December 8th, 2008: Initial Test Results
Once again, I’m way behind with the blog. There’s a great deal to report on.
In spite of some fairly awful weather at times, we had some great cruising over the course of the summer on Sweden’s west coast. The boat sails beautifully with its laminated sails (from the China Sail Factor – www.china-sail-factory.com), and is incredibly comfortable to live aboard. It’s also extremely efficient, especially the Alpenglow lighting (www.alpenglowlights.com). At anchor, the solar panels on the hard top (made by Malo – it will fit any of the Malo range) kept up with the entire house load, including running my lap top for most of the day. The AirX wind generator (www.windenergy.com) put the icing on the cake. I was tickled to death.
We succeeded in getting the electric propulsion system up and running just before the end of the boating season. There is a 16 kW, 144-volt DC electric motor from BluWav (www.BluWavsystems.com) powered by a large bank of Thin Plate Pure Lead (TPPL) Odyssey batteries from EnerSys (http://www.odysseyfactory.com), and also by a 22 kW, 144-volt DC generator from Polar Power (http://www.polarpowerinc.com). The system integrator is Emotion Hybrids (www.electricmarinepropulsion.org).
We were only able to do a very limited amount of testing because of teething problems with the generator’s control system (since resolved by Bjorn at DC Marine – www.dcmarine.se – thanks, Bjorn!), but nevertheless I was able to draw some tentative conclusions:
- When operated in diesel-electric mode – i.e. the generator is running any time propulsion is called for, and the batteries are not part of the equation – it is almost impossible to make the system more efficient from a propulsion point of view than the conventional inboard diesel. This contradicts a good deal of published literature.
- Almost all people developing electric propulsion systems are using permanent magnet (PM) generators because of their high efficiency. The voltage on such a generator is a function of its speed. Typically, the necessary output voltage is determined by the charging needs of the batteries in the system (e.g. absorption or float voltage), and as such the speed of the generator is determined by battery state of charge rather than load. This makes it extremely difficult to operate the generator efficiently at light loads (because for efficiency you need to slow the generator down to match its output to the load) unless you add some kind of an inverter stage to the generator’s output which allows you to regulate the output voltage independent of generator speed. This makes an expensive generator even more expensive.
- Even if you add the inverter stage to the generator and slow its speed to match the load, although you will improve its efficiency as compared to the generator without the inverter stage, the efficiency will still fall off at light loads. To optimize efficiency, you have to design the system such that when it is running the generator always runs at peak efficiency (i.e. relatively highly loaded). This requires using the batteries as a ‘buffer’ such that light propulsion loads are met from battery power at least until the battery state of charge (SOC) has dropped to the point where the charge acceptance rate (CAR) is high enough, when combined with any propulsion and house loads, to load the generator to its ‘sweet’ spot. This kind of a system constitutes a hybrid electric boat. It’s the only way to move forward with this technology.
- The CAR on conventional lead-acid batteries (wet cells, gel cells and AGMs) at higher SOCs is such that it is very difficult to design a hybrid system to operate more efficiently than a conventional inboard diesel. Problems are compounded by the high losses when charging and discharging batteries (on the order of 30%). By comparison, the Odyssey TPPL batteries have much higher CARs to much higher SOCs, with lower losses through the battery. I can already improve efficiency at low boat speeds over the conventional installation, but at higher speeds the conventional system is still more efficient (and always will be). I have not yet clearly defined the ‘cross over’ point. My goal is to get it up around normal cruising speeds, in which case the hybrid will be more efficient most of the time, although the absolute fuel savings will still be quite small.
- There are other battery technologies – notably lithium-ion – that have much higher CARs than TPPL with much lower charge/discharge losses (lithium-ion is close to 0%), but these are currently not viable in this application. I am watching this closely as they will immediately shift the cross-over point to higher boat speeds.
- When on the hook, proper use of the batteries as a buffer can result in the energy for house loads on the hybrid boat being created far more efficiently than on the conventional boat (in many cases, at least twice as efficiently). The greater the house loads are as a proportion of the total energy needs on a boat, the greater the opportunity to improve efficiency with the hybrid.
- So far, I have only considered propulsion efficiencies and house loads assuming that all the energy consumed on board comes from a fossil-fueled engine (either directly, or via battery charging). The hybrid, however, offers the opportunity to integrate alternative energy sources, notably solar, wind and, above all, regeneration (from spinning the propeller when under sail). A recently-launched Lagoon 500 catamaran, using the same BluWav motors as we have (one in each hull) in an Emotion Hybrid system has been generating 3 kW at 9 knots. The more alternative energy that can be plugged into the equation, the greater the efficiency of the hybrid as compared to a conventional installation. Integration of alternative energy sources is the key to making hybrid technology viable in boats.
- Effective control of a hybrid requires managing all energy sources and consumers to optimize efficiency without compromising battery life through overly aggressive cycling. The more you think about it, the more complicated it gets. This energy management side is very poorly developed at the present time.
There’s a great deal to still be researched and developed. Luckily, the group of companies that have come together around the ‘Nada’ project has been awarded a 2.2 million euro grant by the European Union ‘Framework Program 7’ to study these issues in depth and come up with optimal marine hybrid systems. We’ll get going in earnest in 2009.
With this kind of funding, and the additional resources it is drawing in (for example, the Bosch Engineering Group has expressed an interest in working with us on the energy management module – what we call the EMM), I am becoming increasingly confident that we will be able to create cost-effective hybrid systems that I will feel comfortable recommending to boatbuilders and owners (when I started this project, I had a much less optimistic outlook).
Over the winter Malo should be outfitting ‘Nada’ with some really sophisticated instrumentation (torque, thrust and rpm meters, in particular, and higher resolution fuel monitoring equipment) plus datalogging capabilities. We’ll start next year by collecting baseline data on the relative efficiencies of a wide range of propellers (fixed, feathering, folding, the Gori, and the Autoprop). It’s going to be a very interesting year.
In the meantime, we discovered something really noteworthy with this summer’s limited test program. We had a propeller with just a few barnacles on it. As compared to a polished propeller, it increased the fuel consumption for a given boat speed by up to 50%, and dropped the boat speed for a given shaft speed by up to 30%. At the moment, the most cost effective way to improve efficiency under power is to regularly clean the propeller!