1. Introduction 
Presently the overall capacity of electricity generation systems based on geothermal
energy installed within European countries is 1.13 GW (EU 0.8 GW). These geothermal
power plants contribute with approx. 7.1 TWh/a (EU 6.0 TWh/a) to the European
electricity supply. The by far biggest share of this electricity generation (about 98
%) results from high enthalpy fields which can only be exploited at specific
locations with promising geological conditions. Only to a very small extend low
enthalpy resources are presently used. But these low enthalpy resources have by far
the biggest electricity generation potential throughout Europe and worldwide. 
However, techniques for exploiting low enthalpy resources for the production of
electricity are currently only partially established. Firstly there is the challenge
to lock up the deep underground to achieve flow rates at temperatures high enough to
realise an economic viable project. Secondly the surface technology (i.e. power plant
technology) needs to be further developed and optimised in order to achieve high
efficiency rates with low investment costs and thus low electricity generation costs. 
In the past, different technical approaches to convert low temperature heat into
electricity have been developed. Some systems are available on the market, others are
still in a demonstration phase and others only exist as system studies (i.e. on
paper) so far. The overall goal of these development efforts was and is to achieve
the highest possible efficiency rates taking the given thermodynamic constraints into
consideration. Therefore for the use of low enthalpy resources mostly binary cycles
are chosen. These cycles have on one hand the advantage of the turbine not getting in
touch with the brine in case of e.g. too aggressive brines; on the other hand – and
even more important – binary cycles are able to convert low temperature heat (i.e.
even temperatures below 100 °C) into electricity. However, a simple conclusion about
the most promising binary cycle respectively power plant technology can not be drawn
based only on the criteria “efficiency”. Additionally a wide range of further aspects
has to be taken into consideration. In general thermodynamic aspects need to be seen
together with technical feasibility and availability as well as economic,
environmental and social aspects related to the site specific conditions. 
On this background the aim of the EU-funded ENGINE-workshop was to analyse and assess
possibilities and limitations of the currently available power plant technology using
the energy retrieved from low enthalpy geothermal sources. To get an overall view
representatives of research and industry as well as project operators and planners
have been brought together and presented their individual view. Besides some partly
controversial conducted discussions, the basis for fruitful exchange of information
and experiences was thus created. The slides of the presentations are available under
http://engine.brgm.fr/. 
Besides the R&E-needs irrespective of a specific cycle or power plant (e.g. material
selection for sealing, turbine parts, heat exchangers etc., optimising of the working
fluids) the main findings and discussion results of the workshop can be summarised as
follows to give an impression of the overall picture. 
	2. ORC or Kalina Cycle? 
	Geothermal electricity generation from low enthalpy resources is realised in binary
plants. Thereby the heat of the brine is transferred via heat exchanger to a working
fluid, which evaporates already at low temperatures (e.g. organic substances).
Currently two types of binary cycles suitable for the frame conditions given in
geothermal power plants for low enthalpy resources are available on the market: the
Organic Rankine Cycle (ORC) (i.e. a conventional Rankine cycle running with a
specific working fluid evaporating below 100 °C) and the Kalina cycle (i.e. also
basically a Rankine cycle being fed with a working fluid consisting of a mixture of
two substances like e.g. ammonia and water). To use of such a mixture as working
fluid has the advantage that the heat can be transferred more efficiently between the
brine and the power plant cycle; this reduces possible losses. Therefore the Kalina
cycle promises higher efficiency rates within a certain temperature window and hence
might be advantageous at temperatures below 130 to 140 °C. But the use of a mixture
of two substances with a varying mixing ratio within the working fluid streams is
only possible with a more ambitious and expensive technology compared to the use of
only one substance. This is one of the reasons why there is only one Kalina cycle in
operation within a geothermal power plant so far. In contrast there are numerous
Organic Rankine Cycles under operation worldwide. Apart from these differences
between the two cycles “ORC” and “Kalina cycle” these cycles do have more in common
than being contrary. Each cycle has for a certain application at a specific spot
specific pros and cons. But both cycles also show a significant optimisation
potential in terms of the design of the working fluid, the cycle and turbine designs
as well as the cooling system. Therefore the question is not to choose an ORC or a
Kalina cycle. Rather the optimisation task is to find the right cycle for the
reservoir characteristics and the other project specific demands given at a certain
site. 
	3. Axial or radial turbines? 

The turbine used within an ORC or a Kalina cycle is in most cases an axial inflow
type. This is derived from the conventional water steam turbine industry where axial
turbines are state of technology due to their promising performance within the
respective application (i.e. for the use in big e.g. coal fired power plants). The
design parameters of the turbine however within cycles driven by low enthalpy
resources can vary decisively compared to a “classic” turbine used within a water
steam cycle (e.g. enthalpy drop, stream and rotor velocity). Therefore investigations
have shown that radial inflow turbines can lead under specific conditions to higher
efficiencies. Considering the importance of optimising the efficiency of such cycles
under the conditions defined by the geothermal reservoir without raising the overall
complexity of a cycle radial turbines could be a promising opportunity. Therefore the
question is not to use axial or radial turbines. The point is to choose the turbine
type promising the highest efficiencies at lowest risks – without any ideology and
predefined opinions. 
	4. Air or water cooling? 

The working fluid of the ORC or Kalina cycle could be cooled down with cooling
systems driven by air or water. Air cooled plants have the disadvantage to face
seasonal changes in cooling temperatures. Therefore such systems can often not
guarantee maximum power from the plant throughout the year. Additionally the running
fans need electricity and space; also noise emissions need to be considered at a
location close to populated areas. In contrast water cooled plants in most cases can
realise lower and over the year more constant condensation temperatures and
pressures. They therefore allow for a larger enthalpy drop in the turbine and thus
slightly higher efficiencies. However the mass flow of water in the demanded quality
required by the water based cooling system needs to be provided considering legal
obligations e.g. of the temperature level and the available amount. This can also
culminate in high provision costs due to power demand for pumps and e.g. water
conditioning systems. Therefore - at a certain location - the question about air or
water cooling is not really the most important one because in most cases a location
specific compromise has to be found anyway. If e.g. enough water is cost efficient
available in most cases a water cooling system will be implemented due to economic
reasons. But often this is not the case. Then there is only the chance to go for an
air cooling system or even a combined system. 
	5. Fancy or proven technology? 

Fancy (“high efficiency – high risk”) or proven (“low efficiency – low risk”)
technology is a matter of the viewpoint respectively of the philosophy. Aiming for
low risks one can get good and reliable power plant technology on the market
characterised often by relatively low efficiencies; this is e.g. the case for a
standard power plant based on an ORC. Accepting a slightly higher risk one will find
on the market cycles which promise considerably higher overall efficiencies with the
disadvantage that these cycles do exist so far maybe only as a demonstration plant or
even only on paper. Therefore the question is not to go for fancy or proven
technology. The question is what technological risk a project can / will accept for
the profit the project strives for. 
This question is in most cases not answered by the project developer; rather the bank
or the investor providing the credits decides what risk might be taken. Due the fact
that the risk finding the reservoir conditions needed for an economic viable project
is in most cases quite high most projects go for a proven and well known power plant
technology with a high availability in order to minimise the existing risk. This
attitude makes it very difficult for new and innovative technologies to break into
the market. Therefore fund raising of public money for demonstration projects is
often an important step in order to prove technical feasibility of new technologies
to allow them the market access. 
	6. Power or CHP? 

Converting low enthalpy resources to electricity produces considerable amounts of
waste heat because of necessarily low thermal efficiencies due to the – according to
Carnot – given thermodynamic constraints. The logical consequence – regarding the
relatively high investments of geothermal power production from low enthalpy
resources – is therefore to try to additionally sell this heat on the local heat
market and realise combined heat and power (CHP) projects like i.e. in Húsavik,
Iceland, or in Neustadt-Glewe, Germany. In order to further optimise this economic
win-win-situation under the given economic and political frame conditions it might be
even more promising to run a geothermal CHP plant heat leaded instead of aiming for
the highest power output. In Unterhaching, Germany, for example, running the system
with the overall installed electrical capacity for maximising the electricity output
is the strategy for the summer, whereas during the winter supplying the demanded heat
- even by reducing the electricity generation - has priority. Therefore the goal
should always be to find a way to sell the heat locally respectively to identify a
location where a heat demand is given to improve the economic performance of a
geothermal power plant especially for low enthalpy resources. 
	7. Lessons learned 

The available know-how from existing geothermal power plants – regardless if with ORC
or Kalina cycle, with axial or radial turbine, with water or air cooled, if power or
CHP, with fancy or proven technology – shows that electricity production from low
enthalpy resources in Europe is still a fairly young technology which lacks further
experience. This is the case for the development of geothermal resources (i.e. the
underground part) as well as power plant systems (i.e. the part on the surface).
Nevertheless there are quite a lot of projects planned and considerably more
experience will be available in the years to come. On this background the lessons
learned so far can be summarised as follows. 
	• Optimisation was mainly thought to be a question of thermodynamics. But regarding
geothermal projects this is only one part of the whole picture, in which technical
and economic aspects as well as the site specific frame conditions need to be
included in order to provide high availability and economic feasible power plants.
Therefore the simple discussion about the pros and cons of ORC vs. Kalina cycle, of
air vs. water cooling, of fancy vs. proven technology and of power vs. CHP in terms
of a further development of geothermal energy use will not lead to any results. The
main task of project developers instead is to identify the site specific conditions
and clarify the perception of risks which can be taken. 

	Thereupon the project as total needs to be optimised aiming for economic feasibility
free of predefined opinions. 
	• New and innovative technology is always connected with technical and financial
risks (“No risk, no fun!”). This is also the case for systems generating electricity
from geothermal resources in general and for low temperature power plants in
particular. But with an increasing technical effort (and higher costs) and innovative
ideas the efficiency (and thus the income) of a power plant cycle can be improved.
Before being able to break into the market these technologies need to be proved which
is generally not possible on a purely commercial basis. Here the government is asked
to support the market access of such new and innovative technologies which are
definitely needed for further establishing geothermal electricity production in Europe. 
	• Another approach to promote geothermal electricity production from low enthalpy
resources – and also evidence that promoting geothermal energy use needs open minded
project developing – was stated as a combination with other sources of energy (like
e.g. biogas plants). New concepts of combining different energy options supplying
heat on different temperature levels can result in a higher overall efficiency and
thus profitability and hence be decisive for realising geothermal based electricity
production. 

Low enthalpy resources for geothermal energy production show the potential to
contribute substantially within the energy system (i.e. within the heat and
electricity market) throughout Europe. Thereby it is necessary to develop this
technology to contribute for a more sustainable energy system in the future. On this
background the goal is to successfully develop (i.e. technologically promising,
economically viable, environmentally benign and socially acceptable) geothermal power
plants. Project development and optimisation is hence a task of having a look on the
overall picture.

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