Complete photovoltaic-based power supply for Germany – III. Supply with constant solar power, storage and networks

Heiner Eckert Grafik Photovoltaik Gleichstromkabel Solar Energie

Each energy form has specific characteristics with regards to supply, application and efficiency. It is here in particular, that the otherwise unachievable benefits of an electricity-based primary energy supply are evident. Energy per se can be described as a mass of “free” electrons in atoms, usually metals, and molecules, or in the form of photons as radiation energy, linked by the Planck’s quantum effect h [Js=Ws2

E = h*f[Ws]

Where E: Energy [Ws], h: Planck’s constant [Ws2], f: Radiation frequency] [1/s]

This gives rise to the photo-electric effect where electrons are knocked out of materials, such as doped silicon, by the photons in the solar radiation. Einstein received the Nobel Prize for physics in 1921 for his discovery in 1905, and this effect is the basis of photovoltaics (PV) and therefore solar energy (refer back to the two previous articles about optimal technical generation of solar energy: Articles I. and II.). 

The conversion of electrical current into mechanical energy is virtually quantitative in an electric motor (>90-99%, depending on heat lost) [1]. In comparison, the system efficiency of Otto and diesel engines is significantly lower with just 25-30%, while fuel cell-based mobility also only reaches 45% [2]. 

ENERGY STORAGE

The rather cumbersome route involving PV → electrical current → hydrogen → fuel cell → electrical current →electric motor is based on using hydrogen as an energy storage medium. This is no longer necessary when increasingly powerful and inexpensive batteries are available, especially when considering the poor efficiency of that route. However, hydrogen may play a significant role for CO2 avoidance in large-scale plants for metal and cement production. Stationary hydrogen technology could be usefully implemented in discontinued nuclear power station premises. 

A certain amount of scepticism is required regarding any mobile applications using this transparent and odourless gas. This is due to: 

  • The extreme danger of explosion (explosion limits 3-80%) in enclosed areas such as underground garages and tunnels 
  • The low ignition temperature (textile friction is sufficient) 
  • The high flame velocity (8x higher than natural gas)
  • The high-pressure hydrogen tanks (700 bar!) 
  • The lack of heat conductivity of the almost invisible flames (high UV radiation percentage – no sensation of heat directly at the fire area) 
  • The huge explosion clouds [3]. 

The above-mentioned properties of hydrogen can result in nasty surprises, even in minor car accidents.

The greatly varying PV outputs during daily and seasonal cycles means that compensation measures are necessary for a constant power supply. Based on the property described above of “free” electrons, a battery is inherently the optimal and best storage media for this system. Lithium-based high-performance cells are practically irreplaceable for mobile systems, such as those used in the traffic sector, due to the high power density and low weight. 

Research continues apace with the focus on “solid glass batteries”, which do not contain liquid electrolytes, and which therefore minimise the risk of fire [4]. 1 kg lithium in an electric car can generate 10 kWh current and therefore a range of approximately 100 km. If you compared 50 kg lithium with the equivalent 50 kg fuel or diesel in a combustion engine, lithium would provide a range of approximately 5,000 km. Just this comparison alone highlights this unique feature of lithium. Lithium batteries with a high level of performance are already available on the market. 

Tesla built a giga-power plant with an energy content of 130 MWh and an output of 100 MW. Just one plant supplies 30,000 households in Australia with electricity, Figure 1 [5].

constant solar power, Tesla giga-power plant with 130 MWh energy content and 100 MW output.
Tesla giga-power plant with 130 MWh energy content and 100 MW output.

The elasticity (factor 8-fold) of high-performance lithium batteries is exhausted after approximately 10 years, but this does not affect the storage capacity. This is entirely sufficient to act as a storage unit for household use (output approx. 3 kW) or for other applications that require less power. Batteries based on the metals sodium, magnesium and aluminium are also suitable for pure storage purposes without high peak loads; such metals are readily available and easily accessible earth elements. 

The development of stationary sodium-sulphur batteries is also making rapid progress [6]. A lack of resources is therefore not to be expected in the future, which means that other methods for energy storage are moot, apart from special solutions. Essentially, all energy storage solutions that use thermodynamic methods are unfavourable, because their efficiency cannot rise far beyond 30%, due to the second law of thermodynamics.

NETWORKING

As the sun shines at some point in time all around the world, this would presuppose an elegant method to maintain a constant power supply which per se would not need a power storage function. Eurasia ranges from the west coast of Spain to the east coast of Japan across 150 latitudes, which is equivalent to an interval of 10 hours and a distance of 12,000 km. If we laid a direct cable from Spain to Japan and operated PV units along its route, each location along this route could extend their fictive sunshine duration by 10 hours. This is a major factor, as the day and night equinoxes on March 22 and September 21 would each provide 22 hours of sunshine (instead of 12 hours) for global PV: 100%: 24 x (24+10) – 100 = 42 % more sunshine falling on the PV units. 

The additional electricity from China would raise morning outputs in Europe right up to midday levels, whereas the evening hours in China would be compensated by electricity from Europe. As direct current cables already transport electricity over several thousand kilometres, a 12,000 km long cable would be a challenge, but not a technical adventure. China is currently building roads and railways along the silk road, and this would be ideal for power lines as the route for the direct current cables is already prepared, the cables would only need to be laid. The route itself is naturally an ideal location for PV (see Complete PV-D-II) and also lends itself to improving political relationships.

REFERENCES

  1. https://www.pro-physik.de/nachrichten/elektromotoren-specken-ab
  2. https://www.energieagentur.nrw/brennstoffzelle/brennstoffzelle-wasserstoff-elektromobilitaet/wirkungsgrad
  3. https://www.fwvbw.de/fileadmin/Downloads/Einsatz_Wasserstoffleitfaden.pdf
  4. M. A. Kraft et al., “Lithium Superionic All-Solid-State Batteries”. Journal American Chemical Society 2018140, 16330-16339.
  5. https://cleantechnica.com/2017/11/23/tesla-completes-worlds-largest-li-ion-battery-129-mwh-energy-storage-facility-south-australia-notfree/
  6. https://www.energie-experten.org/erneuerbare-energien/photovoltaik/stromspeicher/natrium-schwefel-batterie.html