB as the testing staff members were of the opinion that the rice needed to cook vigorously before putting the entire pot inside the HRB.

When the gas is reduced to 50% and 33% of the full gas amount, the HRB presents a dramatic saving in the amount of gas used. The saving of full gas time can be calculated by comparing the time the gas is turned on for the traditional method with that of the HRB.

For a 50% gas reduction, the HRB presents a saving of full gas time of approximately 76%, while a reduction of 33% gas produces a saving of full gas time of 69%.
Saving of full gas time= Time for water and rice to boil+Simmer time)-(Time for water and rice to boil in the HRB/(Simmer time for traditional method)

(Figure 2)
Fig 2 - Heat Retention Box Cartin.jpg

The precise difference between full gas and low gas could not be determined in this simple demonstration, but in both cases the amount of gas saving was between 70% and 75%. The percentage saving of gas will be similar to the savings when firewood and electricity are used to cook. This means that only 25% to 30% of the firewood currently used will be required compared with the traditional cooking when the HRB is used.

The HRB cooking demonstration found that most energy was saved when the washed rice was placed in 1 litre of cold water before being cooked on full gas and only once vigorously boiling placed inside the HRB for a minimum of 45 minutes.

A wide range of food can be cooked in the HRB (potatoes, lentils, beans, pasta, and stew meat) with similar energy savings. The HRB works well for items which are boiled or steamed; however it does not work for items which are fried.

The two constructions with the three layers of PE-backed reflective foil gave the best results. The hardboard box with the door was the easiest to work with. The three reflective foil HRB resulted in softer and tastier rice.

Higher Altitudes

Since for most food types cooking stops at 60-65°C, the quality of the HRB is determined by the length of time the temperature stays above 65°C (Pressure cooking at high altitudes, 2013). In high mountain areas, the boiling temperature of water becomes lower with higher altitude.

For altitudes above 1500 m, the use of a pressure cooker is highly recommended as it further saves energy consumption by 15-20% and saves time.
People living at an altitude of 1500 m already need to consider a 5°C lower boiling temperature for food preparation. When placing the hot pot with food in the HRB, the pot temperature is about 95°C (= 5°C lower than at sea level) and drops quicker to 65°C.

The HRB for higher altitudes therefore has an improved thermal insulation characteristic of RC = 2.2 m2.K/W. This allows a heavy pot with food, totalling 6-7 kg, to stay warm above 65°C for at least 4 hours. Temperature drop tests will show that with a larger hot mass inside, the temperature will stay high longer.

(Figure 3)
Fig 3 - Heat Retention Excersie Bubur - Printed.jpg

(Figure 4)
Fig 4 - HRB Cook Club - Printed.jpg

Test Results

Detailed calculations on the functioning of the reflective foils with and without PE backing found that the most effective air space between the foils was between 15 mm and 17 mm. The design was changed to include a second air space of 15 mm increasing the total wall thickness.

The tested HRB with three reflective foils and two air spaces of 15 mm had a total pot weight of 5kg or 7kg respectively. The following table demonstrates the approximate temperature drops for weight of pot and starting temperature. The 105°C is for the pressure cooker.

It is important to note that the results will vary with different quality reflective foils. The measuring tolerance is 1°C.

Table 3 Showing the relationship between size (weight) of food and temperature drop


The results correctly demonstrate that when the weight of the total cooking pot with food is larger, it will retain its heat longer inside the HRB. They also indicate that the food continues to cook in the HRB for at least two to three hours; food also remained very warm for four to five hours when placed directly from the cooking pot first into the HRB.

Most microbes are destroyed in the beginning stages of cooking when the food is initially heated to boiling temperature, keeping the food above 60° C avoids any development of salmonella (Low Temperature Cooking, 2007.

Setting up Cooking Clubs

The HRB is demonstrated to the community through cooking clubs; community members are able to trial the equipment and understand the energy savings before purchasing it. The project lends one unit of equipment) for a month or two, after which the club members can buy it or return it.

Groups of six to eight people are taught how to use the equipment by cooking their own type of traditional meals and are able to sample the dishes afterwards.
The group members are encouraged to compare dishes and recipes cooked using the equipment. The best recipes are compiled into a booklet, which is then supplied along with the HRB or pressure cooker.

The group members have access to designs of the equipment so they are able to construct the technologies themselves or through a local craftsman who can be trained by working with an experienced HRB producer who also supplies the necessary materials.
HRB works well for items which are boiled or steamed; however, it does not work for items which are fried. The HRB is an excellent energy saver for both small and large families having 5-7 members or more. For smaller families, the total quantity of food cooked will be much less and some skill needs to be developed to make it equally energy saving, as the heat retained best when the mass is large.

The HRB described has dimensions 450 mm x 450 mm x 450mm. This is large enough to contain a six litre pressure cooker and a three litre cooking pot as well as other combinations within the free internal space of 350 mm x 350 mm x 400 mm.

Step by step can be found at www.nienhuys.info page ICS.

List of References:

Biomass Energy Centre, Typical calorific values of fuels, http://www.biomassenergycentre.org.uk/portal/page?_pageid=75,20041&_dad=portal&_schema=PORTAL, accessed June 2013.

Countryfarm lifestyles 2008-2013, Instructions for building hayboxes for fireless cooking with recipes, http://www.countryfarm-lifestyles.com/hayboxes.html, accessed May 2013.

Lacalamita, T, et al., 2013. Slow Cooking Temperatures, http://www.dummies.com/how-to/content/slow-cooking-temperatures.html, accessed June 2013.

Nienhuys, S, 2012a. Basics of thermal insulation in high altitude areas of the Himalayas, Technical working paper, number 1, http://www.nienhuys.info/mediapool/49/493498/data/HA_TechWorkPaper-1_Thermal_Insulation_Basics_January_2012_.pdf, accessed 17 July 2013.

Nienhuys, S, 2012b. House improvement general. Introduction part 1, Applicable for high altitude areas such as the Himalayas of Pakistan, India, Bhutan, Nepal, Afghanistan and Tadjikstan, http://www.nienhuys.info/mediapool/49/493498/data/House_Improvement_General-HA_1_.pdf, accessed 17 July 2013.

Nienhuys, S, 2012c. Improved Cooking stove, ICS, Technical Working Paper – Number 10, http://www.nienhuys.info/mediapool/49/493498/data/HA_TechWorkPaper-10_ICS_April_2012_.pdf, accessed 17 June 2013
Nienhuys, S, 2012d. Thermal Insulation Technical Working papers #2 and #3, http://www.nienhuys.info/thermal-insulation.html, accessed June 2013.

Nienhuys, S, 2000. Research Report on BACIP Wood Stoves for High Mountain Areas, http://www.nienhuys.info/mediapool/49/493498/data/Stoves-Research.pdf, accessed August 2013.

Partnership for Clean Indoor Air, 2006, Test results of Cook Stove Performance, http://www.pciaonline.org/files/Test-Results-Cookstove-Performance.pdf, accessed June 2013.

The Engineering Toolbox, 2008, Emissivity Coefficients of some common materials, http://www.engineeringtoolbox.com/emissivity-coefficients-d_447.html, accessed June 2013.

Pressure Cooker Recipes, 2013, Pressure Cooking at high altitudes, http://missvickie.com/library/altitude.html, accessed August 2013.

BBC Food, 2007, Low Temperature Cooking, http://web.archive.org/web/20070831161204/http:/www.bbc.co.uk/food/tv_and_radio/fullonfood_meat.shtml accessed June 2013.
Last edited by marcus@hedon.info .
Page last modified on Saturday December 14, 2013 13:05:55 GMT.
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