Integrating Hot Water, Hot Iron, and Long-cycle-life Batteries to provide Off-grid Solar PV Cooking Energy Storage in Malawi

By Robert Van Buskirk, Kachione, LLC.

This is the third piece of a series of blogs by the Kachione team who are pioneering Solar Home Systems inclusive of cooking. Kachione LLC is a clean cooking provider in Malawi which has received grant funding from the MECS Challenge fund. In this blog, Robert Van Buskirk shares lessons learnt in the process of integrating three types of energy storage in Kachione’s efforts to lower the cost of off-grid solar PV cooking in rural Malawi.

Kachione LLC is dedicated to the goal of providing solar electricity access to the lowest income households in rural Malawi—including cooking with solar photovoltaic (PV) electricity. In this blog, we describe how this goal has defined our approach to addressing energy storage issues in a solar home system (SHS) that includes electric cooking.

Because we serve very-low-income communities, the primary design objective of most of our technological innovations is minimizing the cost of energy delivered. We have therefore prioritized cooking energy storage technology options from least-cost to most-costly as follows:

1. Direct-use DC Solar (DDS) electricity use: The cheapest battery is the battery that you don’t have to buy.  By using solar panel electricity with no intervening battery, a solar system can cook with no battery costs incurred.  DDS cooking is very inexpensive, but somewhat inconvenient. Unlike some thermal solar cookers, the cook does not have to stand in the sun while cooking as a wire can bring the pot into the shade, but it does still mean that one has to cook at certain time of the day.  A highly insulated DDS cooker can slow-cook dishes for an evening meal, or a DDS cooker can directly cook dishes during the middle of the day.
2. Hot Water for Cooking and Washing: Solar electricity can replace some portion of household biofuel use when solar electricity from a solar PV panel heats hot water in an insulated container and then that hot water is used later for cooking or washing. Hot water is an inexpensive way of storing PV energy output that otherwise might not be used in a solar DC system with cooking
3. Hot Iron in a Pre-heated Cast Iron Cooker: We find that a cast iron pot can weigh 6kg or more and costs approximately $5/kg at wholesale (container-scale) prices delivered to Malawi. One kilogram of cast iron stores about 15Wh of heat energy when its temperature is raised 120 deg. C.  Thus a pre-heated cast iron pot raised to 220 deg. C has energy storage that can be used for cooking food that costs about $0.33/Wh.
4. Long-cycle-life Batteries: We find that currently very-long-cycle-life lithium titanate battery cells with a cycle life of greater than 10,000 cycles have a factory door (container-scale) cost of about $0.20/Wh. Factory door assembled battery costs are about $0.50/Wh

Naturally, the most convenient, flexible and reliable form of energy storage is also the most expensive.  To address this issue, Kachione LLC first introduces customers (after enabling them to achieve lighting for their house) to a low-cost SHS with cookers that initially utilize the lowest cost forms of cooking energy storage (i.e. DDS and hot water).  The process is then conceived of as allowing customers to gradually upgrade their SHS to include larger and larger long-cycle-life batteries and cast iron cookers to extend their ecooking opportunities.

Note that because customers may be upgrading their cooking energy storage over many years, it is important for any cooking energy storage capacity in our SHS systems to be able to last 5 to 10 years or more.  If the energy storage components of the SHS lasts only a few years, then within those few years, customers won’t be able to afford to increase the cooking capacity of their SHS. Instead, they will be spending whatever little money that they have on maintaining the energy storage capacity of their existing SHS rather than upgrading their system capacity so that they can eventually cook all of their food with solar electricity rather than wood.

DDS and Hot Water Solar PV Systems

A simple DDS cooker that can also heat water can be rather simply made by attaching resistive heaters to a metal pan and then nesting the pan in a larger pan and insulating the space between the pans.

We observe customers using DDS cookers primarily for heating water which is then used in cooking or for washing and bathing.

One version of the DDS cooker uses an aluminum manual pressure cooker as the inner pot.  With the pressure-cooker DDS cooker, customers tend to use the cooker for both heating water and cooking beans.

Building a 10-Cell LTO Battery

In order to increase cooking convenience beyond what can be provided by DDS cookers, energy storage in a battery can be considered.   To keep the costs really low, beyond the more commonly found Lithium Iron Phosphate batteries Kachione LLC has developed prototype cooker batteries using lithium titanate (LTO) battery chemistry. LTO batteries have a cycle life that is greater than 10,000 cycles which theoretically should enable such batteries to last more than 10 years even when cycled several times per day.   LTO batteries can also charge and discharge rapidly without substantial degradation, and have a great safety record.

A picture of an initial prototype 10-cell LTO battery is shown in figure 3. The average voltage of an LTO battery cell is about 2.4 volts so this corresponds to a 24-volt battery.  The factory door price of a 10Ah LTO cell can be as low as $4/cell, so the per Wh cost of LTO battery cells is about $0.17/Wh.  (To put this in context, Lead Acid at around 500 cycles of 50% depth of discharge (DoD) can be at about $0.4/Wh (due to the low useful DoD and cycle life), and Lithium Iron Phosphate at around 3000 cycles and 85% DoD is about $0.15/Wh). 

In contrast, the apparent factory door cost of assembled LTO batteries with battery management electronics appears to be at or above $0.5/Wh.  This implies a retail distribution cost of about $1/Wh.

How do we make long-cycle-life batteries affordable to rural Malawians? We at Kachione LLC are developing the capacity to assemble LTO batteries from individual pouch cells in Malawi. Such assembled batteries will also have the attendant control and battery cell management electronics customized for cooking applications in Malawi.  

Summary and Conclusion

By using in-country assembly of custom LTO batteries, we at Kachione LLC are now in the process of getting the retail cost of long-cycle-life LTO battery storage to be substantially lower than $1/Wh.  If half of the storage needs of a cooking event can be provided by hot water and hot iron rather than an LTO battery, then the average cost of total stored cooking energy in an SHS system can drop to less than $0.5/Wh in Malawi with the possibility of similar cost declines in other African countries. 

Amortizing a $0.5/Wh average energy storage cost over more than 5000 cooking events over the course of 10-year system lifetime, means that the energy storage cost per unit cooking electricity can be less than $0.10/kWh.

We at Kachione LLC believe that over time, customers will increasingly value and recognize the benefits that that accrue when an SHS with cooking attains an electricity storage cost of less than $0.10/kWh.  We believe that meeting such a cost target will then eventually make off-grid solar electric cooking systems convenient, affordable and accessible to virtually all households in rural Malawi: including those with the lowest incomes. 

And for us, THAT is the primary goal and objective.

Wish us luck!

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Featured image, top: A simple Malawi-made DDS cooker that is made by attaching resistive heaters to an inner pot and nesting it inside a larger pot with insulation. [photos courtesy of Gilbert Robert and Robert Van Buskirk, Kachione LLC].