Sky imaging: a necessary complement to storage for efficient operation of hybrid solar-diesel off-grid power plants

[From HPAC Engineering]. In recent years, solar-diesel hybrid applications have become more and more interesting as the technology allowing for integrating photovoltaic (PV) energy into diesel power plants has improved considerably and PV investment costs have decreased.


Typical daily solar+storage power diagram (source

Technically, the main challenges have arisen from the unstable energy generation of PV power plants and the fact that traditional diesel gensets are not very flexible and can hardly deal with loads of less than 30%-40%. The whitepaper shows how accurate forecasting can help address these challenges and boost hybrid system efficiencies.


Besides storage, a second solution for dealing with intermittencies in a solar-diesel hybrid project is prediction. If sudden output changes in the solar power plant can be forecasted, then additional spinning reserve can be provided for certain time periods although not constantly.

Sky-imaging-SteadySunSolar power forecasting provides the ability to adjust the spinning reserve dynamically. In addition, knowledge about the duration of power output changes allows the optimization of the operation of the assets, both for the diesel gensets and the storage system. In the end, solar power forecasting can significantly reduce the Levelized Cost of Energy (LCOE) of solar-diesel hybrid systems.

The forecasting of the irradiation and the production of the PV plant is extremely reliable in the short-term, with sky-images cameras (like the one pictured here supplied by SteadySun) taking hemispherical photos every minute, while a forecasting algorithm provides power production forecasts for periods from 1 minute to 1 hour.

You may download the whole white-paper (10 pages) here [pdf], with contribution from The Energy.


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Solar Action Alliance | All what you need to know about solar PV

Solar Action Alliance is a company of environmentalist who want to spread the word about the use of solar energy and how to reduce the consumption of fuel for power generation.

Their website is full of (free) resources about the state of adoption of solar energy in each State, plus solar irradiation maps, regulations and links to a blog about case-studies across the USA and overseas.

According to their website their first initiative is “a free program which is designed to talk you through the nitty-gritty of solar installations including the best products to buy, the amount of money you need to kick-start your solar panel rooftop project, how your state’s government offers incentives in various forms and all the ways you gain financially from your solar initiative.”

We guess it’s worth take a tour of the website here.

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JuiceBox Energy Storage start-up in the US market

Energy storage start-up JuceBox has entered the competition for storage systems among Tesla, Sonnen and (soon to come) ABB with what they claim is an easier-to-install product.

The first JuiceBox system, an 8.6 kWh lithium-ion storage system is addressing residential and small scale commercial buildings. The JuiceBox is integrated with a full-featured commercially available inverter/charger and can be deployed in parallel for higher power and energy needs.  The system is designed to support grid-tied, grid isolated in the event of grid failure, and off-grid configurations.

The system is also ready to provide grid services such as demand response, potentially improving the business case for installation.

The battery chemistry selected by JuiceBox is a lithium-ion Nickel Manganese Cobalt (NMC) supplied by Samsung. The JuiceBox system delivers a minimum of 4000 cycles and 10-years of operation with this cells with 4 kW rated power in 2 hours discharge and 5,5 kW of maximum power

All JuiceBoxes are connected to the Cloud through a robust, secure cellular connection. There are no fire walls or router issues to deal with for installers and homeowners.

As of the price tag, according to an Energy Storage release “End users currently pay in the region of USD $12,000 for the JuiceBox system, including a $9,900 battery pack with Samsung SDI cells and a 30% Federal tax credit, Schneider Electric inverter, installation, wiring and so on.”

Assuming installation and overheads account for $1,500 the storage system alone is priced at $8,400 – which is less than $1,100 per kWh of real power (8 kWh) – a big deal below Sonnen and Tesla, according to the prices releases here. Good news for installers and consumers.

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Leasing Solar Power to a Million Rural Customers in Tanzania | by Eff Grid Electric

Off Grid Electric, an American company called Mpower in Tanzania, which has built a reputation for delivering cheap “solar as a service,” will lease kits for $10 that generate enough power for lighting, mobile phone charging, and running a TV.

Among the statements from  teh post we highlighted these:

“We redirect funds they were already paying for kerosene and batteries to a solar lease payment”

“The company boasts that it’s delivering the best possible equipment, rather than the cheapest hardware or the biggest profit margins.”

Read the full – amazing – article from the source here: Leasing Solar Power to a Million Rural Customers in Tanzania | ChEnected

Watch their video:

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What is PID and how can you reduce solar power loss?

Potential induced degradation (PID) is a phenomena that has only recently become a concern in the photovoltaic industry. PID impacts the ions of a solar cell and results in the degradation of the o…

Source: What is PID and how can you reduce solar power loss?

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Off-grid power to remote locations: a balanced mix of renewable and fossil sources

There are articles around the web assuming that off-grid power can be supplied by 100% renewable sources coupled with storage facilities. In most actual situations that’s not the case.

When approaching the sizing of a power supply system two parameters are essential: [1] the availability rate and [2] the variations of the power source over the year and over the day.

The reason is that you cannot ensure 100% availability of a source which is itself not 100% available, unless you increase the size of the storage to a degree which makes the system either not financially sustainable or technically impossible to be operated efficiently.

The availability rate differs widely depending on the users served by the power system. Typically, the required availability is around 95-98%. In some cases (e.g. oil pumping systems, mobile telecom infrastructures) the rate must be close to 100%. When talking about solar power, data sources provide the typical reverse-bell daily irradiation diagram, but no clues are available about the variations (actually the mean square variation) around the average value.

You must then make assumptions according to historical weather data to define the number of continuous days of full clouds your system would be required to face in order to grant the users with the required availability rate.

Another point of concern is the seasonal variation of the irradiation.

If the PV generator is sized to supply enough energy during the winter (or low-sun) season, you must expect to “waste” energy during the summer – unless the user has seasonal power-loads consistent with the seasonal variations of the solar irradiation (like e.g. seasonal farming).

It won’t be even thinkable to have batteries sized to store months of energy for only a 2-cycle duty per year (unless you are capable to find other means to store the energy locally).

There is also a relationship between the size of the solar generator and the storage size. The batteries provide for two services: [1] to store the energy produced by the PV plant in excess of the energy supplied instantaneously to the user and [2] to supply the loads with energy as the PV plant is not producing. The point is that the lifespan of electrochemical batteries is affected by the number of charge-discharge cycles as well as by the completeness of each cycle. The more the cycles are complete (according to the set-point provided by manufacturers) the more the batteries last. So the number of cycles has also to be factored in the economic simulation.

What it turns out in most of the cases, is that the optimal system’s solution requires a programmable source of power – typically a diesel or LNG genset.

The genset has two functions: to recharge the batteries during low-sun periods (and/or to feed power directly to the loads) and to contribute to reach the required availability rate. The good point is that given that the batteries may supply (for short periods) a power capacity higher than the generator’s peak power, the generator’s size does not have to bear the full loads’ capacity.

Simulations with a spreadsheet help come to the optimal mix of solar power and fuel-powered generator in a proportion generally around 80/20 of the total energy supplied.

Which is indeed by far cheaper – and environmental welcoming – than a 100% diesel-fuelled power system.

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Guidelines for grid-connected energy storage systems by DNV-GL

DNV-GL-guidelines-storage-2016[18-01-206] Guidelines for grid-connected energy storage systems by DNV-GL have been issued at the end of 2015. The comprehensive document covers the safety, operation and performance of grid-connected energy storage systems. These aspects have been assessed for electricity storage systems in general, with emphasis on lead-acid, Li-ion and redox flow technologies and Li-ion capacitors.

The proposed guidelines are limited to common requirements, based on worldwide accepted regulation and best practices like IEC, ISO and IEEE standards. Any electricity storage applications at the high, medium and low voltage grid level as well as home energy storage are considered within scope of the document – which is available for free download formthe DNV-GL website here >>.




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New investment model for the off-grid-energy sector in Africa

shelter new House

A new investment model for the off-grid-energy sector in Africa that seeks to bring securitization to the off-grid sector in Africa has been developed by the Dutch investment firm Oikocredit International teams with London solar innovator BBOXX.

Backed by New York merchant bank Persistent Energy Capital LLC, the program aims to raise up to $2 billion over the next five years to turn solar in Africa into an asset class, and creating contracts for thousands of solar rooftop arrays to sell as bonds to investors.

Read more from the PV magazine >>.


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Enel Green Power, Powerhive to develop solar grids in Western Kenya

[11-01-2016] From PVTech. Enel Green Power is partnering up with mini-grid technology solutions provider and developer Powerhive to construct and operate mini-grids in 100 villages located across Kenya.

Agrialma 1

Farm-building roof-top solar PV (credit Solar Farm Srl)



The grids will require an investment of around US$12 million over the span of 2016 — 93% provided by Enel Green Power and 7% from Powerhive.

The project, which will be developed by Powerhive, will be comprised of solar mini-grids with a total installed capacity of 1MW and will be built in the Kenyan counties of Kisii and Nyamira. Once completed, the grids will provide clean energy to 20,000 homes, businesses, schools and health care centers, powering around 90,000 people in the process.

Francesco Venturini, CEO of Enel Green Power, said: Kenya’s rich and differentiated technology mix in the renewables industry offers a quite unique platform for the business development of Enel Green Power in Africa. This country couples a low electrification rate, still in the range of 30%, with one of the highest mobile phone penetration rates of the region, thus making this union ideal to implement innovative solutions able to integrate the electrification of rural areas with generation from renewables, storage facilities as well as advanced billing systems.”

Integrating these grids with energy storage facilities will give these systems the ability to balance supply and demand, reducing concerns over offset variations in customer loads and unpredictable spikes in power generation.

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Demystifying microgrids

glob-off-grid-mapMicrogrids are multiple generation points and loads under one control system that is optimising the generation and the power distribution to the users. Microgrid refers to distributed energy resources and loads that can be operated in a controlled, coordinated way, optimizing the use of the grid resources.

The main features of microgrids are:

  • they can be connected to the main power grid, operate in “islanded” mode or be completely off-grid;
  • they can be supported by storage systems and grid-regulation systems
  • compared to centralised generation systems connected with multiple-voltage grids,  they account for more reliable and better quality power supply, grid resiliency, source independence almost no grid losses;
  • can be tailored to each customer’s requirements;
  • microgrids are scalable and there is no cap as to size; can power remote mines, industrial and commercial sites, villages, communities, clusters of farming facilities.

Microgrids can deliver energy at a lower cost compared with the energy supplied from far-away generation points, since they are optimized for the the local conditions. Microgrids work at best when exploiting local sources of fuel – typically renewable sources – and geografically concentrated loads.

The trade-off between a centralised system and a microgrid is based essentially on the assesment of transmission costs and generation costs.

An interesting post about how India is planning to develope microgrids can be found here:

A new frontier for the development of microgrids is the financial sustainability of a business model in which there is not a major off-taker signing a PPA to grant the power purchase for the time required to pay back the investment.
Marco Bonvini

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