Mesh microgrids – a more efficient off-grid future

by | Dec 19, 2017 | 0 comments

At Okra our engineers have been slaving away for the last 12 months to develop a plug & play grid in a box allowing companies to easily create their own microgrid networks.

The reason we decided to build this technology is because Stand Alone Solar Home Systems (SHS) have the following issues:

30% is wasted

On average more than 30% of power generated from SHS is being wasted – meaning a higher number of generation assets must be purchased compared to those needed within a distributed network.

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Shortened battery life

When batteries are frequently being discharged to a low level this damages the battery and shortens its life. In a distributed network algorithms can ensure batteries aren’t heavily discharged by sharing power in the network.

Blackouts are common

Due to variation and unpredictability in household energy consumption, on certain days battery storage inevitably goes to zero resulting in blackouts.

Okra is a plug & play device which enables a network of solar generation and storage – this improves power utilisation efficiency by 50%, increases lifetimes of batteries (most expensive part of a network) and removes the risk of blackouts occurring for off-grid households.

To back up the above claims we’ve run some simulations and modelling using data from our pilot in Prey Pdao, Cambodia. Firstly looking at the data from the five houses we can see a high amount of variation in how much energy each household consumes on any specific day and also significant differences in the total amount of energy each household consumes:

Under SHS this variation results in a high amount of power being wasted when batteries fill up on low consumption days and possible blackouts on high consumption days. However with a distributed microgrid network when one household has low consumption, excess power can be shared to the network and on days when consumption is high additional power can be drawn from the network. This can be achieved while ensuring no individual battery is excessively discharged (maintaining lifetime of batteries).

For our simulations we’ve assumed there are 50 households in the village, each has randomly been assigned one of the load profiles of the five houses from our pilot. We’ve assumed that under the SHS model each house is fitted with a 100Wp panel and a 65Ah Battery. To confirm our assumptions we’ve developed a model in Python where we can simulate hourly generation and consumption for each household while taking into account various power losses associated with sharing power and charging/discharging batteries.

Through our simulations we can monitor the state of charge (SOC) of batteries, amount of power wasted, and the number of blackouts that occur under both SHS and an Okra network. Our simulations suggest under a distributed microgrid model we would only require 33 100Wp panels to provide the same (actually better, because of reduced chances of blackout) quality of power to households. Our simulations suggest we can reduce battery size per house from 65Ah to 50Ah. This means a lower capital expense for the distributor when setting up an Okra network compared to SHS.

Here is a summary of some key figures from the simulations we’ve run:

SHS OKRA
Total Power Generated (Kwh) 7000 4690
Total Demand During Blackout (Kwh) 150 0
Total Power Wasted (Kwh) 3000 578
Upfront Capital Expense* $10,400 $9,917

We can see that for SHS there is a significant amount of power being wasted – this typically happens as it is hard to accurately size a system for an individual household which will typically lead to either the system being too large (High Amount of Power Wasted) or too small (High number of blackouts / short battery life).

Another advantage of Okra’s technology is that by using smart algorithms we can control the depth of discharge of the batteries at each household, ensuring no single battery is too deeply discharged, and as a result does not get damaged and have a longer life..

Using the manufacturer’s specifications for lifetime of batteries at different depths of discharge we can compare to the daily depth of discharge for batteries in an Okra network vs Stand-Alone solar systems to predict how often batteries need to be replaced.

This table shows the number of batteries that need replacing each year under both SHS & Okra:

Year SHS – Battery Replacements Okra – Number of Battery Replacements
1 10 0
2 10 0
3 10 0
4 50 50

This means that over the first 3 years of operation for the SHS 30 new batteries must be purchased due to being heavily discharged. Under Okra because each batteries depth of discharge can be controlled by sharing power in the network ensuring the life of the batteries is optimised. The below chart compares the daily minimum state of charge for one particular houses battery under both Okra & SHS.

Currently SHS is the most widely used solution that Desco’s (distributed energy service companies) use to electrify households in off-grid communities. This often leads to a number of issues such as high amounts of power being wasted, households experiencing blackouts and batteries having to be replaced frequently due to damage. These simulations show that by using Okra’s distributed micro grid technology Desco’s will be able to provide off-grid households with a far more reliable supply of electricity at a lower up front cost with longer lasting batteries.

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