POWERMANAGEMENT
Figure 5. Solar micro- converter block diagram
However, it is not as accurate as the optimized P&O at low-irradiance conditions (<30 percent).
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References
[1] Photovoltaic Grid Parity, Bill Sweet, November 2009, IEEE Spectrum Energy Wise News. [2]
www.solarbuzz.com [3] Performance Evaluation of Solar Photo Voltaic Modules Under Field Conditions Through a Quick Diagnostic Tool, Proceedings of ISES World Congress 2007 (Vol. I – Vol. V), Springer Berlin Heidelberg.
[4] Energy comparison of MPPT techniques for PV Systems, Roberto Faranda and Sonia Leva, WSEAS
TRANSACTIONS on POWER SYSTEMS, Issue 6, Volume 3, June 2008
[5] Renewable and Efficient Electric Power Systems, by Gilbert M. Masters. Pub. John Wiley & Sons, Inc.
The economic benefit of module electronic assemblies can be analyzed only in terms of recovered electricity cost. Energy recovered for the entire string as a result of one mismatched or shaded module can be significant. A micro- converter can recover as much as 50 percent of the power from a partially shaded module whose voltage contribution might otherwise be reduced to zero. While a shade condition might exist for a couple hours during the day, mismatched modules are a permanent condition. Once you estimate the amount of energy recovered, savings can be calculated in dollars. Equation 1 calculates the annual savings G ($), from ESAVED or the energy recovered per day (kW h), and RATE or the local electrical tariff ($/kW h):
This evaluation is the benefit provided by just one module electronic assembly fitted to one PV module in the string. Figure 4 reflects the typical benefit a micro-converter provides and after accounting for power conversion efficiency. It is equivalent to three-to-seven percent of the module’s total energy generated. The amount recovered is higher in the string for adversely mismatched modules or those periodically in partial shade. Once module electronics are added, they can provide other functions beyond power conversion and MPPT such as safety, monitoring and management. In a traditional string installation the individual string series elements can not be disconnected in the event of a fire.
The local fire department knows to disconnect the grid, but this only turns off the string inverter output. Many cities have inspection requirements for installation, but disconnects are not currently required. At some point as system numbers increase awareness kicks in and insurance companies get involved. MCU-controlled systems can accommodate these regulations. Monitoring and management are desirable functions. Module problems like fouling the surface or encroaching shadows from trees and new construction can be detected and service can be requested.
Conclusion
Electrical savings must be offset by annualized cost of the module electronic assembly. The loan installment to purchase the assembly, A ($), assuming it is paid off over its useful life is calculated using the capital recovery factor (CRF) [5] in Equation 2:
Electrical savings must be offset by annualized cost of the module electronic assembly. The loan installment to purchase the assembly, A ($), assuming it is paid off over its useful life is calculated using the capital recovery factor (CRF) [5] in Equation 2:
In the final analysis the economic justification requires that A ($) ≤ G ($).
Example: Assuming energy recovered per day is 0.06 kWh, Equation 3 shows the break-even cost for a micro-converter assuming fixed 12¢/kW h local retail tariff, and a loan interest of seven percent APR over a 25-year lifetime?
Is the solar energy glass half full? It depends on your perspective. If you only look at the cost per watt, it does not compete well and likely will not get there soon. However, if you consider the smart grid and the ability to manage many distributed power sources of intermittent performance, then solar energy may be the answer. Installing a 4 kW solar system on your house or business is certainly more attractive than installing a 4 kW pulverized coal plant.
Solar when combined with energy efficient use can be even more attractive. Solar energy is not new, but the attention it is getting is. Previously, installation focused on cost with little concern for management at a module level. When the integrated module electronics is brought down to a cost of about $30, then solar module management will become accepted. We think the glass is half full and on its way to adding more in the glass.
This article first appeared in Power Systems Design Europe, Power Systems Design North America and Power Systems Design China December 2009
www.solar-pv-management.com Issue III 2010
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