Something for Nothing?

This is a little add-on to my earlier vlog/blog/report "Why Waste Money?" that looked at narrowboat power unit efficiency. An epilogue, so to speak.

80% Efficient?

I read a report about a huge luxury yacht that described the generator as “80% efficient”, and I thought that can’t be true. Or can it? The trick they used was to include recovered heat in the calculation of efficiency. The yacht recovered heat from the engine cooling system, from a heat exchanger in the exhaust and even from the oil cooler on the transmission.

All diesel engines get hot (they are, after all, heat engines, and the clue is in the name!). Putting the engine in a soundproof box is a nice idea, but sound insulation is also good heat insulation, so the heat stays inside the box. Generators designed for marine use are typically going to be mounted inside the boat, so they need to remove this heat in some way. Many include air:water heat exchangers to transfer the heat from the air around the engine into the cooling water. At the same time, the alternator will get hot due to losses, and, if the alternator is water cooled, this heat is also passed out with the cooling water.

BetaGen 10 as a Hair Dryer?

On Perseverance we are lucky to have a BetaGen 10 generator. The BetaGen 10 has an air cooled alternator, which sucks ambient air across the alternator coils and then over the engine. The heated air is blown out of a duct at the back of the generator by a small fan. This may seem less sophisticated than described above, but read on…

I measured the resulting stream of warm air and found a temperature rise of about a 25C. I used a crude pitot tube to measure the flow velocity and a water manometer reading of 24mm corresponds to a flow of 20m/s. Given the cross sectional area of the duct I made this 4.25 cubic metres per minute, or 0.087 kg/sec. With the temperature rise, this is a bit over 2kW. Think running a hair dryer on its hottest setting.

I considered ducting this warm air into the cabin, but I was uncomfortable with the safety issues of joining the engine bay to the living accommodation, especially as I sleep on the side of the bed closest to the flames! I therefore asked Jim at Alexander Boats to include vents so that the hot air can be ducted into the cratch. In summer the excess heat won’t matter as we can roll the cratch side panels out of the way, while at other times of the year we will have gentle heating for free. It should certainly help us to dry wet clothes.

Something for Nothing?

OK, you don’t get anything for free in this life. The fan draws 105W but as the heat output is about 2,000W so it’s a good trade-off.

I did make a slight modification to the generator. As delivered, the 240VAC fan is connected to the alternator output, so it runs all the time that the alternator is turning. However, when the generator starts the engine and alternator are cold so there is no hot air to be blown out, and, as I indicated on the blog about testing, after shutdown there is still considerable residual heat in the generator. I therefore converted the fan to be externally powered and added a thermostat so that it runs whenever there is hot air to move.

The performance of this generator alone (electrical energy out divided by fuel energy in) shows an efficiency of 22%. Adding the hot air stream increases the overall efficiency to 28%. If I add the calorifier into the calculation (let’s say a 55L calorifier heated 60C in an hour) adds a further 3.8kW giving a final efficiency of 40%.

OK, I know you can’t keep heating the calorifier once it is up to temperature, and I know I haven’t included any allowance for solar power and, yes, I do I appreciate that a blast of hot air into the cratch on a summer afternoon may not be ideal, but at least these sums give you some idea of the energy balance on the boat. The bottom line is that using waste heat is a good thing to do, but we still use most of the energy from the fuel we burn to heat the canal.

Arctic Jacuzzi

The result was nothing like the claimed 80% of the yacht I mentioned at the start, but if all the reclaimed heat goes into hot water and heating, I have visions of the yacht in arctic waters with the wealthy owners basking in a hot tub, just to make use of all that hot water!

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