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Letters and emails should contain complete name, address and daytime phone number. Letters to the Editor are submitted on the condition that Silicon Chip Publications Pty Ltd may edit and has the right to reproduce in electronic form and communicate these letters. This also applies to submissions to "Ask SILICON CHIP" and "Circuit Notebook".

Grid-connected inverters
have MPPT

In response to one of your correspondent’s query about solar power, as far as I am aware, most if not all grid-connected inverters use MPPT (Maximum Power Point Tracking). In fact, it is said that grid-connected solar power is the most efficient generation of PV power.

I have a 1504W system on my roof and I have seen it generate close to 1600W, which is the limit of my inverter. I have eight 24V 188W panels.

The peak output seems to happen on cloudy days when the Sun suddenly appears. On fully sunny days, the output is lower, with the temperature rise of the panels causing a significant reduction. I seem to get between 1200W and 1300W under those conditions with full exposure. The highest daily output currently (in October) is just under 10kWh per day.

Horst Leykam,
Dee Why, NSW.

Compressed natural gas
has drawbacks as a fuel

I agree with your editorial comment that Australia should use more of its abundant natural gas as a substitute for petrol and diesel fuel used in transport. But having seen both CNG and LNG heavy trucks up close and personal, I concluded both are brutes of systems compared to liquid fuel.

CNG is stored in large carbon fibre cylinders at 350 bar. LNG is stored in a large Thermos flask at -162°C. In an accident, both seem like the Hindenburg on steroids to me. The energy densities are 25% and 60% of diesel respectively, so a lot more volume must be distributed, at a higher cost. The energy to compress and to liquefy are about 1.5% and 25% respectively.

In engines it is much harder to burn NG efficiently than liquid fuel. Diesel engines use about 40% less fuel than spark ignition ones, due to their respective physics. NG can only be used in diesel cycle engines by inducting NG with the incoming air and setting it off with a small squirt of diesel. This is not as efficient or as controllable as liquid injection, with modern systems injecting multiple small bursts of fuels as the piston moves down in the power stroke. Liquid injection is not practical with CNG or LNG.

Therefore, both CNG and LNG are less efficient “well to wheel” and are much more inconvenient and expensive to distribute compared to liquid fuels.

May I suggest that the optimum way to use NG as a fuel (or biomass, shale or coal for that matter) is conversion to synthetic liquid fuel, preferably diesel, and to distribute and use that through existing infrastructure and engines. This would have instant acceptance and cost around $30 per barrel. Plants producing more than 400,000 barrels per day exist in South Africa, Qatar and China, for example – see http://en.wikipedia.org/wiki/Synthetic_fuel#Projects_under_construction and http://www.chemlink.com.au/gtl.htm

John Thompson,
Albury, NSW.

Multimeters can lie
in valve circuits

With respect to the article on therm-
ionic valves in Vintage Radio (September 2010), there is not a lot of information on the basics of taking voltage readings to be found outside trade teaching and from what I have seen of some of the new generation, not a lot of understanding. It needs to be explained. The article on valves did not and could not be expected to address it; it is a subject in itself.

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