Simulation and analysis of a solar-assisted trigeneration system

UNIVERSITY OF TRIESTE 
 
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Simone De Luca – Master thesis        - 19 -
  
Abstract 
This work aims to assess energetic and economic performance of a solar-assisted 
trigeneration system that provides heating, cooling and electricity to a public hospital 
building. This analysis is based on a transient numerical model. 
The core objective is the development of a cost-effective control strategy for the solar field, 
which collects the thermal energy supplied by concentrating collectors and delivers this 
amount of heat to a cogeneration unit, represented by an Organic Rankine Cycle machine. 
Heating and cooling production are based on wasted thermal energy from the power unit, 
with the latter provided by an adsorption chiller. 
Such set of technologies represents an innovative solution for building size energy systems 
with few installed plants worldwide. Main critical issues are the variability of the solar 
energy source, the limited power cycle efficiency at low evaporation temperatures and the 
technical and control requirements for coupling low-grade rejected heat with an efficient 
cooling cycle.  
Energetic advantages of combined cooling, heat and power are expected to be remarkable 
even in small-scale applications, but the economic feasibility is still questionable. Therefore, 
a performance and economic analysis of the system is carried out using numerical simulation 
results. The final goal of this analysis is the definition of an optimal design for further 
development of the studied system. 
In order to achieve the proposed objectives, some tasks have been completed. 
Firstly, technical features of solar trigeneration systems are examined in order to define the 
state of the art of such systems. That investigation aims to rate the actual performance of 
such systems and understand which kind of improvements can be applied through an 
optimization process. 
After the theoretical study of each main component, a transient numerical model of the 
whole system is implemented using the TRNSYS simulation software. The thermal oil loop is 
completely modelled within this work, while heating and cooling circuits are simulated using 
an existing numerical model adapted to the studied system specifications. 
An overall control strategy is also elaborated, with particular attention to the minimization 
of the backup heater fuel consumption and the maximization of average solar output 
throughout the year. The control logic is implemented in the same transient model.  
Simulation results are used to evaluate performance of the obtained system. Electrical and 
thermal outputs are evaluated in terms of annual and seasonal amounts of energy, while 
consumption is considered through Final Energy and Primary Energy indicators. A comparison 
with two reference systems, for heating and cooling production, respectively, is the base for 
the evaluation of energy and economic savings related to the studied system. 
Last analysis aims to assess the economic feasibility of the project, by means of calculating 
an approximate return of investment, using actual costs of the components. Since the 
involved technology is costly, a further optimization of the system is considered to achieve 
higher economic performance by changing some model parameters. 
Chance of investigating this innovative trigeneration system is given by the European 
research project BRICKER.
UNIVERSITY OF TRIESTE 
 
_________________________________________________________________________ 
Simone De Luca – Master thesis        - 25 -
  
1 Introduction 
1.1 Aim of the project 
This thesis aims to develop a detailed numerical model of a trigeneration system generating 
space heating, cooling and electricity for a public hospital building, through the usage of 
concentrating solar technology. 
In particular, the work is centred on the optimization and the control strategy development 
of the high temperature solar loop, which delivers the input energy to the cogeneration unit, 
represented by an Organic Rankine Cycle (ORC) machine. 
The high temperature circuit uses thermal oil as heat transfer fluid and includes a number 
of parabolic through collectors and an auxiliary gas boiler. 
Heating and cooling production are based on wasted thermal energy from the power cycle, 
with the latter provided by an adsorption chiller. 
Such set of technologies represents an innovative solution for building size energy systems 
and few examples can be found in literature. Main critical issues of a solar-assisted 
trigeneration system are the variability of the renewable energy source, the limited ORC 
efficiency at low heat source temperatures and the technical and control requirements for 
coupling low-grade rejected heat with an efficient cooling cycle. 
Even well-known benefits of combining heat and power generation from the same energy 
resource might be affected by the mentioned technical difficulties and, for this reason, the 
development of an effective system control is a core part of the work. 
Energetic advantages of trigeneration systems are expected to be remarkable even in small-
scale applications, but their economic feasibility is still to be probed. Therefore, making use 
of numerical simulation results, a performance and economic analysis of the system is also 
carried out. The final conclusion of this analysis is the definition of an optimal design for 
further development of the given system. 
Chance of investigating such an innovative energy system is given by the European project 
named BRICKER, which involves the design and construction of a demonstrator trigeneration 
system. 
1.2 Methodology 
The methodology applied for achieving the proposed objectives, is briefly described in this 
section. 
The first step is the analysis of technical features of solar trigeneration systems in general, 
and main system components in particular, such as ORC, parabolic collectors and adsorption 
chiller. That is necessary in order to understand which kind of improvements can be applied 
to the studied system and what are the main problems to solve. Furthermore, through an 
initial state of the art review, it is possible to rate what are the actual performance of such 
a system and its room of improvement, achievable by means of an optimization process. 
After the theoretical study, the numerical model of the whole system is implemented using 
the TRNSYS transient simulation software: system components are considered separately, 
each one modelled with a different subroutine, while mass and energy fluxes are accounted
UNIVERSITY OF TRIESTE 
 
_________________________________________________________________________ 
Simone De Luca – Master thesis        - 26 -
  
with connections between them. A preliminary study on the required modelling tools is 
carried out before setting up the simulation.  
Concerning simulation activities, the thermal oil loop is completely modelled within this 
work, while heating and cooling circuits are simulated using a numerical model previously 
designed for other purposes and adapted to the studied system specifications. 
An overall control strategy is then elaborated in order to optimize the trigeneration system 
performance, with particular attention to the minimization of gas boiler consumption and 
the maximization of average solar output throughout the year. The control logic is 
implemented in the same transient model and appears crucial also for considering each 
component operating range.  
Instantaneous and yearly simulation results are used to evaluate performance of the 
obtained system. Electrical and thermal outputs provided by the system are evaluated in 
terms of annual and seasonal amounts of energy, while consumption is considered through 
Final Energy and Primary Energy indicators. 
A comparison with two separate reference systems, producing same amounts of heating and 
cooling is done, while generated electricity is assumed to be self-consumed by the system. 
In this way, energy savings related to the investigated system operation can be quantified. 
Last analysis aims to assess the economic feasibility of the project, by means of estimating 
economic savings related to a building-size solar trigeneration system and calculating an 
approximate value of the return of investment, based on actual costs of the components. 
 
In this chapter an overview of actual trigeneration technology is provided, with a focus on 
systems similar to the one which is matter of study. 
1.3 Trigeneration systems for small scale applications: an overview 
Cogeneration is defined as the simultaneous production of heat and power from the 
combustion of fossil or biomass fuels, as well as from geothermal and solar thermal 
resources. The same term is also applied to plants that use waste heat from thermal power 
generation processes. 
Trigeneration is the integration of a cogeneration system with a thermally activated cooling 
technology, with the aim of providing cooling loads during summer period. 
For the sake of simplicity, cogeneration and trigeneration concepts may be referred to as 
CHP (Combined Heat and Power) and CCHP (Combined Cooling, Heat and Power) in the 
following chapters. 
The fundamental concept involving cogeneration is that current day prime mover
1
 have such 
low efficiencies, that a fraction between 50 to 70 % of the fuel energy is being converted to 
heat rather than shaft power.  
As seen in Figure 1.1, the greater advantage of CHP systems is represented by their high 
overall efficiency:  the reuse of wasted thermal energy implies that less primary fuel is 
                                            
1. A prime mover is defined to be a machine that transforms energy from thermal, electrical or pressure form to 
mechanical form, typically an engine or turbine [1.1].
UNIVERSITY OF TRIESTE 
 
_________________________________________________________________________ 
Simone De Luca – Master thesis        - 27 -
  
consumed to obtain the same amount of electric and thermal energy of two separate 
generation systems. 
 
Figure 1.1: Comparison between cogeneration and separate production of heat and electricity 
An ideal cogeneration situation occurs when there is an equality between power and thermal 
demands, such as in industrial applications involving the utilization of the waste heat for 
some process related need such as drying, steam production, or auxiliary heat to furnaces. 
In this field, cogeneration has been utilized for over a hundred years. 
However, when exploiting CHP systems for residential purposes, heat demand may vary with 
seasonal changes, resulting in dramatically decreasing of CHP efficiency in summer when 
the need for heating is minimal, especially in hot climates. 
On the other hand, many regions exhibit a summer season with high demand for cooling and 
air conditioning, with the result of whether discomfort or high electrical consumption. 
These are remarkable motivations for coupling a Thermally Activated Cooling (TAC) 
technology with a combined heat and power unit, performing trigeneration. 
It is proven that, if properly designed, this system approach can achieve up to 50% greater 
system efficiency than a CHP plant of the same size [1.1]. 
The trigeneration concept has come up since a few decades, in the same time when CHP 
system began to be applied for residential consumers, usually involving large units with 
district heating, funded and operated by a municipality (utility cogeneration). 
Same as cogeneration, CCHP has been widely utilized in large-scale centralized power plants 
and industrial applications [1.1], but the concept of integrating various units to form a 
trigeneration system was first introduced in the early 1980s for municipal cooling and heating 
[1.2]. 
 
Today CHP has become economically feasible and is being addressed on a very large scale, 
while trigeneration systems are still limited to the industrial sector. 
According to 2015 REN21 Global Report [1.3], about 8% of the world’s electricity generating 
capacity is in CHP facilities, with a total installed capacity of 325 GW and an average overall 
efficiency of 75–90%. Growth of the employment of this technology has been remarkable: 
in 1983 just about 5% of the power in the United States was cogenerated while the 20% has 
been reached in 2010 [1.4]. In the same year in Europe, where cogeneration has been
UNIVERSITY OF TRIESTE 
 
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Simone De Luca – Master thesis        - 28 -
  
historically well established due to higher energy cost, the total CHP systems capacity 
installed has exceeded 105 GW with Germany leading with 22% of the EU overall capacity, 
followed by Poland and Denmark with 9%. Looking at single-country statistics, 46% of the 
electricity generation in Denmark is provided by CHP systems, the 30% in Latvia, Finland and 
Netherlands [1.2].  
With regards to trigeneration systems instead, high investment costs are limiting spread 
across the market. 
 
Nevertheless, CCHP has attracted considerable interest in the last years for small-scale 
decentralized applications, below one MWel, with the development of different options 
regarding cooling technologies and cogeneration units.  
In this sense, potential trigeneration users might be multi-residential dwellings and 
communities, office buildings, hotels, hospitals, shopping malls, universities, restaurants. 
In facts, small-scale CCHP systems can be effectively exploited within distributed energy 
supply systems, which are a mature and effective technology for cogenerated applications; 
this concept implies that multiple connected sources of both power and heat are taken into 
account in order to satisfy the energy demand of a certain area.  
Distributed generation is composed of small-scale systems, usually privately owned and 
operated, and represent a new and different business energy model, that has already been 
recognized as an efficient and reliable alternative to the traditional energy supply [1.5]. 
In this way there is no need of centralized thermal production, as all users can be connected 
together through a district heating network and the heat delivered from production units 
may be exchanged to one another or sent to the usually included thermal storage; also 
electrical power production can be decentralized using cogeneration technologies and smart 
electrical grids. These grids exploit information and communication technology with the aim 
of minimizing environmental impact, improving reliability, reducing costs and improving 
efficiency [1.6].  
Distributed generation permits to improve cogeneration systems efficiency for utility 
applications, balancing fluctuation of heating and electricity demand by the end-users. 
In order to avoid the rejection of high amount of heat during summer season, the addition 
of cooling systems based on heat sources, might lead to further enhancement of 
decentralized systems. 
There is a large literature about the market potential of small-scale trigeneration systems: 
On one hand the need of stand-alone power systems in remote and isolated areas of 
developing countries, on the other the need of small and efficient energy systems for grid 
connected applications in developed countries. 
Furthermore, distributed systems are an efficient way to exploit renewable energy sources, 
and match with the deregulation and privatization of the electrical generation sector that 
has been recently carried out worldwide [1.7].  
 
Considering what has been presented so far, a consistent market for trigeneration systems 
can be found worldwide. What it is not yet been described is the technology allowing 
efficient and affordable solutions for such applications. 
Different heat and power generating technologies have been considered in literature to serve 
as prime movers for CCHP applications. These could be divided into two categories: 
combustion-based technologies, such as internal combustion engine, Stirling engine, gas 
turbine, as well as Rankine cycle, and electrochemical-based technologies, i.e. fuel cells.
UNIVERSITY OF TRIESTE 
 
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Simone De Luca – Master thesis        - 29 -
  
Some of these technologies are commercially mature with wide availability in the market 
(combustion engines and turbines), while others are still in a development stage including 
Stirling-based units, organic Rankine cycle (ORC) based-systems and fuel cell-driven units.  
Concerning all the prime mover technologies listed above, the exploited energy source could 
be both renewable and non-renewable, which some advantages in the first case. 
In many applications indeed, cogeneration and Renewable Energy Sources (RES) complement 
each other, in the sense that several renewable technologies can be operated in 
cogeneration mode; some examples are biomass, geothermal and Concentrating Solar Power 
(CSP). Since both cogeneration and renewables as stand-alone technologies offer proven 
low-carbon benefits, they can perform a double reduction in CO2 emissions if taken together 
[1.8]. 
Another aspect that goes in favour of RES is that low energy conversion efficiency does not 
necessary imply high costs since the energy source is costless.  
A brief description of the most diffused PM for trigeneration systems will be provided within 
this section, in order to understand why it is interesting to couple an ORC with a solar energy 
source for small-scale trigeneration systems. The comparison is made between combustion 
engine, gas turbine and ORC technology. 
 
Due to their constant efficiency above 30%, a relatively high electrical power output and a 
limited initial investment cost, internal combustion engines represent the most used prime 
mover for CHP and CCHP applications in the range of 100-5000 kW. Combustion engines can 
be fuelled by diesel oil, natural gas or gasoline, as well as the product of gasification 
processes (syngas) or biogas. They are a scalable technology that comes from automotive 
and naval transport applications, that has been adapted to energy generation purposes. 
Waste heat from the engine can be recovered at different levels and a different temperature 
ranges: from exhaust gases at 200–400 °C, from jacket water cooling and oil cooling systems 
at 90–125 °C. These multiple and different heat sources increase the complexity of the 
required heat recovering system. 
Generally, these micro-CHP systems have an overall efficiency up to 80%. However, these 
systems are very noisy and need frequent maintenance due to their large number of moving 
parts; in addition to this, the high level of NOx emissions within the exhaust make them 
unattractive for small residential applications. 
 
Gas turbine systems are compact units composed of a combustion chamber, a compressor, 
a turbine and an electrical generator, all rotating on the same shaft. In CHP applications an 
additional heat recovery system is used to recover heat from the high temperature exhaust 
gas. They can be fed with the same wide range of fuels described for internal combustion 
engine-based CHP applications, but with slightly lower polluting emissions. 
Large size gas turbine systems (from hundreds kW to hundreds MW) are very common, 
producing heat and electricity for several commercial and industrial applications, whereas 
micro-turbines (30-400 kW) have recently become a widespread technology also to serve 
small-scale applications, such as distributed energy systems. 
Some advantages of micro-turbine technology are compactness and little required 
maintenance. Moreover, since hot gases are released at temperature of about 250 °C, they 
could be easily exploited for thermally activated cooling technologies, such as sorption 
chillers. Nevertheless, micro-turbine applications for building-size systems are still very 
limited due to their relatively low electrical efficiency and sensitivity of efficiency to 
changes in ambient conditions.
UNIVERSITY OF TRIESTE 
 
_________________________________________________________________________ 
Simone De Luca – Master thesis        - 30 -
  
Last technology to be investigated is represented by organic Rankine cycles. Rankine engines 
convert heat into mechanical work making use of the thermodynamic Rankine cycle, which 
can exploit different working fluids: steam Rankine systems utilize water whereas organic 
Rankine systems use synthetized organic working fluids. 
Traditional steam Rankine cycle can be exploited only in large-scale applications in order to 
reach high efficiency and reasonable investment cost; that is because of their low electrical 
efficiency at partial load and long start-up time. 
Regarding small-scale applications, Organic Rankine Cycle (ORC) emerging technology 
appear to be a robust and cost-effective co-generative solution, thanks to lower operational 
temperature and pressure, as well as high level of safety. Moreover, such systems offer the 
possibility of utilizing low temperature heat sources, e.g. biomass combustion, waste heat 
and solar energy. 
In the range between 200 to 2 MWel ORC systems has demonstrated their technological 
maturity: more than 140 biomass fired ORC plants have been installed in Central Europe, 
making ORC a well-proven and commercially available technology within decentralized CHP 
systems market [1.9].  
Regarding small power applications, instead, ORC is still in development or in early 
commercial phase since reducing ORC unit size penalizes both efficiency and specific cost; 
nevertheless, a number of companies is currently producing ORC modules with net power 
outputs below 200 kWel [1.10]. 
Electric and CHP efficiency may vary a lot with regards to ORC, depending mainly on system 
size. Usually, larger ORC units utilize operating fluids with higher boiling point, such as n-
pentane, toluene and siloxane, which allows both higher electrical efficiency and the chance 
of having rejected heat above 60°C, very useful for utility cogeneration. These are 
commonly known as high temperature ORC and present remarkable potential for CCHP 
systems. On the other hand, small-scale organic Rankine systems are often based on low 
temperature ORC, with low boiling hydrocarbons (R-134a, R-245fa, R-152a) as operating 
fluids. These units show lower efficiencies and sink temperatures around 25°C that makes 
them suitable only for bottoming cycles. 
CHP and CCHP systems with ORC as PM have received increasing attention especially for 
domestic and buildings applications but, in order to make them more efficient, a high level 
of process control and automation seems to be necessary, in particular for extremely 
variable-profile sources such as RES. 
 
A comparison of the most common trigeneration prime movers is presented in the table 
below, with regards to applications up to 500 kWel; as internal combustion engines and gas 
turbines can be used for totally different size application, their cost and capacity may differ 
a lot case by case. With regards to the whole set of systems, some technology improvements 
that lead to great increase in power efficiency - high tech materials, high operating 
temperature and pressure, etc. - can be cost-effective only for large scale applications. This 
means that co-generated exploitation of these technologies appears to be even more 
important for small-scale applications.