68 Introduction to Renewable Biomaterials
the bottom of the ocean at high pressures and low temperatures. When either of these
components is disrupted the structure of gas hydrates breaks into its individual com-
ponents. Generally, the lower the water temperature the lower the pressure required
to maintain the stability of methane hydrates. Methane hydrates pose very different
risks of extraction compared with other fossil fuels. The latter are extracted from rather
stable and well-defined geological structures; methane hydrates on the other hand
are expected to be extracted from deep oceans where their stability is maintained by
pressure and low temperature. Proposed extraction methods are very likely to rely
on local disruptions of this to release the gas. If any of the parameters of this process
is not set up correctly, there is a catastrophic risk of destabilising the entire deposit
and releasing toxic methane to the surface and subsequently atmosphere. Ignition of
destabilised hydrate is likely to cause an explosion that might result in a destructive
tsunami.
3.2.3.8 EROI – How Much Fuel in Fuel?
The history of the human development to date has been a history of replacing
low-quality fuels and working with higher quality ones. Civilisations expanded with
their move towards better resources: from human muscle power to draft animals to
water and wind mills to coal and ultimately to petroleum [5]. Now, for the first time,
civilisations need to deal with the opposite process: how to replace high-efficiency
energy means with lower efficiency ones. The main reason is the depletion of
good-quality fossil resources. There is nothing particularly surprising in this finding;
it is the fundamental characteristic of humans and a component of how our economy
works [5]. Whatever the resource, humans start its utilisation from the best available
and move towards lesser available ones until they prove to be useless or too expensive
in monetary or work input terms. The economic reality dictates that an individual
needs to use best resources at the lowest cost possible first otherwise the competitors
will gain an advantage and price an individual out of the market.
An indicator that is used to measure the efficiency of a particular source of energy is
called energy return on investment (EROI) [5]. EROI is a ratio describing the content
of energy that one obtains from an activity compared to the energy it took to generate
that activity; in other words, it describes how much usable energy is left in the resource
after it is obtained. Investment of a joule of energy in a resource of high EROI produces
many of joules of energy that can be spent on another activities [5]. Good energy sources
have high EROI factors, whereas poor energy sources are closer to 1. EROI equal to 1
is the ultimate physical barrier, which determines usefulness of a resource for energy
production; it means that the process generates no net energy and is therefore point-
less. The relationship between useful energy extracted from the resource and its EROI
is exponential. As long as the EROI of an energy source is above 10, its EROI makes
little difference as over 90% of the energy remains useful. Lower values of EROI, how-
ever, fall into the region called ‘net energy cliff’ where even slight differences in EROI of
a resource make huge impacts onto how much useful energy can be actually obtained
from this resource [7] (Figure 3.2).
First oil fields that were exploited in the beginning of twentieth century provided EROI
in excess of 100 [9]. Once these fields became depleted, the less accessible resources
were used and EROI of 50–30 was characteristic for the oil extracted in the 1970s [5].
Nowadays, conventional oil fields offer EROI in the range of 18–11 [5, 9] and are being
