Métodos y técnicas

The dwt loss is a parameter to estimate the effect of the alternative fuel storage systems on loss of cargo carrying capacity due to their typically lower energy density. This analysis does not take into account the technical issues relative to the conceptual design and engineering of a hydrogen-powered ship, this analysis instead focuses on the extra space requirement that might be needed in comparison with a conventional fuel oil tank. In the case of hydrogen in combination with fuel cells the power density of future ma- rine fuel cells systems should also be included, as ships require a high power installed on board and a fuel cells system’s volume and weight increases incrementally with the power installed (Krˇcum et. al.,2004). Data on large fuel cells power density were not found in the literature, although a possible design solution would be to allocate different module of fuel cells in different location on board ship in order to limit the space requirement. How manufacturers would cope this is highly uncertain. In this thesis, it is assumed that hydrogen storage system density is expected to have the biggest impact on the loss of cargo capacity because of the very low volumetric density of hydrogen storage system, therefore, only the fuel storage system affects the loss of cargo capacity. Although, more research should be dedicated to explore the applicability and space requirement of fuel cells in the maritime context, a robustness analysis discussed in section6.5 will explore the sensitivity of the results in comparison with variation of the hydrogen volumetric energy density. This variation can also been seen as the inclusion of marine fuel cells space requirements.

To evaluate the loss of cargo carrying capacity, first the volume occupied by the hydrogen storage system is calculated (VH2), second the volume occupied by a HFO storage tank is calculated (Vref) as a reference of comparison against which the extra volume is estimated. The dwt loss is then calculated converting the extra volume ex- pressed in m3/kW h in tonnes/kW h, assuming that each m3 on board ships is equal to 0.8 tonnes of cargo capacity (see formula4.3), and dividing all per the energy stored on board in KWh (Est).

dwt loss = (V H2 − V ref ) ∗ 0.8 Est

(4.3)

The volume occupied by a fuel storage system can be expressed as a function of the amount of fuel required on board and the volumetric density of the fuel storage system.

So, formula4.3 can be written as:

dwt loss = (SH2/γH2) ∗ 0.8 − (Sref/γref) ∗ 0.8 Est

(4.4)

The kilograms of fuel stored on board is Sx, while γH2 is the volumetric density of

the liquid hydrogen storage system, assumed to be equal to 40 kg/m3 based onKunze and Kircher(2012). In contrast, γref is the volumetric density of a HFO tank, and it is

assumed to be equal to 930 kg/m3 based onMonique and Vermeire(2012).

The assumptions on the amount of hydrogen that would be stored on board are very uncertain, because it would depend on the type of ship, the size and the specific design, but also on the demand for energy produced on board, which in turn depends on the desired range (hours that a ship would sail with a full tank), the power installed and the voyage conditions during a full tank voyage. The kilograms of fuel stored on board (Sx) can be estimated with the formula 4.5.

Sx = Eout∗ sf cmm (4.5)

Where:

Eout is the demand for energy used on board.

sf cmm is the specific fuel consumption of the main machinery option.

The demand for energy used on board can be estimated with the formula4.6.

Eout = n X i=1 (R ∗ Ti) ∗ (P ∗ Li) (4.6) Where:

R is the range of the ships in hours

Ti is the time in % of R in which the ship sails in each mode i.

Li is the engine load in % of the P (power installed) in which the engine is working in

each mode i.

The voyage conditions during a full tank voyage can be assumed, precisely they are: the time spent in a specific engine mode and the engine load. They were assumed equal

Chapter 4. TIAM-GloTraM 82

to the one found in the TEAMS model ofWinebrake et. al. (2007) and are presented in table 4.5. Also, they are assumed to be constant, so they do not change according to ship type and size and type of fuel on board.

Table 4.5: Assumed engine mode and the engine load condition during a voyage that would consume the full tank

Idle Manoeuvring Precautionary Slow Cruise Full Cruise Mode [T] (%) 1.25% 1.75% 5.00% 7.00% 85.00% Load factor [L] 2.00% 8.00% 12.00% 50.00% 95.00%

The energy stored on board in KWh for hydrogen can be expressed as:

Est =

EH2 ηF C

(4.7)

EH2 is the energy produced on board with the hydrogen and fuel cells system com- bination and ηF C is the assumed efficiency of fuel cells. It is, therefore, possible to write

equation 4.4as: dwt loss = E H2_{∗ sf c} H2F C∗ 0.8 γH2 −E ref_{∗ sf c} ref∗ 0.8 γref  ∗ ηF C EH2 (4.8) dwt loss =( Pn i=1RH2∗ Ti∗ PH2∗ Li) ∗ sf cH2F C∗ 0.8 γH2 − ( Pn

i=1Rref∗ Ti∗ Pref ∗ Li) ∗ sf cref ∗ 0.8

γref ∗ _Pn ηF C i=1RH2∗ Ti∗ PH2∗ Li (4.9) dwt loss =  RH2PH2∗ 0.8 ηF C∗ βH2∗ γH2

−Rref∗ Pref∗ sf cref ∗ 0.8 γref  ∗ ηF C RH2∗ PH2 (4.10) dwt loss =  0.8 βH2∗ γH2

− Rref ∗ Pref ∗ sf cref ∗ ηF C∗ 0.8 RH2∗ PH2∗ γref



(4.11)

It is possible to write RH2and PH2as a proportion of Rref and Pref using the factors

f 1 = RH2/Rref and f 2 = PH2/Pref. Also, the efficiency of fuel cells can be expressed

dwt loss =  0.8 βH2∗ γH2 −sf cref ∗ (ηref + f 3) ∗ 0.8 f 1 ∗ f 2 ∗ γref  (4.12)

By varying the factors f1, f2, and f3 the loss of cargo capacity can be evaluated as shown in figure 4.4. If f1=1 and f2=1 this means that we assume the range and power installed on board a hydrogen-powered ship will not change in comparison to a reference ship with HFO. If f3 varies then it means that the efficiency improves and a new surface is generated in the figure. On the other hand if f1 and f2 are minor than one, it means that the range and power of the hydrogen-powered ship is in percentage lower than the reference ship with HFO. For example if f1=0.8, it means that the range of the hydrogen-powered ship is 80% of the range of the reference ship.The variation of the factors generates surfaces in figure4.4, that shows how these range, power and efficiency affect the loss of cargo capacity. If the range and the power decrease significantly there would be no loss of cargo capacity. When the surface is under the zero level, it means that there would be more space available for cargo for the hydrogen-power ship than in the reference ship with HFO. also shows that if range and power on board ships is reduced then the loss of cargo capacity due to the lower density of hydrogen storage systems decreases and eventually results in having no impact on extra space requirements. In this thesis it is assumed that range and power of a hydrogen power ship will remain the same as the reference ship with HFO on board. Using equation 4.11 the dwt loss was estimated to be 0.52 tonnes/kWh. Also, the effect of LNG tank specifications was calculated with the same method, therefore assuming the volumetric density of the tank equal to 420 kg/m3 the dwt loss for LNG is estimated at 0.06 tonnes/kWh as reported in table4.6.

Table 4.6: Assumptions and estimates of dwt loss for LNG and Hydrogen storage systems on board

Type δ (kWh/kg) Vol. δ (Kg(f)/m3) dwt loss (te/MWh)

LNG tank 13.89 420 0.06

Liquid H2 tank 33.33 40 0.52