The total emergy flow is obtained by summing all the emergyflows relative to the inputs, with the exception of sun, rain andwind, since they can be considered as co-products: the maximum ofthe three inputs is the one taken (Odum, 1996). Inputs are classifiedin three categories: local renewable (R), local non-renewable (N)and imported (F). In order to calculate the energy flows of wind Fig. 1. Geographical location of the Orbetello Lagoon.and waves, we have used the following formulas (Eq. (1) and (2))(Odum, 1996):
Wind energy
=
area
·
air density
·
drag coefficient
·
wind speed3 ·
time (1)
Wave energy
=
shore length
·
(1/8)
·
water density
·
gravity
·
wave height squared
·
velocity
·
time (2)
In addition to the UEV indicators for macroalgae production andfor biofuel production, also %Renew (100 × R/(R + N + F)), the ratiobetween the local emergy of renewable resources and the totalemergy of the product, multiplied by 100, and the emergy yieldratio EYR, the total emergy per unit of imported emergy (R + N + F)/F)are calculated.2.2. Energy return on (energy) invested (EROI)EROI is the ratio of available energy obtained from an energyproduction process compared to the energy required from societyto produce that energy.
EROI
= energy gained
energy required to get that energy
There is no unique way to calculate EROI, and this makes itdifficult to compare results (Murphy and Hall, 2010). Mulder andHagens (2008) categorized EROIs according to what was included inthe analysis and how non-energy resources were considered. Wecalculate EROIs based on direct and indirect fossil energy inputsand direct energy outputs. This corresponds to what Mulder andHagens (2008) call a simple second order EROI