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How a solar power and battery system can be designed and managed

Increasing numbers of private consumers are choosing to invest in solar panels for their homes, and the market is expanding considerably. One way to increase your personal usage of solar energy is to use battery storage systems. RISE has investigated the possibilities of how best to combine these for small houses in order to make it a sound investment.

When the sun is at its brightest in the middle of the day is when demand for electricity from most households is at its lowest. In order to be more self-sufficient in terms of energy in a country like Sweden, it’s necessary to have the capability to store energy for use when demand is greatest.  Even though batteries for storing solar energy are currently available on the market, to date few private consumers have invested in these, although we are seeing increasing interest.

– “I believe in batteries as the future technical solution for round-the-clock storage of solar energy. The most economic approach today is to use solar power for private consumption, so that you avoid purchasing it from the grid,” says Patrik Ollas, who is an industrial doctoral student at Chalmers employed by RISE.

Another reason for investing in batteries is that they provide a back-up if the grid goes down at times of increased risk of natural disasters and power cuts. The cost of batteries has fallen in recent years, in part thanks to technical development within the vehicle industry. It is also possible to use old car batteries for storing solar energy, so-called second-life applications.

Managing the battery with the aid of weather forecasts

In order to make battery storage a more economic investment, Patrik Ollas and his colleagues at RISE are researching the possibilities for optimising batteries by means of, among other things, better control using so-called forecasting in a project funded by the Swedish Energy Agency. In collaboration with Uppsala University the team are studying how weather forecasts can be used to predict how much solar power will be produced over a future period of time.

– “It’s about predicting, with the help of weather forecasts, how much solar power will be generated during the coming days. We are also working on models that with the aid of machine learning will be able to predict the pattern of consumption for a house over a future period of time. With the aid of AI you should be able to predict that the pattern of consumption will look a certain way at a certain point in time,” says Patrik.

The energy produced during the day can be stored and then used during the evening, night and the following morning

By finding out what the pattern of consumption will be, how much solar power will be produced and what the electricity price will be over the next 24 hours, you can plan control of the battery to ensure usage is as economic as possible.

Systems need to adapt to users

RISE researchers are also looking at how to design a system with solar power and batteries depending on the consumption pattern of the house. They are working with electricity company Herrljunga Elektriska, which has shared consumption data from 3,000 users. The project team are trying to sort the users into different categories depending on their pattern of consumption. One category is for families with peaks in the mornings and evenings. Another category is for those who are at home and active during the day, and yet another for those who have high consumption at night. The aim is to try to determine how to design a system with solar power and batteries for these differing types of users.

– “When investing in solar power at present, fairly simple assumptions are made when looking at the type of system to have. The factor of particular interest is the consumption profile. For those with a high level of consumption in the middle of the day when the sun is shining, direct solar power is of greater relevance. However, for a typical family with consumption peaks in the mornings and evenings, a battery system would be more relevant, since the energy produced during the day can be stored and then used during the evening, night and the following morning.”

External services for the electrical grid

Another way to make investments in battery storage a better economic prospect is to have the battery provide services to the external electrical grid. One example of this is so-called frequency regulation. The electrical grid in Sweden has a frequency of 50 Hz, but this fluctuates during the day depending on actual production and consumption.

– “We are looking into the possibilities of having the battery linked up and supporting the grid by charging and discharging, and receiving remuneration for this service from the energy companies. Even though the rules and regulations are not geared to this at present, it would be interesting to see how much you could earn from a service like that.”

Direct current can save energy

Much of the research relating to battery storage takes place in the so-called Research Villa at the RISE facility in Borås, which enables testing of how the models function in reality. It has also been used to test a system with solar power and battery storage in combination with a direct-current system for saving energy. The solar cells produce direct current, and energy can be saved by not having to convert it into alternating current.

– “One of the challenges with direct-current distribution in buildings is that there are no products on the market designed to run on direct current. So it’s not the technology that’s the problem here, but rather the soft values,” concludes Patrik.

Read the final report from "Probabilistic Forecasting for Battery Managment" at Energimyndigheten:…

Patrik Ollas

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