Using two-stroke engines as gensets
Two-stroke engines have been used to generate electricity in power stations, but are rarely used this way on ships. A study by MAN Diesel & Turbo and PBES could change that.
As the industry reacts to global energy trends, traditional power and propulsion arrangements are being challenged. Greater access to cleaner or wholly renewable energy sources, combined with rapid advances in energy storage, are already driving change in small, short-sea and coastal ship designs. A new study by MAN Diesel & Turbo, an engine designer, and battery system specialist Plan B Energy Storage (PBES) has examined how two-stroke engines could be used to generate electricity on bigger vessels – potentially replacing traditional, four-stroke auxiliary engines.
The conventional set-up for deep sea-going cargo ships has been one (or maybe two) two-stroke engines for propulsion, with multiple auxiliary engines or gensets to cater for onboard electricity demand. But according to René Sejer Laursen, MAN Diesel & Turbo, the increasing use of alternative fuels – notably LNG but also including ethane, methanol and LPG – is driving interest in the two-stroke engine as an electricity generator.
“For many of these fuels there are no gensets available that can efficiently burn the fuel,” he says. Therefore, many users of two-stroke engines burning alternative fuels have fitted their engines with power take-in/out systems or shaft generators to supply electrical power, either instead of or as well as the traditional auxiliary engines.
Laursen continues: “In addition, there is a constant need for improving the efficiency of the total plant onboard gas-fuelled ships and LNG carriers. This includes the electricity production, and thanks to the better efficiency and reliability of the two-stroke engine, owners have occasionally suggested using the two-stroke engine for electricity production by applying a generator on the end of the engine, similarly to what is done for power plants with two-stroke engines.”
In the past, the idea of using a two-stroke engine to generate electricity was easy to reject simply because the slow load ramp up and down: in some cases, the engine will simply not be able to deliver the required electrical load and when needed. But Laursen explains that, on a stationary power plant, the two-stroke engine works well because it is not the only provider of electricity. The two-stroke engine is normally a provider of base load in conjunction with other faster reacting generators, such as four-stroke engines, handling the fluctuations.
The emergence of batteries in the marine market has made MAN reconsider the idea. With their fast dynamic response, batteries could provide both energy storage for electricity generated through the two-stroke engine, as well as offering ramp-up support where low-speed engines might not be quick enough.
Laursen notes: “Today we see significantly improved performance of batteries and a drop in cost. Batteries have been introduced on several smaller vessels such as electric ferries, tug boat, fishing boats, and offshore supply vessels operating on short routes. The service feedback has mostly been great and it has proven to be safe, so many projects in smaller ships are underway.”
MAN Diesel & Turbo wanted to study how batteries might support the ramp up and down of electricity production from a two-stroke engine. After contacting PBES, which offers lithium-ion energy storage systems and has had wide success in the marine market, they decided to use a 174,000m3 gas carrier as a concept study.
“The electrical load on these ships is rather high,” says Laursen, “and the installed electrical power is normally approximately 14MW, corresponding to four 3.5MW dual-fuel genset engines. One of the engines is solely redundant, so operational load is 8-10MW maximum. We concluded that these engines could be replaced by two 7S35ME-GI, each delivering 6MW.”
This configuration delivers lower installed power – at 12MW as opposed to 14MW – while still offering full redundancy by using a cylinder cut-out. Two-stroke engines also generally have a significantly higher reliability and require less maintenance than four-stroke engines, adds Laursen.
Sizing a battery to support the engine during load ramp-up and taking charge during load ramp-down requires detailed knowledge of the electrical behaviour of LNG pumps, heavy-duty compressors and other electrical equipment onboard. For this reason, in the first instance the battery was sized based on an estimation to handle the worst-case scenario.
LOAD RAMP UP
Laursen says: “A full load ramp-up would require a rather large battery pack installation. This is expected to be limited, or eventually removed, by studying the real operational profile of the pumps and compressors. Further, the ramping up and down control strategies of both the compressors and of the LNG pumps could probably be smoothened out by fitting the load ramp up for the engines.”
The study eventually concluded that that the batteries should be sized for dynamic load, with a relative low number of cycles over the lifetime of the batteries. An estimate of the C-rate - a measure of the rate at which a battery is discharged relative to its maximum capacity – was also required to size the batteries. Here it was found that the ramp-up phase was not the key factor, explains Laursen.
“Due to the lower acceptable C-Rate during charging it is more the ramp down that decides the size. With a 6MW engine that decrease from 100% to 0% of maximum continuous rating at a rate of 5% per minute, a battery size of roughly 3MWh per engine would be needed - or 6 MWh in total per vessel - if the usage is cut off instantly.”
The auxiliary engines therefore need to be ramped down intelligently to a lower output level before the batteries take over and are charged. After ramp down, the batteries could be used for smaller peak shaving in connection with departure and during the voyage. Sizing the batteries is still a cost issue, so it was concluded that in case of sudden cut-off of power, a feed-forward signal from main switch panel to the regulator of the engine would enable the engine to begin ramp down earlier, reducing the battery support needed.
“With this solution, the need for the batteries could eventually be almost eliminated - but only if the operation load profile has been cleverly designed,” says Laursen. “The need for the batteries can be limited to almost solely act as a safeguard preventing over-speeding of the engines.”
The installation of a two-stroke genset in an LNG carrier engine room requires special attention. On four-stroke engines most of the auxiliaries, including the lube oil sump, are integrated in the engine. The four-stroke engine can be deck-mounted on elastic absorbers almost anywhere in the engine room. This is not the case with two-stroke engines and a generator.
For an LNG carrier, the study found that the most suitable place to install a two-stroke genset would be in the bottom part of the hull between the two main engines – MAN 5G70ME-GI engines in the study examples. Laursen says: “It will be possible to get a solid support here, and if the engine is mounted counter rotating they can also damp out vibrations. The engine also needs to be equipped with top bracing, so support for that needs to be prepared.”
The biggest benefit of using a two-stroke engine as a genset on an LNG carrier lies in the fuel cost savings. The study identified savings of around one tonne of (HFO equivalent) fuel per day for a 174,000m3 gas carrier using two 5G70ME-C9.5-GI main engines and two 7S35ME-GI two-stroke gensets. This corresponds to a fuel saving of 1.3%.
Laursen concludes: “We have found that a technical solution is available for a two-stroke genset in combination with the use of batteries. However, it requires a big effort from the yard to prepare the ship design for the two-stroke genset. Nevertheless, the benefits of using an ME-GI as a genset are clear; higher reliability, lower maintenance costs and lower fuel consumption.”
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