Thermodynamic indicators

Energy-based indicators. There exist numerous metrics that characterise the performance of industrial processes, with regards to their energetic performance:

• the energy efficiencyη, defined as the ratio of the desired product to the spent resources,

in terms of energy;

• the energy intensityιh, defined as the ratio of the resources consumed on-site, to the

energy content of the desired product;

• the energy wasteωh, defined as the ratio of the energy content of the waste streams per

unit of product;

• the specific power consumption, defined as the power consumed on-site per unit of product.

The first indicator is widely used in the power and gas industry, while the three latter are mostly used in the chemical sector.

Exergy-based indicators. Similar indicators to the energy intensity and waste parameters may be developed on an exergy basis. For instance, performance parameters related to the exergy destruction and losses [54, 76, 181, 182] were developed to illustrate the possibilities for improvement and indicate the components and sub-systems on which attention should be focused:

• the exergetic efficiencyε, which reflects how the system under study performs compared to a thermodynamically perfect one:

εk= ˙ Ep,k ˙ Ef,k = 1 − ˙ Ed,k+ ˙El,k ˙ Ef,k (3.31)

The fuel and product exergies are not necessarily equal to the exergy flows entering ˙Ein,k

and leaving ˙Eout,k.

• the exergy destruction ratio y_d∗, which illustrates the relative importance of the kth

component compared to the whole system, in terms of exergy destruction:

y_d,k∗ =Ed,k˙_˙

Ed (3.32)

• the exergy loss ratio y_l∗, which indicates the relative importance of the kth component

or material stream compared to the whole system, in terms of exergy losses:

y_l,k∗ =El,k˙ ˙

El (3.33)

• the irreversibility ratioλ, named exergy loss ratio in Kotas [54, 182] and derived from

the exergetic efficiency definition proposed by Grassmann [45], which represents the fraction of the total input exergy that is destroyed through irreversibilities:

λ = _˙I˙ Ein

(3.34)

• the efficiency defectδk, which corresponds to the fraction of the total input exergy that

is destroyed in the kth component or subsystem:

δk=

˙ Ik

˙

Ein (3.35)

The concept of irreversibility rate, as mentioned in Kotas [54], is strictly equivalent to the concept of exergy destruction used in other works in the field of exergy.

Based on the conclusions drawn from an advanced exergetic analysis, it is possible to use two alternative performance indicators, in addition to the ones used in a conventional exergy assessment:

• a modified exergetic efficiency, denotedε∗_k, which focuses on the avoidable part of the

components with different functions: ε∗k= ˙ Ep,k ˙ Ef ,k− ˙E_{d ,k}UN (3.36)

• the potential for enhancing the system performance by improving the kth component,

denoted ˙E_D,kAV, , and which consists of the avoidable endogenous exergy destruction,

summed to the avoidable exogenous exergy destructions in the other components, caused by the component under study:

˙ EAV, d ,k = ˙E AV,EN d ,k + n  r=1,r =k ˙ E_{d ,r}AV,EX,k (3.37) 3.8.2 Economic indicators

The economic aspects are assessed by calculating the investment Cinvand operating Copcosts,

using the cost correlations of Turton et al. [79] for the first ones. The operating costs are related to the number of operators, the replacement and maintenance of the several components, and the taxes paid because of the emissions of carbon dioxide.

If the integration of an additional process is investigated in a retrofit situation, for instance, with the implementation of a steam cycle, the investment costs are taken to be the additional investment costs. The supplementary operating costs are neglected, assuming that there is neither an increase of the number of operators, nor a higher operator’s salary.

In this specific case, the economic performance can be assessed with regards to the potential

fuel gas savings and reductions in CO2-taxes, or, in other works, with the relative increase in

exported gasδNG:

δNG=m˙NG− ˙mNG,ref ˙

mNG,ref

(3.38)

The economic value of the exported gas streams cannot be precisely estimated. For instance, for the case of the Draugen platform, which is one of the facilities investigated in this thesis, the exported gas is sent through the Åsgard pipeline system. It is mixed with natural gas from the other petroleum fields located in the northern part of the North Sea, and these flows have different chemical compositions (e.g. light- and medium-weight hydrocarbon contents) and physical properties (e.g. viscosity and heating value).

The mixed streams are then treated at the Kårstø plant, in which they are split and refined into a large variety of hydrocarbons (natural gas and liquid petroleum gases) that are exported worldwide. Calculating the economic value of a single natural gas stream is therefore difficult. The flow rates and compositions of the gas streams from the other facilities should be known, and there are high economic uncertainties on the market. On the contrary, the relative increase

of the export gas flow is a clearer and less controversial performance indicator, which depends solely on the facility under study.

For these reasons, economic indicators such as the net present value or the payback time, which combine in a single metric the capital and operating costs, are not considered in this study. They would require a precise knowledge of the economic benefits made by the platform operators for exporting additional gas, which are difficult to estimate for the reasons men- tioned above, and which most likely would not be given by the companies for confidentiality reasons. The decommissioning costs have not been included in the economic evaluation, since these costs are site-specific and vary from one plant to another.