Transitional DC HYBRID Power Transmission and the Future of Decentralised Solar
Over the past decade, the price of photovoltaic (PV) solar panels has so significantly reduced that it is now cost-competitive with grid prices. This has resulted in the rapid uptake of this technology in many countries around the world. In Australia, a combination of a high energy price and good access to solar from all major population centres, has created a unique environment whereby energy from rooftop solar now costs less than buying energy from the grid. However, the widespread uptake of these systems has revealed some technical network limitations that have serious implications for the global uptake of decentralised solar energy generation. A fresh approach is needed to the way we utilise PV solar energy. Viable, cost-effective transitional solutions need to be implemented that address the most critical weaknesses in existing systems.
Over the past decade, the price of photovoltaic (PV) solar panels has so significantly reduced that it is now cost-competitive with grid prices <Romm 2013>. This has resulted in the rapid uptake of this technology in many countries around the world. In Australia, a combination of a high energy price and good access to solar from all major population centres, has created a unique environment whereby energy from rooftop solar now costs less than buying energy from the grid <Peacock 2013>. However, the widespread uptake of these systems has revealed some technical network limitations that have serious implications for the global uptake of decentralised solar energy generation. A fresh approach is needed to the way we utilise PV solar energy. Viable, cost-effective transitional solutions need to be implemented that address the most critical weaknesses in existing systems.
The Case for Decentralised Solar
It is first important to review the economic reasons why PV solar should be seriously considered for both residential and commercial applications. In Australia it is estimated that even without feed-in tariffs the cost of embedded solar energy is less than 40% of the retail price of electricity <Peacock 2013>. This is the main reason why over the past five years, more than one million Australian premises have installed a grid-connect solar power system. That equates to over 11% of the population living in homes that have grid-feed PV solar on their rooftops. During this time an estimated 8000 PV solar-related jobs were created in Australia and industry invested billions of dollars in renewable energy <Vorrath 2013>. As grid-connect PV solar energy competes against the retail price of electricity which in Australia can be up to six times the wholesale price paid to utility power plants, it has become a very economically viable option.
What are the Problems?
There are growing concerns about the ability of the transmission network to support further increases in decentralised solar installations and the effect that feed-in tariffs have had on energy prices. The network stability issues have caused utilities to restrict renewable grid-feed installations so that the maximum rating of any grid-connect system cannot exceed 30% of the maximum transformer rating supplying that installation or area <Citipower, Powercor 2013>. If a solar panel has a capacity factor of 15%, then energy from this low cost renewable energy source is limited to 4.5% of the network’s maximum energy rating.
Despite the cumulative installed capacity of embedded solar energy in Australia exceeding 2.4GW <Vorrath 2013>, it has still not been capable of deferring any additional investment in transmission system upgrades. This is because the power output of solar panels peaks before the network’s maximum demand time <Simpson 2013, p.20>.
The DC HYBRID Solution
Designing PV solar system architectures that eliminate grid feed and integrate energy generation, storage and consumption within a building, can provide a solution to these problems.
The significant difference between the retail and wholesale price of electricity means that it is more cost-effective to store power on site and consume it during peak demand times than to sell it back to the grid. Using solar energy and storage to reduce peak time demand is critical to reducing the overall cost of energy. This is because the transmission infrastructure required in distributing 90 hours of peak time demand per year accounts for around 20% to 30% of the total cost of electricity <AER 2013, p.15>. Additionally, by directly consuming locally generated energy instead of selling it back to the grid, the need for additional transmission infrastructure upgrades can be deferred.
By using a direct current (DC) bus to transmit power within a building, feed-back to the grid can be eliminated. As solar panels and storage devices are natively DC, such a system can achieve higher efficiency in comparison to one that requires the individual components to interact via alternating current (AC). Many hard-wired loads in buildings like lighting and HVAC systems can run more efficiently when powered directly from DC. By allowing energy generation, storage and loads to all interact via a common DC bus, the theoretical maximum efficiency of these systems can be realised. Even without energy storage, the system will be adequately utilised as long as the DC loads exceed the generation capacity.
There are several ways to configure such a DC-based system. To achieve useful transmission distances, the optimum DC voltage range for this application is 350V to 400V. As it is not yet possible to power all appliances from DC, a transitional system is required that provides compatibility for both AC and DC loads. To address this, I identified a transformer and rectifier design that would allow for any combination of both AC and DC loads to be supplied at their respective optimum efficiency. This configuration allows both the AC and DC transmission networks to share a common multiple earth neutral (MEN) earthing system. As more DC compatible products become available, loads within the building can transition from the AC to the more efficient DC transmission network over the life of the building. Due to IP issues regarding the transformer and rectifier designs, I cannot elaborate further on how this was achieved.
As decentralised PV solar energy generation and storage is becoming more economically viable, it is imperative to review the effectiveness of existing power transmission systems. Decentralised solar is the most cost-effective renewable energy source in Australia and potentially many other countries around the world. Re-examining conventional grid-feed PV solar system architectures is critically important in maintaining the future viability of this industry sector. Offering a PV solar system with flexible, efficient energy management options can encourage industry investment in PV solar and put downward pressure on electricity prices by reducing peak time demand.
References
Romm, J. (2013). “Cost of PV cells has dropped 99% since 1977 bringing solar energy to grid parity”, The Energy Collective, Retrieved 14/10/2013 from http://theenergycollective.com/josephromm/285416/must-see-chart-cost-pv-cells-has-dropped-amazing-99-1977-bringing-solar-power-grid
Peacock, F. (2013). “The economics of solar power in Australia”, Solar Quotes, Retrieved 9/11/2013 from http://www.solarquotes.com.au/blog/2013/05/
Vorrath, S. (2013). “Solar milestone, 1,000,000 PV systems installed in Australia”, Renew Economy, Retrieved 9/11/2013 from http://reneweconomy.com.au/2013/solar-milestone-1000000-pv-systems-installed-in-australia-44201
Simpson, R. (2013). “Solar Photovoltaic’s, what effect is this having on the grid?” Ausgrid, Sydney, Australia, Retrieved on 9/11/2013 from http://www.ausgrid.com.au/Common/About-us/Newsroom/Discussions/~/media/Files/About%20Us/Newsroom/Discussions/Solar%20Effect%20on%20the%20Grid.pdf
Citipower, Powercor. (2013). “Going solar, preapproval requirement up to 30kW”, Citipower, Melbourne, Australia, Retrieved on 9/11/2013 from http://www.citipower.com.au/docs/pdf/Electricity%20Networks/Solar%20Connections/Solar%20Fact%20Sheets_May%202013%20v1.1.pdf
AER. (2013). “State of the energy market 2012”, Australian Energy Regulator, Melbourne, Australia, Retrieved on 9/11/2012 from http://www.aer.gov.au/sites/default/files/State%20of%20the%20Energy%20market%202012%20-%20Complete%20report%20%28A4%29.pdf
