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Why a Minimum of Five Days Autonomy is Important for Off Grid Solar

Posted by SEPCO

5/13/14 9:30 AM

Off-Grid-SolarI am often asked why a minimum of five days autonomy is used when sizing off grid solar power systems for lighting or remote power locations is used, so I figured I’d write a quick post about this to explain. There are a lot of factors that go into sizing off grid solar lighting systems, but one of the most important factors is autonomy. Here’s why:

Sometimes the sun doesn’t come out tomorrow

There are times where it is overcast or rainy all day. There is always some solar radiation hitting the earth no matter what, but you can’t always guarantee it will fully charge the battery back to full. At night, the lights will turn on, and by ensuring there is at least five days autonomy will ensure that your lights will run still even after a couple overcast and cloudy days. No one wants to be left in the dark after some bad weather.

Batteries need to last to make the system cost efficient

Rechargeable batteries come with a number of cycles. To make the batteries last, using only a fraction of the power out of them before recharging them will make sure they last longer than if drained to completion after every use. Using such items as a low voltage disconnect and oversizing the batteries for at least five days, seven or more in areas with low sun, can ensure the batteries will last the full five to seven years. This lowers the maintenance of the system and reduces the number of times you have to replace the batteries.

Temperature will affect your battery

Heat and cold both effect battery life and need to be considered when sizing off grid solar. Sizing for a minimum of five days makes sure there is plenty of power for times of extreme heat and cold, making sure the systems work properly.

After all is said and done, a minimum of five days autonomy will ensure your systems work under almost any condition. Remember, if you are in the far north, sizing for seven days or more would be recommended as the winter provides less sun and harsher conditions than southern areas. So next time you are looking for an off grid solar light or power system, make sure you are getting a properly sized battery bank to keep the system running perfectly all year round.

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Topics: Solar Lighting, Solar Power, Battery Storage, Solar Lighting Design

Battery Storage and Off-Grid Solar Power & Lighting Systems

Posted by SEPCO

7/16/13 9:30 AM

Battery storage plays an absolutely critical role in all off-grid solar power and lighting systems. In essence, all the energy produced by the solar array is stored in deep cycle gel batteries, which in turn provides on-demand power to the light fixture or device (ex. security camera, wi-fi, meter etc.)

One of the most common mistakes that many companies in the industry make is designing a system with insufficient battery storage (i.e. anything less than 4x’s the daily load requirement) which will invariably wreak havoc on a system in two ways. 

First and foremost, the system will fail under bad weather patterns (not a matter of IF the bad weather patterns occur, but WHEN!) simply because cloudy weather significantly reduces the solar array’s energy production capacity. With less energy stored in the batteries each day, combined with a constant load amount, it’s only a matter of time before insufficient back-up battery power storage is depleted, thus causing the system to fail.

Solar Power Battery Storage

Bad weather patterns notwithstanding, a system designed with insufficient battery storage will also experience a much higher ‘depth of discharge’ which simply means that the batteries will forced to work much harder because the daily load requirement uses a much higher percentage of the battery’s useable capacity each day. A high depth of discharge significantly reduces the overall life expectancy of the batteries to the point where instead of changing the batteries every 5 to 7 years, the system owner will be forced to change the batteries every 2 to 4 years.

At SEPCO, every solar lighting system we design has a minimum of 5 times daily load requirement with 7 to 10 times (or even more) daily load requirement in geographic areas with very low solar insolation like Northeast and Northwest US. This ensures not only ultra-reliable year round system performance, but also provides a robust, cost effective 5 to 7 year battery life expectancy.

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Topics: Solar Lighting, Solar Power, Battery Storage

New Flow Battery Propel Solar and Wind Power to the Grid

Posted by SEPCO

6/11/13 9:30 AM

A major obstacle to the widespread use of solar and wind power to supply the grid is the problem of storing the energy produced at a low enough cost. Energy generated by the wind and the sun is subject to unpredictable fluctuations. Until now, the cost of batteries large enough to smooth out these swings in production has made the technology relatively uneconomic.

The US federal government recognizes the importance of the problem and the Department of Energy has set up the Batteries and Energy Storage Hub - the Joint Center for Energy Storage Research (JCESR) - specifically to focus on the issue of power storage for alternative technologies. Any new technology supporting the use of renewable, non-carbon energy sources on a commercial scale is clearly of vital significance for the economy and for a sustainable future. It is the sort of development which players in the power industry need to keep abreast of through such agencies as Environmental Data Resources.

The flow battery The JCESR initiative has borne fruit: it has supported Stanford University’s Yi Cui and his team, in their work on a new type of battery. The new battery is an adaptation of the so-called ‘flow battery’.

Solar and Wind Power

Flow batteries use two different liquids, which can be stored in swimming pool size tanks, as anode and diode. They can easily be scaled up, with available space being the main limitation. The volume of stored liquid is the determining factor, as other components do not have to be changed to increase capacity. However, flow batteries utilize very expensive materials.The liquids used are themselves expensive, and they have to be kept apart by a membrane which is also costly.

A cheaper version Yi Cui’s team has adapted the flow battery to be used with cheaper materials: lithium and sulfur. Lithium and sulfur had previously been used in solid form in batteries but suffered from degradation after repeated use. The new type of battery is a combination of flow technology and lithium/sulfur technology. Like flow batteries, it has the potential for large scale use and durability, but it is much cheaper to make.

Instead of using different liquids for the anode and diode, the new battery uses one type of liquid only, along with lithium in solid form. The design is simpler, and there is no need for an expensive membrane barrier to separate different liquids. The lithium is coated with material to inhibit corrosion, which is much cheaper. The materials used, lithium and sulfur are themselves less expensive than those used in the conventional flow battery.

Next steps The prototype design has worked well in the laboratory. The next step is to produce a larger scale model, so that any problems in design and engineering can be worked on. Then, all being well, the team wants to work on engaging a power supply company in a commercial scale trial of the technology. It is early days for this technology: the work has just been reported in the May 2013 edition of Energy & Environmental Science. If it fulfills its promise of cheap, large scale, durable energy storage, we will have taken a step towards a more sustainable future.

About the Author: Tom Grant is a energy sustainability researcher. He enjoys blogging where he shares his findings to various environmental blogs.

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Topics: Guest Post, Solar Power, Battery Storage

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