Energetic and Economic Assessment of a Solar Thermally Driven Innovative Tri-Generation Unit for Different Use Cases and Climates

Sustainability 2025, 17, 10924 https://doi.org/10.3390/su172410924 Article Energetic and Economic Assessment of a Solar Thermally Driven Innovative Tri-Generation Unit for Different Use Cases and Climates Uli Jakob 1 , Michael Strobel 1,2, * and Luca Ziegele 1 1 Dr. Jakob Energy Research GmbH & Co. KG, 71384 Weinstadt, Germany; [email protected] (U.J.) 2 Unit of Energy Efficient Building, Faculty of Engineering Science, Universität Innsbruck, 6020 Innsbruck, Austria * Correspondence: [email protected] Abstract The energy sector is currently under enormous transition, moving from fossil fuels to renewable energies and integrating energy efficiency measures. This transition can hold opportunities for new and innovative energy systems. This study presents an energetic and economic assessment of an innovative tri-generation unit working with a two-phase thermodynamic cycle. The tri-generation unit is driven by heat and is capable of providing heat at lower level, cold, and electricity to end users. The use cases—residential, day-use offices, commercial retail, and manufacturing industry—are integrated in a dynamic simulation model, indicating the operation mode of the unit. The results show that the trigeneration unit is able to provide heat and cold with an Energy Utilization Factor of 35% to 68%, depending on the use case. Solar thermal has a limited to potential to supply the unit with heat, due to the high temperature of 180 °C and the required unit operation at nighttime. The economic comparison indicates that the driving heat must be as low as possible and that savings through self-consumption is most relevant. Keywords: renewable energies; energy efficiency; tri-generation; combined cooling heating and power; thermodynamic cycle unit 1. Introduction The climate target of not exceeding 1.5 °C compared to the pre-industrial climate was already exceeded in 2024 [1], resulting in more frequent weather extremes and calling for a rapid transition in our energy production and consumption. The EU has made this transition legally binding: climate neutrality by 2050 and at least −55% net greenhouse gas cuts by 2030 under the European Climate Law and Green Deal [2]. The main challenge is the more efficient use of energy and upscaling renewables. The third version of the Renewable Energy Directive (RED III) raises the EU’s renewables share to a minimum of 42.5% in 2030. Additionally, the recast Energy Efficiency Directive (EED) sets a binding 11.7% reduction in EU energy consumption vs. the 2020 reference scenario by 2030 [3]. Variable renewable energy production brings challenges to the electricity and energy systems. The EU is therefore prioritizing “efficiency-first”, high-efficiency cogeneration, and efficient district heating and cooling into policy, and is setting milestones for integration of renewable energies and waste heat through 2050. These frameworks reward Academic Editor: Jesús Ballestrín Received: 30 October 2025 Revised: 24 November 2025 Accepted: 27 November 2025 Published: 6 December 2025 Citation: Jakob, U.; Strobel, M.; Ziegele, L. Energetic and Economic Assessment of a Solar Thermally Driven Innovative Tri-Generation Unit for Different Use Cases and Climates. Sustainability 2025, 17, 10924. https://doi.org/10.3390/ su172410924 Copyright: © 2025 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/). Sustainability 2025, 17, 10924 2 of 22 solutions that deliver multiple energy vectors with primary energy savings compared to separate generation [4]. Innovative multi-generation concepts, like tri-generation, also referred to as Combined Cooling, Heating, and Power (CCHP), that simultaneously supplies electricity, heat, and cooling from a single energy driver, match this need by coupling sectors and balance renewable variability at the point of use. The concept of CCHP units or systems is already presented and discussed in different forms. The typical CCHP concept is based on the combination of a CHP unit coupled with a thermally driven absorption chiller [5]. The CHP unit predominantly generates heat, with an estimated electrical efficiency between 20% and 40% [6]. The surplus heat then can be used to drive a thermally driven absorption chiller, characterized by an estimated thermal COP of 0.6 to 0.8. The electricity generation can be increased if the generated heat is further used in an Organic Rankine Cycle (ORC) unit. However, the electrical efficiency of an ORC unit driven with temperature below 200 °C is relatively low, with ca. 10% up to 25% [7]. Thus, the potential of additional power generation, especially for smaller units is very limited. Additionally, those CHP-based concepts of CCHP need a driving fuel, such as natural gas, biogas, or synthetic fuels. Heat-driven solutions have the benefit that they require fewer steps and energy transfers to provide the driving energy for CCHP compared to fuel-driven technologies. Dupuy et al. identify three types of heat-driven systems [5]: • Single-generation: 1 Output. • Cogeneration: 2 Outputs. • Tri-generation: 3 Outputs. Any CCHP unit relies on individual components, such as generators for electricity supply and heat exchangers for heat and cold extraction. Based on the cold generation typology, components such as an absorber and a generator for the sorption process, or a compressor for the vapour compression cycle, are required. Unlike a CCHP system with multiple units involved, a tri-generation unit is a single unit that is able to provide both heating, cooling, and electricity. The tri-generation cycle developed and patented by Stefano Briola follows such an approach [8]. The patented setup of the tri-generation unit is presented in Figure 1a. No thermally driven sorption cooling is involved in the tri-generation concept; instead, a vapour compression cycle is used. Additionally, the tri-generation cycle uses only one refrigerant that is capable to absorb thermal energy at high temperature (driving energy), extract heat for the user, drive generators through expansion work, and extract cold at low temperature. A fundamental concept of the process design is the isothermal heat transfer in the two-phase area. In this area, the refrigerant is neither fully liquid nor fully gaseous. The thermodynamic cycle is presented in Figure 1b in a Temperature-Entropy (T-S) diagram. The cycle can be divided into an upper sub cycle (steps 1–8) and a lower sub cycle (steps 1–12). However, operating in the two-phase area poses particular challenges to the performance of both compressors and expanders. First investigations on how to tackle these challenges already have been made and presented to the scientific community [9,10]. Sustainability 2025, 17, 10924 3 of 22 (a) (b) Figure 1. (a) Tri-generation unit cycle with the individual components and (b) Temperature-Entropy diagram of the tri-generation process showing operation mainly in two-phase area. Source: [11]. This study will refer to a total of 13 and 15 states of the tri-generation unit, respectively, as presented in Figure 1. The change in refrigerant characteristics between each state is referred to as step. Table 1 briefly describes each step of the tri-generation thermodynamic cycle. The mentioned steps will further be referred to throughout this article. More details on the structure and concept of the thermodynamic cycle can be found in the publications of Briola [8,12]. Table 1. Description of the single steps of the tri-generation process. Step Description 1–2 Compression; rise in temperature, pressure, and entropy 2–2* Heat transfer, heat absorption; isothermal, rise in entropy 2*–3 Compression; rise in temperature, pressure, and entropy 3–4 Heat absorption from ext. heat source; superheating, rise in temperature, and entropy 4–5 Expansion (power generation via generator); reduction in temperature and pressure, rise in entropy 5–5* Heat transfer, heat disposal; reduction in temperature and entropy 5*–6 Expansion (power generation via generator); reduction in temperature and pressure, rise in entropy 6–7 Condenser (heat supply to user); isothermal heat rejection, entropy loss 7–8 Expansion (power generation via generator); reduction in temperature and pressure, rise in entropy 8–13 Mix with state 12 to state 13 1–9 Heat rejection to ambient; isothermal loss of heat, entropy reduction 9–10 Expansion; reduction in temperature and pressure, rise in entropy 10–11 Evaporation (cold supply to user); isothermal heat absorption, rise in entropy 11–12 Compression; rise in temperature, pressure, and entropy 12–13 Mix with state 8 to state 13 13–1 Heat rejection to ambient; isothermal loss of heat, entropy reduction Another challenge in the set-up of this tri-generation unit is the selection of a suitable refrigerant, being in line with the latest legal developments regarding the Global Warming Potential (GWP) of refrigerants. A detailed screening of about 60,000 structures revealed Sustainability 2025, 17, 10924 4 of 22 that the refrigerant R1233zd(E) is most promising, considering thermodynamic, processrelated, constructional, safety, and environment constraints [13]. In addition, the refrigerant must also work with the lubricating oil, which directly has an influence of the performance of the compressors and expanders. The actual thermodynamic and fluid dynamic properties of refrigerant–lubricating oil mixture are very difficult to estimate and hold a wide range of uncertainties. An investigation on the performance of the selected refrigerant R1233zd(E) mixed with a Polyolester oil (POE) lubricating oil showed that enthalpy and entropy of the working fluid are influenced, given a modified characteristic of density and viscosity [14]. This study evaluates the dynamic performance of a novel tri-generation unit based on a two-phase thermodynamic cycle. The tri-generation unit is assessed including heat supply from concentrating solar thermal and analyzed in different climates. Building on the recent EU research project (Regen-By-2), two-phase expanders/compressors enable efficient conversion of renewable or waste heat into electricity, useful low-temperature heat, and chilled water in one plant [11]. The study tests its economic and energetic performance across representative use cases, within residential, day-used offices, commercial retail, and industrial manufacturing in different climates in Europe. The target is to identify where multi-vector supply can outcompete conventional single-generation systems and lower the use of GHG-emitting fossil fuels. Although tri-generation technologies are conceptually well known, their techno-economic feasibility under broad market economy operating conditions and policy constrains remains poor documented. This research aims to fill this gap by providing case study-based investigations. 2. Materials and Methods This study uses software-based simulations to model the tri-generation unit in a simplified way. Software-based simulations are the preferred method of investigation, as it allows analysis of the unit at low stage of the technology readiness level (TRL). 2.1. Modelling of Tri-Generation Unit The study presents a dynamic assessment of the unit, including the impact of environment related to the heat dissipation component (step 13–1 and step 1–9 in Figure 2). The dynamic calculation model is based on the unit modelling by a previous study [15]. The dynamic model is deviated from a stationary model created in EBSILON Professional 15.0 software, which is suitable and favoured for modelling of stationary thermodynamic cycles. A performance map for dynamic behaviour of efficiency rates was developed from 1125 individual stationary calculations to cover, first of all, different boundary conditions in the ambient air temperature and its impact on the heat dissipation in the cycle, as well as different heat source and heat supply temperatures. The model applies the refrigerant Rz1233zd(E) with an isentropic efficiency in expansion components of 0.75, within a range from 0.7 to 0.88 [8,16]. The isentropic efficiency of compressors is 0.65, with the references to the literature stating efficiencies between 0.35 and 0.75 [8], and up to 0.89 [17]. Since both the expander as well as the compressor are working in the two-phase area, their performance and behaviour cannot yet be fully pictured. As this study targets primarily the dynamic representation and modelling of the tri-generation unit, these assumptions serve as a baseline. Generators and motors are pictured with an efficiency of 0.98 and 0.95, respectively. Sustainability 2025, 17, 10924 5 of 22 Figure 2. Modelling of stationary thermodynamic system calculation in EBSILON software. The numbers refer to the different states of refrigerant quality, as given in Figure 1. Source: [15]. The tri-generation unit is capable of providing heat, cold, and electric power simultaneously. It is estimated that by modifying the mass flow in the heat supply section to the user (step 6–7) and in the cold supply section to the user (step 10–11), the level of heat and cold supply can be adjusted and controlled. The dynamic model allows to alter the heat and cold supply in 25% steps (0%, 25%, 50%, 75%, 100%). The control of heat and cold supply has a direct impact on the overall performance of the thermodynamic cycle. A reduced cold supply to the user, meaning a reduced flow rate in the lower cycle, results in a lower power consumption of the compressor and thus to a higher net electricity generation of the unit. Reduced mass flow in the condenser (step 6–7) would lead to a lower heat supply to the user, but also to an increased power generation in step 7–8. Thus, the mass flow control of the cycle’s evaporator and condenser is relevant not only for heat and cold supply, but also for the overall performance of the cycle. The 0.25 steps in mass flow control allow a fundamental management and freedom of the unit’s operation. The dynamic model reacts to the ambient temperature, which directly affects the cycle’s performance in step 13–1 and step 1–9. Since heat must be released in these two steps to the ambient, the refrigerant temperature at step 13 is set to be 5K above the ambient temperature. Thus, subsequently an increase in ambient temperature results in a higher refrigerant cooling temperature (T cool ). As this further affects the refrigerants enthalpy and the corresponding vaporization pressure, the total cycle is affected. The simulation in Figure 3 shows the impact of increased T cool on the upper sub cycle (a) and on the lower sub cycle (b) separately. The illustrations show T cool temperatures varying between 21 °C and 51 °C. Sustainability 2025, 17, 10924 6 of 22 (a) (b) Figure 3. T-S diagram showing the impact of increased refrigerant cooling temperature (T cool ) on (a) the upper sub cycle section and (b) the lower sub cycle section. Source: [15]. At a higher cooling temperature, the pressure drop in step 7–8 turns out smaller, which results in a lower power generation in the installed generator. For the lower sub cycle (Figure 3b), an increased T cool results not only in a higher pressure lift in step 11–12, but also in a reduced potential cold supply, as step 10–11 has a decreased potential in heat absorption. This behaviour is not only modelled for the given refrigerant cooling temperatures but also calculated in 2 K steps for the given range. The results were transferred into a set of equations for dynamic modelling. The previously developed dynamic model marks the backbone of the investigation of this study, as it is the first and only known approach of dynamically modelling the cycle developed by Briola 2017 [8]. The model allows a control of heat and cold supply, managed through a 0.25 stepwise alteration of the mass flow. It also pictures the performance of the thermodynamic cycle of the tri-generation unit dynamically for changing refrigeration cooling temperatures. In the following, this control of heat and cold supply via alteration of the mass flow is referred to as heat/cold supply mode. The described dynamic model calculates the energy efficiency for each energy vector based on the driving heat energy input. Thus, the delivered energy output is always directly related to the energy input. The size of the tri-generation unit is adjusted linearly by changing the heat supply. The efficiency of electric power net output ranges between 4.4% and 14.8%. The net electric power yield strongly depends on the heat and cold supply. The heat supply efficiency (COP) for a heat supply mode of 1 is at about 38%, whereas the cold supply efficiency (EER) for a cold supply mode of 1 is at about 36%. Both COP and EER are linearly linked to the 0.25 stepwise control. Figure 4 shows the electric efficiency, COP, and EER of the tri-generation unit for different T cool temperatures and for a heat and cold supply mode both at 0.5 and at 0.75, respectively. The COP increases with rising T cool as the heat consumption from external source (step 3–4) reduces, while even more heat is available to exploit in the condenser (step 6–7). This behaviour is presented in Figure 3a. The combination of electric energy efficiency, COP, and EER indicates the total conversion of heat input to output of useful energy and is further referred to as the Energy Utilization Factor (EUF). Sustainability 2025, 17, 10924 7 of 22 (a) (b) Figure 4. (a) Conversion efficiency of heat source to electric power/heat/cold for different cooling temperatures T cool for a heat and cold supply factor of 0.5; (b) Conversion efficiency of heat source to electric power/heat/cold for different cooling temperatures T cool for a heat and cold supply factor of 0.75. Each conversion efficiency is represented in individual equations. This set of equations is included in the dynamic simulation software TRNSYS18, which allows modelling the tri-generation in a large environment linked with other components and units. Figure 5 presents the methodology on how the heat supply mode is defined based on the given heat demand and the maximum capacity of the tri-generation unit. Figure 5. Methodology of the heat supply mode calculation and the corresponding heat supply from the tri-generation and the final use of heat. The heat and cold supply modes are calculated in the dynamic simulation for each time step based on the given heat and cold demand. This means during hot season and no heat is demanded, the heat supply mode is set at 0. If heat is required, but less than the tri-generation unit can supply at maximum, the heat supply mode is calculated to the closest factor mode (0/0.25/0.5/0.75/1). Accordingly, this mode is the factor to multiply the maximum heat supply capacity of the tri-generation as the mass flow of the condenser (step 6–7) is factorized accordingly. The same control is applied to both heating and cooling. As a consequence of this control, the final heat or cold supplied from the tri-generation unit may not fit exactly to the actual demand, causing either a shortage or an excess of heat or cold. However, this study follows an approach in thermal energy balance, assuming a lossless thermal energy storage. Any energy supply is calculated by multiplying the conversion factor by the heat source input. Thus, the tri-generation unit size is based on the limitation of the heat source capacity. This heat source capacity is influenced by an Sustainability 2025, 17, 10924 8 of 22 initial sizing for a balanced heat supply to minimize over production or a lack of energy. The calculation of the heat/cold supply modes was then adapted to the reduced unit size. 2.2. Types of End Users The performance of the tri-generation unit depends on the climate and the degree of heat and cold supplied to the user. Therefore, the characteristic of the user and its thermal requirements is a relevant aspect for the tri-generation unit. Four main user types have been identified and included in the dynamic simulation model. The heat and cold demand characteristics were estimated for each of the four user types. • Industrial site: Manufacturing industry that requires mainly heat, but also cold. It is characterized by daily operation, and the heat/cold demand is independent from the ambient temperature. A batch process is applied for both heat and cold demand. • Residential user: This use case represents typical residential users, applicable for both single family and multifamily houses. Heating and cooling demand depends on the outdoor temperature and is mainly operated during daytime. During nighttime, both demands are reduced. • Office user: The office use case represents the typical day-use office. Thermal energy demand depends on the outdoor temperature. Heating is required at cold temperatures, whereas cooling is required for hot outdoor temperature. • Retail and commercial user: This case represents shops and supermarkets, with heating demand based on the outdoor climate. Cooling demand is always on for the storing of fresh products, with peaks in the morning and in the afternoon for filling fresh products. These four described user types can also be combined to represent further use case. The following four combined use cases have been identified and characterized: • Industrial area: The industrial area combines the manufacturing industry and the office of industrial companies. The thermal energy demand of this use case is mainly driven by the constant demand of the industrial manufacturing. In addition to that, climate dependent peaks from the company’s office buildings influence the total thermal demand. • Central urban district: This use case can be found in various city centres. It is a combination of the office and retail user types. The thermal energy demand of this use case is mainly driven by the climate, with cooling demand in hot season and heating demand in cold season. • “Berlin Mix”: This use case combines the industrial, office, and residential user types. “Berlin Mix” is a type of urban structure named after the sector combination that can be historically found in Berlin. The energy demand in this combined use case is dependent on the climate on the one side, but also strongly affected by the demand in the industrial sector. The structure of the “Berlin mix” becomes more significant in modern city planning typologies where mixed used structures with urban manufacturing and the target of gaining new residential space becomes a key solution [18]. • Rural municipality: The rural municipality combines industrial retail user types with residential user types. It represents the combination that can be found in villages, where supermarkets and manufacturing SMEs are located close to residential areas. The four described combined use cases have been modelled in detail and integrated in the dynamic simulation, with the individual demands of each user type separately. The energy demand for both heating and cooling depends on the daily schedule and partly on the outdoor climate. The schedule and the heat/cold supply modes are multiplied with the maximum heating and cooling load, respectively. Table 2 presents the maximum heating and cooling loads considered for each of the four investigated use cases. Sustainability 2025, 17, 10924 9 of 22 Table 2. Description and maximum heating and cooling load for the four investigated use cases. 1: Industrial Site 2: Urban Central District 3: “Berlin Mix” 4: Rural Municipality Case description Industry: Office: Industry: Industry: Heating: 800 kW Heating: 500 kW Heating: 150 kW Heating: 600 kW Cooling: 800 kW Cooling: 700 kW Cooling: 150 kW Cooling: 600 kW Office: Retail: Office: Retail: Heating: 200 kW Heating: 200 kW Heating: 300 kW Heating: 150 kW Cooling: 200 kW Cooling: 400 kW Cooling: 300 kW Cooling: 300 kW Residential: Residential: Heating: 300 kW Heating: 350 kW Cooling: 300 kW Cooling: 70 kW Heating demand 4352 MWh/a 1896 MWh/a 2582 MWh/a 4435 MWh/a Cooling demand 5143 MWh/a 1277 MWh/a 1117 MWh/a 4379 MWh/a Dimensioned heat source capacity of the CCHP plant 2898 kW 2060 kW 2183 kW 3290 kW The tri-generation unit is not sized to answer the maximum heating or cooling load of a use case. Instead, it is optimized on meeting the heat demand by prevent massive over dimensioning. The tri-generation is sized for the lowest difference between heat surplus and not covered demand for an initial operation. The heat/cold supply mode calculation is then adapted, see Figure 5. As heat source, an ideal waste heat was taken into consideration, with no specification of the industrial sector. 2.3. Economic Data To identify suitable applications for the commercial roll-out of tri-generation systems, an economic assessment is necessary. Techno-economic assessments based on simulation results are a fruitful pathway to demonstrate the financial effectiveness of innovative technologies, such as for future hydrogen pipelines [19]. For the tri-generation unit, the economic assessment is performed for all four investigated use cases. The analysis is based on a 20-year calculation model that considers cost of investment, operation, and energy costs as well as potential revenues. The specific investment cost for the Regen-By2 plant is estimated account 3250 EUR/kW without the integration of the heat source. Annual cash flows include revenues from selling all the generated electricity and surplus of heat and cooling. Cost savings from the self-supplied heat and cooling are taken into account for the amount of heat/cold demanded. The energy prices are backed and based on recent EU statistics and regulatory benchmarks. For sold electricity and self-consumed, heat savings of 0.20 EUR/kWh are assumed, consistent with the EU non-household average electricity tariff of 0.1899 EUR/kWh in the second half of 2024 [20], and within the range of reported district heating prices [21]. Given the limited harmonization of districtcooling tariffs, the sale price for cooling is estimated at 0.12 EUR/kWh. An example for cooling sales prices is provided by the Paris cooling network at 0.10–0.13 EUR/kWh [22]. Any surplus heat and cold is sold at 0.08 EUR/kWh, representing wholesale or large-consumer procurement conditions below regulated end-user tariffs. Maintenance and repair costs are calculated with 3% of CAPEX per year. An economy-wide 3% inflation is applied as a modelling assumption. These economic indications allow the calculation of payback period and profitability indicators. Through the annual calculation, it is possible to identify the year of amortization for an investment in the tri-generation plant in different use cases. The motivation of this economic comparison is to demonstrate not only the energetic but also the financial viability of the proposed concept, which is necessary for a large-scale deployment. Sustainability 2025, 17, 10924 16 of 22 are not considered here. An operation of the tri-generation unit in the wintertime via solar thermal only is not feasible. Table 5. Overview and comparison of the climate conditions in the three locations and the required collector field size for balanced heat source supply for the year, January, and July, respectively. Balance Period Uppsala (Sweden) Stuttgart (Germany) Madrid (Spain) Mean ambient temperature 7.51 °C 9.55 °C 14.71 °C Global horizontal radiation (GHI) 960 kWh/m2a 1182 kWh/m2a 1722 kWh/m2a Energy demand Heat Source 11,034 MWh/a 9921 MWh/a 9071 MWh/a Collector Field size (no. of collectors in array × number of arrays) aperture area Year 367 × 10 37,700 m2 320 × 10 32,900 m2 114 × 10 11,700 m2 January - 8215 × 10 842,900 m2 513 × 10 52,700 m2 July 86 × 10 8900 m2 100 × 10 10,300 m2 57 × 10 5900 m2 Hours of solar only operation Year 1504 h (17%) 1548 h (18%) 2393 h (27%) Solar energy generation Year 11.05 GWh/a 9.91 GWh/a 9.06 GWh/a The second investigation on the solar thermal energy supply considers a fixed-field size of 3000 m2 aperture area with no consideration of a thermal storage. Thus, any solar heat surpassing the demand at a given time is not used. The demand of the Berlin Mix is the same as in Table 5. Table 6 shows the results of this second investigation. With such a system, the solar share in primary energy supply remains limited, about 5% (Uppsala), 6% (Stuttgart), and 14% (Madrid). Again, the system shows the best performance for the location of Madrid. The rest of the required heat must be supplied from a back-up system, considered to be a natural gas boiler with an efficiency of 0.9. Thus, a solar thermal collector field of 3000 m2 would lead to a GHG emission savings of 115 tCO2-eq to 290 tCO2-eq. This analysis demonstrates that a back-up system is necessary even if a 3000 m2 field of CST collectors is installed. Table 6. Overview and comparison of the impact of a 3000 m2 solar thermal collector field and the requirement of a back-up system. Uppsala (Sweden) Stuttgart (Germany) Madrid (Spain) Collector field size 3000 m2 3000 m2 3000 m2 Total solar energy yield 564 MWh/a 603 MWh/a 1384 MWh/a Solar surplus heat (not used) 50 MWh/a 45 MWh/a 86 MWh/a Solar energy directly used 514 MWh/a (5%) 558 MWh/a (6%) 1298 MWh/a (14%) Energy from back-up supply 10,519 MWh/a (95%) 9362 MWh/a (94%) 7773 MWh/a (86%) Hours when back-up boiler needed 8585 h (98%) 8578 h (98%) 8465 h (97%) GHG emission savings 114.8 tCO2-eq 124.6 tCO2-eq 289.9 tCO2-eq The study shows that the tri-generation circle is robust against different climates, and the annual EUF shows no great deviation. CST can technically supply the required heat input, but annual balancing becomes nearly impossible in Northern/Central Europe due to low radiation in the winter period. In the southern climate, driving the unit by heat from CST could be an alternative to missing industrial waste heat. At large field sizes (3000 m2), CST acts as a partial decarbonisation measure. Sustainability 2025, 17, 10924 17 of 22 3.4. Economic Analysis The economic analysis includes an initial investment of the tri-generation unit for each case individually as the size of the unit is adapted accordingly. Figure 11a shows that the earliest amortization is given for the use case 4, rural municipality, after 5.4 years. When residential and office are there in a use case only, the longest amortization rate after eight years is given. It shows that a large energy consumption from industry and retail is economically advantageous, as they demand heat and cold simultaneously. Figure 11b shows the economic curves for the Berlin Mix application in the three different climates. Economic-wise, the location is less relevant than the selection of use case. However, it shows that the operation in Stuttgart with temperate climate achieves best performance. (a) (b) Figure 11. Amortization of different use cases (a) for the climate of Stuttgart and (b) for different climates for the Berlin Mix use case. Table 7 shows the investment and the annual average of savings, costs, and income for the Berlin Mix use case for the three investigated locations of Uppsala, Stuttgart, and Madrid. Sustainability 2025, 17, 10924 18 of 22 Table 7. Economic data of the use case “Berlin mix” in different cities with separate climate. Uppsala (Sweden) Stuttgart (Germany) Madrid (Spain) Investment 7415 k€ 7095 k€ 6455 k€ Ø 20 yrs sale of energy earnings 822 k€/a 845 k€/a 720 k€/a Ø 20 yrs energy savings 903 k€/a 827 k€/a 693 k€/a Ø 20 yrs missing energy costs −89 k€/a −47 k€/a −50 k€/a Ø 20 yrs maintenance and replacement −292 k€/a −286 k€/a −260 k€/a Ø 20 yrs annual balance 1273 k€ 1293 k€ 1053 k€ Amortization time 7.0 yrs 6.8 yrs 7.5 yrs The economic analysis does not take energy costs for the heat source into consideration, assuming that industrial waste heat is available for free, even though the practical implementation is not feasible. This assumption works only if the industrial company also operates the tri-generation unit. Rather than focusing on the heat source cost, this first economic assessment focuses on the energy outputs of the tri-generation machine. However, if the heat source cost account for 6 ct/kWh, the tri-generation may not reach an attractive payback period. A cost-effective operation of the unit implies either lower costs or higher energy supply costs to users. Due to lightly different demands in the different climates, the investment costs vary from EUR 7.4 million (Uppsala) and EUR 7.1 million (Stuttgart) to EUR 6.46 million (Madrid). Cumulative 20-year operating cash-in (energy sales and savings) lands near EUR 1.4–1.8 million, with Stuttgart and Uppsala showing higher cash-in. When considering missing energy costs and maintenance costs, Stuttgart has the highest annual balance income with EUR 1.29 million, while Madrid has the lowest at EUR 1.1 million. The payback times fall within a narrow range of 6.8–7.5 years, as presented in Figure 11, indicating that economic performance is not primarily based on the investment cost. For the location of Stuttgart, the tri-generation unit presents the lowest period of amortization. This results, on the one hand, from slightly lower investment costs than in northern climate, but also from the balance between the high heating demand and the still existing cooling demand—which is higher than in northern climate—leading to a surplus production of cooling that is higher than in the warm and cold climates. This case profits from selling cooling energy the most. The tri-generation unit for the location of Madrid is smallest among the three sites, and its operation leads to a reduced generation of surplus energy and thus to less income from selling surplus energy. A sensitivity analysis was performed for the Berlin Mix use case in Stuttgart to illustrate the impact of the specific energy prices, as presented in Section 2.3. For the energy prices of sold surplus energy, an alteration of +25% was considered, causing prices of 10 ct/kWh instead of 8 ct/kWh of sold thermal energy and 25 ct/kWh instead of 20 ct/kWh of electric energy. The same accounts for a price reduction by −25%. Also, the energy prices taken into consideration in savings through self-consumption were included in the sensitivity analysis, also showing an alteration of +/- 25%. The results, presented in the form of the amortization line in Figure 12, show that in this case, a variation in the prices of sold energy is very close a variation in self-consumed energy. Both vectors change the annual performance by +/- 14–16%. The time when amortization is reached changes by about +/- one year. Thus, even if either the economic income of selling surplus energy or the economic savings through self-consumption is reduced by 25%, amortization is still achieved within 7.8 years at the latest. Sustainability 2025, 17, 10924 19 of 22 Figure 12. Amortization line of sensitivity analysis. The diagram shows baseline set-up (black) with and the amortization line of specific energy prices for savings through self-consumption or for selling of surplus energy. The biggest impact on the energy-specific economic performance comes from savings through self-consumption and reduced energy purchase from external sources. Second, it is relevant that further customers outside the initial customer group are in spatial range for energy transport. Otherwise, the distribution of surplus energy is limited or even blocked. Surplus energy comes from overproduction of energy, which is directly aligned to the selection of the heat/cold supply modes, as well as due to the dimensioning of the unit. In the southern climate, the plant dimension has the lowest capacity and the lowest surplus generation. In conclusion, payback around 6–7 years is feasible across the different climates. 4. Discussion The results of this study are derived from a first-of-its-kind dynamic simulation study of a tri-generation unit based on a single thermodynamic cycle. Therefore, the validity of the results is limited on the previous results in modelling the dynamic performance of the tri-generation derived from a performance map generated through static simulations. This affects the calculation of electric efficiency, COP, and EER. The dynamic simulation model of the tri-generation unit is significantly influenced by earlier assumption and input values on component level, such as the isentropic efficiency of expanders and compressors. Furthermore, the model represents the tri-generation applying the refrigerant R1233zd(e) only, which does not allow a direct validity of the performance of the unit with other refrigerants. Heat and cold generation of the tri-generation unit is controlled via a selection of heating/cooling supply mode. Each mode represents the level of mass flow to run through the unit’s evaporator and condenser, respectively. The mode allows an alteration in 25% steps, and the selection is based on the actual heat/cold demand. The given mode arrangement requires further modification to meet the actual heat/cold demand more precisely. A Thermal Energy Storages (TES) is relevant and very important for the unit’s operation. The characteristics and role of TES, namely, size and losses, are not investigated in this study, as the unit is not modelled in a full-system approach but for itself instead. The relevance of TES can be reduced through controlling the heat/cold supply modes in short steps, such as 5% or 10%. Sustainability 2025, 17, 10924 20 of 22 The energetic analysis highlights the different needs of the four investigated use cases that include a combination of individual user types. The thermal energy demand of office and residential buildings depends on the climate and varies throughout the year. As presented in Table 2, the heat source capacity may outstrip the combined thermal energy demand load for both heating and cooling by factor 2–3. This is both a technical and an economic constraint. The unit dimension in this study is sized form the demandside, assuming a heat source is available at any capacity and any time. This simplified approach was selected as the study targets the demand first before assessing the integration to any heat source. There is room for improvement in sizing the unit, as even with an initial sizing, surplus of heat and cold is generated. The economic analysis presents the economic performance of the tri-generation for four different use cases and for the Berlin Mix use case in three different locations in separate climates. It includes investment costs and annual costs in contrast to savings and earnings from supplied energy. The operation of the unit needs further investigations and a detailed economic analysis assessing suitable business models. The economic analysis of this study presents a first pilot analysis that highlights the most relevant aspects but does not cover the full picture. The application of the tri-generation unit is most promising when industry and retail is part of the user types in a use case, as they provide a constant demand for thermal energy. Office and residential users rarely have a simultaneous demand for heat and cold, thus the tri-generation cannot exploit its full potential in simultaneous supply of heat, cold and electric power. Additionally, industry is a promising user type, as industrial waste heat is a promising heat source. For the three investigated locations, the best economic performance is identified in the temperate climate of Stuttgart. Even though any excess heat and cold, as well as all electricity, is sold to external users, the most relevant are the financial savings through self-consumption of heat and cold. The tri-generation unit generates more heat and cold than required in all three locations. An adjusted sizing of the unit will directly affect not only the investment costs, but also the savings and earnings from energy sales. 5. Conclusions This study presents a first approach in dynamically modelling a tri-generation unit that is based on a single refrigerant thermodynamic cycle operating in the two-phase area, able to deliver heat, cold, and electric power to the user. When driven by renewable heat, this unit can provide emission-free energy to users and play a supporting role in the clean energy transition. The research contributes to a better understanding of the critical points in the operation of the presented tri-generation unit, such as the influence of Tcool, its performance to supply different user types, as well as presenting the performance in different climates. The presented tri-generation unit is a promising and innovative solution for the effective use of excess heat or renewable heat. As the unit is still under development at the moment of writing, simulation studies are the preferred way to investigate the unit’s performance and potential in future applications. The study’s highlights are as follows: • Initial assumptions were used to develop a dynamic calculation model of the heat, cold, and electricity supply from the tri-generation unit and applied for annual simulations. • The developed control strategy allows to adapt the unit’s heat and cold supply in 25% steps based on the users’ demand. • Four use cases were identified, and the performance of the tri-generation unit in each case was analyzed. • The Energy Utilization Factor (EUF) is highest for industrial sites and rural municipalities, due to the thermal demand of industry and retail throughout the year. Sustainability 2025, 17, 10924 21 of 22 • Climate influences both user demand and the performance of the unit. • Selling electricity is a significant aspect in the economic performance of the tri-generation unit. Lab-scale testing of a physical prototype is necessary to validate the performance in simulation and to analyze the actual flexibility in start–stop operation. The impact of TES integration on the unit’s behaviour and the set-up in a system are topics of interest and worthy of future investigations to deliver a comprehensive assessment of the technology. The roll-out of the technology, aiming at a higher technology readiness level (TRL), benefits from this study by highlighting relevant aspects for further optimization. Next steps for the roll-out of products to commercialize this tri-generation technology include not only scientific lab tests but also demonstration in real use cases to further assess its potential. Author Contributions: Conceptualization, U.J. and M.S.; methodology, U.J., M.S., and L.Z.; software, L.Z.; validation, U.J. and M.S.; formal analysis, U.J. and M.S.; investigation, U.J. and M.S.; resources, U.J. and M.S.; data curation, M.S.; writing—original draft preparation, M.S. and L.Z.; writing—review and editing, U.J. and M.S.; visualization, M.S. and L.Z.; supervision, U.J.; project administration, U.J.; funding acquisition, U.J. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the European Union’s Horizon 2020 research and innovation programme under grant agreement N° 851541, project REGEN-BY-2. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Dataset available on request from the authors. Conflicts of Interest: The authors declare no conflicts of interest. Abbreviations The following abbreviations are used in this manuscript: CCHP Combined Cooling, Heating, and Power CHP Combined Heating and Power COP Coefficient of Performance CST Concentrating Solar Thermal EER Energy Efficiency Ratio EPW Energy Plus Weather EUF Energy Utilization Factor GWP Global Warming Potential PTC Parabolic Trough Collector SME Small and Medium sized Enterprises TES Thermal Energy Storage TRL Technology Readiness Level References 1. WMO Report Documents Spiralling Weather and Climate Impacts. Available online: https://wmo.int/news/media-centre/wmoreport-documents-spiralling-weather-and-climate-impacts (accessed on 25 September 2025). 2. European Green Deal. Available online: https://www.consilium.europa.eu/en/policies/european-greendeal/?utm_source=chatgpt.com (accessed on 25 September 2025). 3. Renewable Energy Targets—European Commission. 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