WHR systems turn up the heat

11 Oct 2017
There’s a rich seam of waste heat waiting to be mined

There’s a rich seam of waste heat waiting to be mined

Waste heat recovery systems, WHRS, have been almost the exclusive province of bigger ships – till now. Innovation is set to broaden their appeal writes Stevie Knight.

Steam yields a lot of power, but it’s not all about the heat. What drives a Rankine Cycle turbine is the way gas moves across the blades as it expands and cools on its way toward the condenser vacuum. So, although water is cheap, it’s not the only game in town.

As Mia Elg of Deltamarin explains, although Steam Rankine Cycle (SRC) installations are the generally accepted approach for faster, larger vessels with exhaust temperatures above 300C, it’s not so useful for smaller or slower paced ships with efficient two-stroke engines.

Further, as vapour temperature drops sharply on its way through the turbines, SRCs usually include dual-pressure boilers and superheaters which bump up the temperature as it passes through the later stages. This raises efficiency and stops condensation damaging the blades – but it traditionally makes for a bulky, complex installation.

Therefore marine Organic Rankine Cycle (ORC) systems are just beginning to pick up a following. Instead of steam, ORCs use organic fluids, vaporisation being tailored to the heat range in hand explains Elg. As a result, the tech lends itself to lower pressures and temperatures while avoiding condensation issues, adding up to simpler, single-stage turbines.

This approach makes good use of scavenge air as the compressor outlet is usually running at 100C to 160C. Further, despite coming out at between 80C to 100C, jacket water heat has quantity on its side and it’s continuously available while the engines are running. This heat source is entirely suited to ORC processes, says Elg, estimating a yield “of between 7% and 10% efficiency”.

The solutions are developing apace. Most marine ORC systems on offer are modular units yielding around 150kW apiece: this lends the tech a useful set of characteristics including the ability to scale up easily and automatically turn on and off in response to the thermal energy available – which is directly related to higher engine load and therefore ship speed.

But within this general outline, the game’s wide open.

For example, Calnetix’s ORC makes use of some very advanced technology.

The company has married a radial turbine with a high speed permanent magnet (PM) generator; this avoids both electrical excitation and mechanical shaft losses – and there’s no gearbox explains Calnetix’s Venky Krishnan.

The generator is also supported on active magnetic bearings “so it floats in air” says Krishnan: this allows the unit to be completely sealed and, as a result, “there’s nothing that can leak the working fluid”. Further, the kit is being containerised to sit on the deck, and although permanently plumbed in, it’s not taking up engine room space.

Obviously, it’s rather expensive but as Krishnan underlines there’s no maintenance, so Calnetix’ installation yields a broader advantage than the system’s 7% or so efficiency. Last year a Hydrocurrent module (developed with MHI) was installed on the Arnold Maersk where it’s consistently putting out 125kW from 85C jacket cooling water.

Climeon, on the other hand, utilises direct condensation, a novel process for the marine industry. This means a little of the cooled working fluid is further chilled, then sprayed back into the condensing chamber “creating millions of tiny droplets... which gives you a huge reaction surface, condensing the gas immediately” says Climeon’s Johan Larsson.

Although it’s a much less intense process than found in SRC systems, ORC installations still pressurise then superheat the working fluid. However, Climeon avoids this by switching refrigerant for a fluid with a particularly low vapour point, dropping the pressure from the more usual 10bar-plus to 2.5bar. “The benefits for a low pressure system are twofold,” claims Larsson. Firstly, “it’s easier to gain high turbine efficiency” he says, explaining that although it’s counterintuitive, it is actually more difficult to gain higher efficiency when there’s a larger pressure difference. The second point is pragmatic: “The components cost less,” he adds.

In fact according to Larsson, the unit’s efficiency reaches around 10%, at the top end of Elg’s estimate and “over half of the [Carnot] theoretical limit of around 18%”.

Avid, on the other hand, is focused on the bottom line. Vahid Walker says: “although you can play tunes with various exotic designs... you have to face the associated costs”.

Walker points out that while there are a number of potential low-grade heat streams that could be plaited together, there are good reasons for keeping it simple. The biggest investment is the heat exchanger, “and the more sources, the more of these you need,” he says.

He adds: “Unlike a lot of ORC demonstrator projects, we are not looking at the green credentials, we are looking purely at commercial returns”. To prove the point, their first installation will not be on a cruise ship, but a regular cargo vessel.

Snipping the price is partially a matter of having the capabilities of a large company behind you – in Avid’s case, a firm that’s been involved in the electric and hybrid vehicle market for some time. “We have a multidisciplinary team here, so rather than going to marine suppliers and buying in at ridiculous prices, we can make our own components – and get them certified by the class societies,” says Walker.

It’s a long business and Avid are only just off the starting blocks but the advantage will be real enough: “We might not want to churn out millions of parts – we can’t compete with Grundfos on production runs – but, as we can absorb the engineering investment, we can better tailor everything to suit.”

It’s early days for ORC innovators and admittedly, there’s still only a couple of existing marine installations, but there’s more on the order books: Calinetix has another three Hydrocurrent units under manufacture for further Maersk line vessels and Climeon has been contracted to supply 18 modules for Virgin’s three new Voyage cruise ships.

EXHAUST GAS TURBINES

There are a number of established exhaust gas combinations already out there but more ideas are just around the corner.

For example, both ABB and MAN Diesel & Turbo have a combination power generation solutions which have found a regular niche onboard vessels like the Maersk Triple E class containerships, especially as this can feed the reefer demand. MAN’s Bent Ørndrup Nielsen explains this consists of setting a power turbine in a bypass at the receiver, diverting around 11% of the highest flow. Next in line comes the SRC: this also takes advantage of preheating from the scavenge air and jacket water.

According to Nielsen, together a PT/SRC on a big (25MW-plus) container vessel can gain around 8.5% to 9% efficiency - although the final case has to be determined by the operational loads, oil-determined payback times, electrical demand and last but not least, space as it’s a huge installation “with the dual pressure steam boilers almost the same size as the main engines”. However, Nielsen adds that given all this, it “can be used to answer the EEDI regulations” especially coupled with a PTI that feeds excess to the propulsion.

But there is much to be gained by integration with other systems. For example, while the amount of working steam exiting the Exhaust Gas Recirculation (EGR) is just too low for a steam turbine, entwine the boiler with the EGR circuit itself and the hotter flow inside the system changes the picture entirely. The solution is, he says, “under development”.

Further, MAN is investigating how to combine this technology with low pressure Selective Catalytic Reduction (SCR) systems. While engines optimised for Tier III emissions can be adapted to work with the SCR and steam turbine (though not the power turbine element), special tuning gives Tier III engines running in Tier II mode the potential to bypass the SCR “as the engine itself will be taking care of the Tier II requirements” explains Nielsen. This gives operators flexibility to choose the most economic response.

But, how do these steam/power turbine installations perform when the engines are operating off their design point? After all, the power plant of a ship on water is subject to all kinds of fluctuations. And - could ORC systems be an alternative for the exhaust gas stream of ships that fall slightly below the ‘ideal’ case for the traditional answer?

This is just the question asked by Jesper Graa Andreasen (one of Fredrik Haglind’s DTU research team) in a study supported by the Danish Maritime Fund. He looked at a base case of a typical Maersk containership with a 23-MW two-stroke MAN diesel engine, which puts it on the lower end of the scale of vessels suitable for an SRC/PT installation.

It was compared with an ORC system tailored to a 300C outflow with an exhaust gas recuperator that boosts the initial temperature of the (carefully chosen) working fluid:  both cases included the requirement for service steam.

The results were interesting: “What we saw in general was that at design loads, the more complex SRC reached higher efficiency, but when you drop the load off the engine, the ORC cycle was better able to maintain power for longer.” In fact, using 0.5% sulphur fuel the results showed that while at full, 100% engine loads steam had a 6% efficiency advantage over an ORC using the most suitable fluid, even at 80% this had dropped to negligible. And at 45% load, the ORC was almost doubly efficient.

He explains most of this is because turbine throttling – necessary to keep the minimum temperature in the boilers constant – is less of an issue with the once-through configuration used in the ORC unit, compared to the SRC unit which uses drum boilers.

So, will ORC technology start to push into traditional SRC territory? Andreasen is convinced that it’s worth pursuing, although Elg points out there’s still a way to go before commercial designs are available “as utilising exhaust gases directly in a ORC might lead to best process efficiencies, but ... technical solutions do not really yet exist for the temperature range and exhaust gas volumes found in most ships”. Despite this, she adds “it’s an interesting area”.

BIG PICTURE FOCUS

Mapping onboard waste heat is complex, but Mia Elg points out that if you don’t have the big picture, “it’s possible to end up in sub-optimizing the system... getting a good turbine efficiency but poor overall performance”.

Some of the issues might be less straightforward than assumed: a cruise ship might seem like the perfect candidate for WHRS “until you realise it spends much of its time in port” she points out.

Further, how the demands are cascaded can make or break the deal: “With every device you put in a ship, you are losing a certain amount of the useful energy potential... For example, an exhaust gas boiler with standard dimensioning can destroy the potential for another application,” she explains.

However, given a simulation tool like Deltamarin’s (which can be loaded with the whole gamut of data from plant designs to heat load profiles and operational patterns) it’s possible to visualise where overall thermal efficiencies can be gained, yielding results which, she says, are “surprising and certainly not trivial”.

Deltamarin’s efficiency proposals cut across everything from small to large innovations: adjusting the flow rate on a cooling water circuit “can make a big difference to your control of the waste heat recovery” says Elg.

Still more radical is a variation on the usual SRC installation which folds the boilers, cooling water recovery and various type of electricity generating elements together. Even if a high steam flow or temperature isn’t available she explains “we can still take the best of both” by making a flexible entity out of smaller counter-pressure steam turbines, efficient gas boilers with a superheating feed from the exhaust and several ORC units operating at various temperature levels. And, she adds, “both footprint and cost are minimized”.

Further, for some boiler applications it might be worthwhile dumping the pump and its associated energy losses. It’s not just the energy demand (which at anything up to 20kW per unit can certainly add up), pumps will continue working when their heat source cools. So Elg explains that recent cooperation with Alfa Laval has focused on natural circulation boilers which avoid making the system colder when the engines stop running.

It’s another new idea for the designers to play with.

FIRST IN LINE
 
When it comes to utilising waste heat, Geir Erik Samnøy of Presentwater points out that one shouldn’t overlook the obvious – for cruise ships “which are mostly floating hotels with a substantial thermal energy demand”, the boilers and watermakers have to be first in line to pick up the potential from the waste heat flow whether that’s the exhaust or jacket water cooling. It’s not just energy efficiency he says, it’s cost effectiveness: “There are fewer components – and for operators, it’s all about the payback time”.

However, as Samnøy explains, the sector still needs updating as cruise patterns are changing: “The ships are reducing their speed, staying longer in port, going to more sophisticated – often more extreme – destinations like the Arctic. This puts a squeeze on the heat and water supply.”

Therefore Presentwater is making headway with 15 retrofits under its belt already. It’s not only the older vessels that are benefitting: “Even if we go for quite fresh cruise ships under three years old, we still see between 3% and 10% fuel savings. It’s usually because the yards and owners have stuck to known, tried systems that they know work - but they won’t necessarily meet the new demands.”
 
The enhancements include modifications of pipes and valves to optimise the balance, but it also leans heavily on better automation... “You really don’t want the onboard crew to constantly intervene to maintain effectiveness during different modes of operation,” he says.
 
Despite this, the company has a heat-to-power generator “on the way”. So far under wraps, the Motorship has found out it’s not based on the more common Rankine Cycle but on the Ericsson Cycle which yields higher efficiency through isothermal compression and expansion.