5 November 2018

Study can help policy makers and grid operators

Solar panels in the city: consider well which neighbourhood you want to install them in

Most of the world's population lives in cities, and these are responsible for 70% of all greenhouse gas emissions. Photovoltaic solar panels can make a significant contribution to reducing these emissions. However, the energy production from solar panels fluctuates considerably, and they cannot be installed just anywhere. So how does one know which are the best neighbourhoods for investing in solar energy and what factors one needs to take into account?

Energy scientists Geert Litjens and Bhavya Kausika together with professors Ernst Worrell and Wilfried van Sark were the first to make a long-term study of all the factors that influence the installation of solar panels on city rooftops. The conclusion of their study published in the journal Solar Energy is that it makes quite a lot of difference in which neighbourhood one chooses to deploy solar energy. In one neighbourhood, for example, the ‘rooftop conditions’ may be less favourable, while another neighbourhood may in theory produce surplus electricity. Moreover, the production of solar energy may be constrained by the capacity of the local electricity grid. And here also, the study shows, the situation differs from neighbourhood to neighbourhood.

The Utrecht researchers took the city of Utrecht as their starting point, but their research methodology can also be applied to other cities. This study can be of great use to policymakers in accelerating the energy transition. It will help grid operators when they want to know what the consequences are for their electricity grids.

Differences between neighbourhoods

There are many factors that influence the amount of solar energy a neighbourhood can produce: not only the number of square metres of rooftop area, but also the slope of the roofs, the orientation (North-South, East-West, or somewhere in between), and the presence of tall buildings in the neighbourhood. It is because of these factors that the historic city centre cannot produce as much electricity as the suburbs. So there is a limited benefit for investing in systems for storage of solar energy in the city centre, as most of the solar energy produced is directly consumed. People who live in the city centre would be better advised to supply any surplus solar power they may produce to a neighbour with a ‘less favourable roof’, say the researchers. In contrast, storage systems would be sensible in the suburbs where solar energy production is higher. And for maximum effect, policy should focus on districts where solar energy would make a large contribution to reducing greenhouse gas emissions. For instance, a neighbourhood made up of bungalows would obviously be able to produce more solar energy than a neighbourhood comprising smaller houses in the city centre.

Electricity supply and demand need to be in balance, or transformers and other equipment may fail or break down


Suburban neighbourhoods have larger roof areas per household than city centres, so they can produce more solar energy. However, people's demand for electricity is more or less the same, whether they live in the suburbs or the city centre, as they all have to cook, heat their houses etc. This means that the surplus electricity generated in the suburbs could be supplied to other districts in the city. However, a local surplus does have consequences for the grid, as electricity supply and demand need to be in balance, or transformers and other equipment may fail or break down. Grid operators can anticipate this by increasing grid capacity. The results of this study can help grid operators to better estimate in which neighbourhoods the current grid needs to be changed if the full potential of solar panels is to be used.

Digital map

For all 88 neighbourhoods in Utrecht, the researchers examined structural aspects, such as building height and roof slope and orientation, as well as socio-economic factors such as family composition per dwelling and the number of cars in the neighbourhood. They used the structural data to calculate the potential surplus of solar energy that a neighbourhood could produce, or the potential production shortage. They also explored the possibilities for storage. All these data resulted in a digital map of the city that shows per neighbourhood the potential (or lack of potential) for, and consequences of, installing solar panels.

Text to be continued below map

City map of Utrecht with self-consumption ratio and self-sufficiency ratio

Self-consumption ratio (a) and self-sufficiency ratio (b) per neighbourhood if 50% of residential rooftop area is fitted with solar panels. Self-consumption ratio is the percentage of solar energy used directly in the household where it was generated. Self-sufficiency ratio is the percentage of electricity consumed generated by solar power.

The figures show that, as far as the city centre of Utrecht is concerned, for example, the rooftop area is too small to be able to meet the demand for electricity from solar power alone. This neighbourhood shows a high self-consumption ratio but a low self-sufficiency ratio, so little surplus electricity is produced and storage is not recommended.

In contrast, a neighbourhood such as Leidsche Rijn has a relatively large rooftop area available for solar panels. In this neighbourhood, almost 40% of the electricity consumption can be produced by solar panels. The self-consumption ratio is also 40%, so the remaining 60% of the electricity produced could be exported to the grid. This surplus solar energy can be stored and so can increase the self-consumption ratio and self-sufficiency ratio in the neighbourhood.

Supplementary energy sources

Our cities do not have enough roof area to supply the residential demand for energy, let alone the needs of industry and transport, so additional sources, such as offshore wind parks and solar parks outside the city are vital to the energy transition. This makes a ‘climate neutral’ city difficult to achieve. However, the researchers are keen to emphasise, we must continue to encourage investment in solar panels. Moreover: every little bit helps, even solar panels on roofs with a less-than-optimum orientation, where the yield may be 15% lower than from an ‘ideal’ roof.

For this study, the researchers did not examine the potential for solar panels on building façades. This brings so many extra variables, such as the glass surface area, that a separate study is needed.

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This study is part of the strategic theme ‘Pathways to Sustainability’ from Utrecht University, in which scientists from various disciplines are working together with external partners to develop a sustainable society.

G.B.M.A. Litjens, B.B. Kausika, E. Worrell, W.G.J.H.M. van Sark, ‘A spatio-temporal city-scale assessment of residential photovoltaic power integration scenarios’, Solar Energy 174 (2018), pp. 1185 - 1197. See https://www.sciencedirect.com/science/article/pii/S0038092X18309496.