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However, the contribution of RES to the total energy supply in small islands is often limited
by grid stability concerns. In fact, though a large number of renewable sources is in general
available [4], being that most of small islands are not or are only partially connected to
continental electrical networks, appropriate management of energy supplies is required to
reach a perfect balance between production and demand and sufficient electric grid safety
standards. In a recent interesting paper a detailed survey was conducted for the French islands
[5], pointing out that RES potential is often underused, accounting for a share between 5%
and 45% (with a 20% average) in terms of installed capacity and between 2% and 30% (with
a 10% average) in terms of annual production. In particular, difficulties emerge with
increasing penetration of intermittent energy sources (like solar PV and wind energy),
characterised by high intermittence levels and stochastic character; when such sources are not
available, in fact, other generation units have to compensate the lack of power and,
consequently, sufficient reserve margins must be ensured (units supplied by fossil or
predictable-renewable sources and already in part-load operation). Although advances in
energy storage technologies are expected to weaken the problem in the medium-term, most of
the energy systems currently installed on small islands have the following in common:
- A higher rate of exploitation of deterministic renewable energy sources (depending upon
their local availability), such as hydropower [6] and biomass [7], compared to the
aforementioned intermittent ones. In a very few cases, an appropriate mix of stochastic and
flexible/predictable renewable energy sources has allowed to achieve very high RES
penetration levels [8];
- The largest share of energy supply by diesel or heavy fuel oil, used in internal combustion
engines. This result, that has been verified by an extensive survey at European Union (EU)
level examining a significant number of small islands out of the 286 located within the EU
territory [9], is due to a number of factors, such as: (i) the relative ease with which fuel can be
purchased and supplied (compared to less attractive alternatives, such as natural gas to be
transported either by pipelines or liquefied by tankers), (ii) the flexibility of engines
(especially when designed by a modular approach) in meeting daily and seasonal variations in
energy demand.
In spite of the high share of installed power generation capacity by diesel oil-fuelled Internal
Combustion Engines (ICEs) and of the highly strategic outcomes that could be achieved for
small islands [10], the penetration of Combined Heat and Power (CHP) in small islands is
very low. This is mainly due to the following factors:
1. The difficulties in exploiting the recovered heat to supply a useful heat demand.
Residential and non-domestic energy users are often disseminated over large areas (eventually
characterized by complex orography), with a very low demographic density, thus making the
viability of district heating (DH) or cooling (DC) networks hard to achieve;
2. The heat requests highly fluctuate on a seasonal basis. In many Mediterranean
islands the heat loads for space heating are quite low, both because of the mild climate and the
small share of permanent residents (compared to the annual peak of presences), especially in
sites with a touristic vocation.
In a recent document assessing the CHP potential in Malta [11], it is clearly stated that
feasibility of a small-medium scale district heating (and cooling) network is a preliminary
condition to favour the penetration of cogeneration or trigeneration; in the same work two
main barriers to the spread of district heating are identified: (i) the prevalent use of electric
heating in standalone residential users, due to the absence of natural gas grids, (ii) the short
space heating season due to the mild climate.
Despite district heating represents a well consolidated technological solution, with a high
penetration especially in densely populated areas and cold climates, intense research activities
have been conducted in the last few years as concerns possible improvements or identification

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