mandag 9. september 2013

The new combination of solar cells and super caps

Av Øyvind Sunde Sortland, Thomas Holm, Guillherme Gaspar, Denny Ehrler, Sandro Hommel and Peng Liu, NorRen Summer School 2013


Motivation

The share of electricity generation from solar cells is increasing rapidly and already has a high share in countries like Germany and Italy. A high share of photovoltaic (PV) electricity generation poses challenges for grid operators to provide stable electricity supply due to the highly intermittent solar radiation, varying over time scales of minutes as shown in Figure 1. The output power of PV systems can drop from maximum to a very low value and increase just as suddenly due to passages of clouds.


Intermittency of power production measured for a 2 kWp PV system during a day.

The minute scale fluctuations in PV power supply are particularly detrimental to grid stability because they are not predicted by weather forecasts. Grid operators would thus benefit from a smooth power supply profile, which provide an opportunity for integrating short-term storage in the PV systems. This post provides a preliminary assessment of the potential for supercapacitors (Electrochemical Double Layer Capacitors, EDLC) to complement batteries for short-term load leveling before feeding the grid. Integrated household and neighborhood systems of renewable energy production and storage also have potential to provide local load-leveling.

 

Potential and challenges of supercapacitors in load-leveling 

Supercapacitors are normally used for short-time charging (range of seconds) while batteries are considered only for longer-term charging (range of hours). Radiation is intermittent most of the time, which feeds an intermittent power supply for several times a day. This could be an opportunity for capacitor applications, which besides short-time charging time, can last for hundred thousands of cycles. The lifetime of supercapacitors also fits the normal lifetime for a solar module (from 15 to 20 years), and does typically not need replacement within the lifetime of the module, which is typically necessary for batteries with a lifetime of up to 10 years. The main disadvantages are the low energy content and cost per Wh, the latter being one order of magnitude higher than the batteries. In a daily range of time, the batteries are more reliable because of their larger energy density. According to this, a hybrid system composed by supercapacitors and batteries could be a good solution for a PV system incorporation to the grid. in order to cover the intermittent of the power supply in a short range of time and also accumulation from day to night.


Comparison of key parameters of supercapacitors and batteries.

According to Kim (2010), an efficient combination of solar cells and supercapacitors requires DC/DC conversion and a maximum power point tracking (MPPT). MPPT allow both the PV system and the supercapacitors to operate at different voltages for maximum combined efficiency. A single module combining solar cells, a DC/DC converter with MPPT control and a supercapacitor bank has potential to smooth the short-term power drops of PV electricity production shown in Figure 1, by discharging the supercapacitors during minutes of low PV power production and recharging the supercapacitors after the PV power output is restored. The high temperatures generated in solar cells poses a design challenge for such a combined system as the DC/DC converter and supercapacitors should operate close to ambient temperature in order to retain the specified lifetimes. A proposed combined module in Figure 2 incorporates a water heat exchanger for the combined purpose of heating water for domestic use and cooling the module.

Proposed module for combined PV energy generation, water heating and supercapacitor bank
(area of supercapacitor materials exceeds area of solar cells by orders of magnitude).

Such combined modules also minimize increases to installation costs, which currently amounts to over 55% of the total costs of typical rooftop PV systems. However, supercapacitors are currently not an economical alternative for load-leveling as the capacitor size needed to deliver energy equivalent to one minute PV power production is an order of magnitude more expensive than current PV modules.

 

Household energy storage system

Another opportunity to avoid grid stability problems by fluctuating energy feed in could be household energy storage system. This system offers the possibility to store the generated energy from the PV-system. Thereby it is possible to use your own generated energy.


Proposed electrical components in household power generation and storage system.

When the sun is shining, the generated energy will be firstly used to supply the consumers in the household. If there is more generated energy than necessary, the energy will be stored in the storage system. Only if the storage is fully charged, the energy will be fed into the grid. If there is not enough solar energy for supplying all appliances, the energy will be delivered from the storage system. Only when the storage is empty you need energy from the grid. Another advantage is the possibility to build a standalone system if there is no grid connection.

In the future, energy price policies are expected to change. There will no longer be just one energy price for consumption. There will be maybe three or four different prices dependent on daytime or season. Then it will be possible to do a price optimization by having a household energy storage. If the price is low, the storage will be charged. If energy is required in times of a higher price it will be provided by the storage system. The second step could be to join into the active energy market. The energy can be stored in the storage system and be sold when the price is high.

 

Conclusion

Very rapid fluctuations of the power output of solar cells are detrimental to grid stability in an energy mix with a high share of PV power generation. Supercapacitors have technical potential for load-leveling of unpredictable fluctuations in time scales of minutes and can complement battery storage over larger timescales. A module combining solar cells, domestic water heating and supercapacitors is proposed for its potential to deliver stable power according to forecasts. Currently, supercapacitors are not an economic alternative for load-leveling of PV systems. A household PV generation and storage system is an alternative in which the intermittent PV power is not directly fed to the grid, but rather used or stored locally in the household in time scales of hours and above. Such energy storage opens additional trading opportunities for the households, which corresponds to load-leveling of the grid.

 

References 

Kim, Y., Chang, N., Wang, Y., Pedram, M. (2010). “Maximum Power Transfer Tracking for a Photovoltaic-Supercapacitor Energy System” in 2010 ACM/IEEE International Symposium on Low-Power Electronics and Design (ISLPED), ISBN 978-1-4244-8588-8, pp. 307 – 312.

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