System and method for forecasting photovoltaic power generation system degradation

1. A system for forecasting photovoltaic power generation system degradation, comprising the steps of:

a power meter capable of assessing measured photovoltaic production for a photovoltaic system operating at a known location over a set time period;

a monitoring infrastructure capable of assessing measured solar irradiance data for the known location over a reference time period that minimally overlaps with the set time period;

a configuration specification for the photovoltaic system; and

a digital computer comprising a processor and a memory that is adapted to store program instructions for execution by the processor, the program instructions capable of:

simulating time-series photovoltaic production by the photovoltaic system using the configuration specification and the solar irradiance data for the reference time period;

deriving adjustment factors by minimizing error between the time-series simulated photovoltaic production and the measured solar irradiance data;

creating adjusted time-series simulated photovoltaic production by multiplying the time-series simulated photovoltaic production by the adjustment factors;

calculating normalized ratios of the adjusted time-series simulated photovoltaic production to the time-series simulated photovoltaic production for a current time period and a time period previous to the current time period; and

selecting a degradation time period and calculating degradation of the photovoltaic system as a function of the normalized ratio for the current time period and the normalized ratio for the previous time period.

2. A system according to claim 1 , the program instructions further capable of:

calculating the degradation for consecutive degradation time periods; and

forecasting long-term degradation by evaluating a trend in the degradation for the consecutive time periods.

3. A system according to claim 2 , the program instructions further capable of:

evaluating the trend as the average of the long-term degradations for the consecutive time periods; and

evaluating the trend as the mean of the long-term degradations for the consecutive time periods.

4. A system according to claim 2 , the program instructions further capable of:

omitting the degradation for a first year of operation for the photovoltaic system.

5. A system according to claim 1 , wherein the long-term degradation Degradation t for the current time period t is determined in accordance with:

Degradation

t

=

1

-

Adjusted

⁢

⁢

Simulated

t

-

1

/

Simulated

t

-

1

Adjusted

⁢

⁢

Simulated

t

/

Simulated

t

where Adjusted Simulated t-1 represents the adjusted time-series simulated photovoltaic production for the previous time period; Simulated t-1 represents the time-series simulated photovoltaic production for the previous time period; Adjusted Simulated t represents the adjusted time-series simulated photovoltaic production for the current time period; and Simulated t represents the time-series simulated photovoltaic production for the current time period.

6. A system according to claim 1 , wherein the previous time period represents a first year of operation for the photovoltaic system, the program instructions further capable of:

normalizing the normalized ratio for the current time period by dividing by the normalized ratio for the previous time period.

7. A system according to claim 1 , the program instructions further capable of at least one of:

minimizing the error by evaluating relative mean absolute error;

minimizing the error by evaluating mean bias error; and

minimizing the error by evaluating root mean square error.

8. A system according to claim 1 , wherein the configuration specification is derived, the program instructions further capable of:

obtaining ambient temperature measured for the known location over the set time period; and

searching for optimal values for each variable in a configuration specification for the photovoltaic system by optimizing each variable, one at a time.

9. A system according to claim 8 , the program instructions further capable of:

selecting a candidate value for the variable being optimized;

simulating photovoltaic production for the photovoltaic system using the ambient temperature, the solar irradiance data, and the candidate value;

calculating error between the simulated photovoltaic production and the measured photovoltaic production; and

choosing the candidate value as the optimal value for the variable being optimized upon the error meeting a minimal threshold of error.

10. A system according to claim 1 , the program instructions for simulating time-series photovoltaic production being further capable of:

generating a set of sky clearness indexes as a ratio of each irradiance observation in a set of irradiance observations that has been regularly measured for the known location over the set time period, and clear sky irradiance;

forming a time series of the set of the sky clearness indexes;

determining irradiance statistics for the photovoltaic system through statistical evaluation of the time series of the set of the sky clearness indexes; and

building power statistics for the photovoltaic system as a function of the photovoltaic system irradiance statistics and the configuration specification.

11. A method for forecasting photovoltaic power generation system degradation with the aid of a digital computer, comprising the steps of:

assessing through a power meter measured photovoltaic production for a photovoltaic system operating at a known location over a set time period;

assessing through a monitoring infrastructure measured solar irradiance data for the known location over a reference time period that minimally overlaps with the set time period;

referencing a configuration specification for the photovoltaic system; and

operating a digital computer comprising a processor and a memory that is adapted to store program instructions for execution by the processor, the program instructions capable of:

simulating time-series photovoltaic production by the photovoltaic system using the configuration specification and the solar irradiance data for the reference time period;

deriving adjustment factors by minimizing error between the time-series simulated photovoltaic production and the measured solar irradiance data;

creating adjusted time-series simulated photovoltaic production by multiplying the time-series simulated photovoltaic production by the adjustment factors;

calculating normalized ratios of the adjusted time-series simulated photovoltaic production to the time-series simulated photovoltaic production for a current time period and a time period previous to the current time period; and

selecting a degradation time period and calculating degradation of the photovoltaic system as a function of the normalized ratio for the current time period and the normalized ratio for the previous time period.

12. A method according to claim 11 , the program instructions further capable of:

calculating the degradation for consecutive degradation time periods; and

forecasting long-term degradation by evaluating a trend in the degradation for the consecutive time periods.

13. A method according to claim 12 , the program instructions further capable of:

evaluating the trend as the average of the long-term degradations for the consecutive time periods; and

evaluating the trend as the mean of the long-term degradations for the consecutive time periods.

14. A method according to claim 12 , the program instructions further capable of:

omitting the degradation for a first year of operation for the photovoltaic system.

15. A method according to claim 11 , wherein the long-term degradation Degradation t for the current time period t is determined in accordance with:

Degradation

t

=

1

-

Adjusted

⁢

⁢

Simulated

t

-

1

/

Simulated

t

-

1

Adjusted

⁢

⁢

Simulated

t

/

Simulated

t

where Adjusted Simulated t-1 represents the adjusted time-series simulated photovoltaic production for the previous time period; Simulated t-1 represents the time-series simulated photovoltaic production for the previous time period; Adjusted Simulated t represents the adjusted time-series simulated photovoltaic production for the current time period; and Simulated t represents the time-series simulated photovoltaic production for the current time period.

16. A method according to claim 11 , wherein the previous time period represents a first year of operation for the photovoltaic system, the program instructions further capable of:

normalizing the normalized ratio for the current time period by dividing by the normalized ratio for the previous time period.

17. A method according to claim 11 , the program instructions further capable of at least one of:

minimizing the error by evaluating relative mean absolute error;

minimizing the error by evaluating mean bias error; and

minimizing the error by evaluating root mean square error.

18. A method according to claim 11 , wherein the configuration specification is derived, the program instructions further capable of:

obtaining ambient temperature measured for the known location over the set time period; and

searching for optimal values for each variable in a configuration specification for the photovoltaic system by optimizing each variable, one at a time.

19. A method according to claim 18 , the program instructions further capable of:

selecting a candidate value for the variable being optimized;

simulating photovoltaic production for the photovoltaic system using the ambient temperature, the solar irradiance data, and the candidate value;

calculating error between the simulated photovoltaic production and the measured photovoltaic production; and

choosing the candidate value as the optimal value for the variable being optimized upon the error meeting a minimal threshold of error.

20. A method according to claim 11 , the program instructions for simulating time-series photovoltaic production being further capable of:

generating a set of sky clearness indexes as a ratio of each irradiance observation in a set of irradiance observations that has been regularly measured for the known location over the set time period, and clear sky irradiance;

forming a time series of the set of the sky clearness indexes;

determining irradiance statistics for the photovoltaic system through statistical evaluation of the time series of the set of the sky clearness indexes; and

building power statistics for the photovoltaic system as a function of the photovoltaic system irradiance statistics and the configuration specification.