City under power: Smart City organizes energy
by Dirk Baranek | 19.06.2019
The concept of the smart city means, among other things, a closely networked system of energy generation and use. Cities must move in this direction in order to positively influence their carbon footprint, because up to now, cities have primarily been places of energy consumption. That's about to change.
Why does the smart city make sense?
It makes economic sense if the generation of energy and its use have a local connection. If electricity is generated where people live, this saves enormous resources. Above all, it relieves the burden on the transmission system and eliminates the need for grid expansion with controversial power lines. That's why it's right for cities to think about how they can generate usable energy on their land.
Since wind turbines are prohibited in the vicinity of settlements and hydropower is only available to a limited extent, efforts are focused on two technologies: photovoltaics and combined heat and power. The smart city generates the electricity it consumes on site. Cities with millions of inhabitants are organizing millions of power generators that supply the city with electricity using large and small plants. Electricity storage with optimized charging and discharging strategies and intelligent solar monitoring systems are fundamental parts of the system to make this smart city work.
Combined Heat and Power: Almost Base-Load Capable
Northern hemisphere cities have a disadvantage in energy supply compared to those in the south: they have to use energy for heating. Sure, that's what a lot of air conditioners run for in the south, but heating buildings requires much larger amounts of energy. These capacities will continue to be supplied largely by fossil fuels in the coming decades. It is therefore all the more important to use them efficiently. This is the core of cogeneration: When heating energy is needed, the systems generate heat as well as electricity at the same time, thus achieving efficiencies of almost 100 %.
Combined, heat and power plants and fuel cell heating systems use natural gas or biogas and, when they heat water, generate electricity with the process heat. These heaters are a welcome source of electricity in the fall and winter. They produce almost without pause, making this electricity base-load capable. Hundreds of thousands of small, highly efficient power plants with an efficiency of 95% instead of a few large power plants on the city's doorstep with a maximum efficiency of 45% - these are the alternatives.
So the smart city is looking to promote heating systems that generate heat and power. At least until the use of fossil fuels is completely phased out.
Photovoltaics: Smart cities have enough space
The generation of electricity from photovoltaics is now at a very good cost-benefit level. So good, in fact, that these plants are economically viable to operate even in climatically rather unfavorable circumstances - not to mention the climate policy necessity of their use. The challenge for cities in building solar generation capacity is the space it requires. Photovoltaics require space. That puts off some contemporaries who can't get used to the public visibility of electricity-generating plants. For reassurance, one can state that not as much space is needed as might be feared.
Here are a few numbers for reference:
- One square meter of photovoltaic system generates about 200 kWh of electricity per year.
- A city of 600,000 inhabitants like Stuttgart needs about 840 million kWh of electricity per year.
- If half of the electricity for Stuttgart were to be generated with PV, this would require an area the size of 2.1 million square meters.
- For comparison: In Stuttgart, an area of almost 38 million square meters is covered with buildings. Another 30 million square meters are used for traffic areas.
Conclusion: Yes, photovoltaics requires space, but certainly not on a scale that cities could not handle. It is more a matter of activating areas in cities that make economic sense and on which photovoltaics fit organically into the urban space.
PV on the old and new roofs of the city
Photovoltaic modules will be found on every roof in a smart city when it makes sense to generate electricity there: private homes, buildings for industry and commerce, public buildings such as schools, sports halls and administrative facilities, depots, etc. The smart city will strive to fully exploit the potential that exists for electricity generation.
Hurthermore, the smart city will be inventive in finding the potential. There will be PV on bus stops, on newly covered traffic areas such as parking lots, or the top floors of parking garages.
New technical options, such as building photovoltaics directly into roof tiles, will mean that electricity will soon be generated on every roof in the city and in many places in public spaces.
Making the most of the Vertical
High-rise buildings will generate electricity with their glass fronts. There are several technical approaches to using high-rise cladding to generate electricity. Semi-transparent solar cells in window glass, for example, use only part of the light spectrum to generate power and pass on the other in the form of dimmed or colored light.
All of these approaches have one drawback: the output of these glass modules is nowhere near as high as that of normal PV modules. The special glasses hardly generate more than 10%. However, the vertical surfaces of high-rise buildings are many times larger than the built-up horizontal roof surfaces.
Another field of action for PV are railings, which in cities are not only on the balconies of houses. Bridges, terrace parapets, etc.: Suppliers are inventive and develop absolutely accident-proof elements that generate electricity.
The Masses Do It
The consequence of these many possible applications of photovoltaics is that many, many small and very small systems will generate electricity and feed it into the grid. When the sun shines strongly, a lot of electricity will be produced; when the sun shines little or not at all, correspondingly less. So the solar city is very dependent on the weather for the generation of electricity.
The use of the generated solar electricity, on the other hand, is not; it always takes place, no matter what the weather is like. Storage systems are used to close this gap, i.e. the time difference between the generation and use of electricity.
Electricity storage systems are necessary in the smart city
The sun does not always shine. Photovoltaic modules generate electricity even when the sky is cloudy, but at night they definitely contribute nothing to the energy supply. To ensure consumption during these times, the smart city will have many electricity storage devices. Whether these will all be lithium-ion batteries, tens of thousands of which are now being put into operation every year, is an open question. Scientists and engineers around the world are working on concepts to bring the storage problem as close to a solution as possible in a way that conserves resources and makes economic sense: heated salt, production of hydrogen (power-to-gas), layer energy storage, compressed air storage and much more.
The future will show which systems can be usefully integrated into urban spaces. However, the fact that there will be batteries in every building basement is a realistic scenario, at least for the near future.
Special Case of Mobility
Cities spend a significant amount of their resources on the mobility needs of their residents. The smart city of tomorrow will have a strong electrified public transport system. Individual transport with private cars will be further pushed back.
However, owning private e-cars also offers opportunities. Because the electric cars of the next generation - and only those have a future in cities anyway - will fulfill two functions: they generate electricity, and they store electricity. The surface area of a car is limited, but large enough to install PV modules on the roof and sides. These are, of course, safe for traffic. More interesting for the energy ecosystem of a city, however, are the batteries that store the energy for propulsion in electric cars. These are preferably charged when the sun is beating down from the sky.
However, the batteries could also release the electricity not needed by the cars when it is urgently needed in the power grid. These two scenarios can be realized with electric cars if users behave accordingly.
This can be achieved by rewarding behavior that is adapted to the fluctuations of the energy market. When the sun pops out of the sky, there is an abundance of electricity, and the price of electricity falls. A good motivation for e-car drivers to plug in their vehicle for charging now. When the price of electricity rises because it is dark and the plants are not producing enough, there is a financial incentive to make electricity that is not currently needed available to the grid. Electric cars are basically motorized batteries that stabilize the smart city's energy system.
City Dwellers Use Energy Flexibly
As with electric cars, it is desirable for city dwellers to adapt their energy use to the situation of the energy system. The principle is clear: draw power when there is plenty of power, and vice versa.
For example, running the washing machine at midday when the sun is at its peak and the abundance of electricity in the system causes prices to tumble. Drawing electricity in the middle of the night, on the other hand, will become more expensive. The costly battery systems from which electricity is supplied at night will have to be refinanced.
Thus, financial incentive systems, which are handled digitally and operate in real time on the daily updated electricity market, ensure adapted behavior.
The Power Grid is Digital
The smart city has a transparent and digitally controlled energy system. Extensive IT is required to shape all generators, all consumers, all storage capacities, and the influence of weather into a meaningful, coordinated system. Solar monitoring and control plays a prominent role in this process.
Real-time data is compared with historical data series to develop reliable forecasts. From this, utilities will derive market signals in the future to influence user behavior.
The smart city will not work without IT, because the electricity market, which until now has been rather static with eternally valid tariffs, will now become a much more dynamic market organism in which almost every inhabitant of the city participates. IT is fundamental to this. Market data is the basis for this.