DC-DC Converter Design for connecting between solar PV array and a 15 Ohm heater

[biker.josh07]

I am designing a DC-DC converter to connect between an array of solar photo-voltaic panels and a 15-25 Ohm resistive heating element in a domestic hot water cylinder. The specs for the panels are 160V open circuit voltage and 15A short circuit current for 1000w/m^2 irradiance. At the Maximum Power Point for 25 degrees Celsius the panels can produce close to 2.4 kW. The panels are being connected to a 15-25 Ohm resistive heating element in a domestic hot water cylinder. The load line for the panels is 7.5 Ohms at the maximum power point. I have been investigating different possible converter topologies. I am deciding between synchronous boost,synchronous buck boost,push pull, and resonant converter topologies among others as well. Requirements are high efficiency, good power handling capabilities,PV mppt tracking capability and ease of control,robustness over a potential lifetime of over 20 years operation,low EMI, and avoidance of unnecessary complexity (a lot of requirements I know). Interleaving is being considered to spread the power requirements.

The system is stand-alone,i.e. the panels are not being grid tied when producing excess energy for the solar panels. If not enough energy is being produced for the hot water heating requirements, then a relay will switch the heating element to the AC mains.

What converter should I go with? Is it necessary to have a converter with a transformer in it for isolation/safety/compliance reasons? I am a university student and have access to a lab, library, Matlab etc.

Attached is the datasheet for the panels. They are being connected in an array of 4 in series x 3 in parallel.

Thanks for any help and suggestions.

View attachment nesl 190w as.pdf

 

[BradtheRad]

For best efficiency you probably want to draw power from the panels 100 percent of the time. This suggests using two or more interleaved converters.

If you must step-up or step-down by a large ratio, then consider a transformer and H-bridge converter.

Question #1:

Re heating element, what are operating specs? Voltage? Amperage?

This will determine how you arrange the solar panels for best match to the load, whether 3x4, or 4x3, or 6x2, etc.

Question #2:

You have 12 panels. The datasheet quotes 200W per panel. Although that calculates to a total of 2.4 kW, did you find that you could obtain that under test with a load?

The reason I ask is because your 160V open circuit is at 0A. Your 15A short circuit is at 0V. However these figures will not deliver power to a real load. Instead with a real load you will obtain some middle voltage and amperage values.

As for maximum power output, the volt and ampere levels must be found by test with various loads. This will help you in designing a power converter.