Fifth-generation district heating and cooling (5th DHC) systems offer promising approaches to decarbonizing space heating, cooling and domestic hot water supply. By using these systems, clustered buildings combined with industrial waste heat can achieve a net-zero energy balance on a variety of time scales. Thanks to the low exergy approach, these systems are highly efficient. As part of the Smart Anergy Quarter Baden (SANBA) project, the thermal energy grid simulation tool TEGSim has been further developed and used to design an ultra-low-temperature district heating (ULTDH) network with hydraulic and thermal components fitted to the specific regional characteristics of the investigated case. Borehole thermal energy storage (BTES) used as seasonal storage ensures long-term feasibility. The annual discrepancy of input of thermal energy provided by space cooling and output of energy demanded by space heating and domestic hot water is supplied by an external low-grade industrial waste heat source. This paper presents the functionality of the simulation and shows how to interpret the findings concerning the design of all components and their interplay, energy consumption and efficiencies.

Particle-based systems have immense potential for combining thermal energy storage (TES) with renewable energy sources. The so-called sandTES system, which is an active TES system, utilizes sand or other small particles as a storage material and consists of a hot tank, a cold tank, and a reversible fluidized bed heat exchanger. In the preferred design, the tubes are arranged in horizontal serpentine tube bundles; thus, the headers are positioned vertically, for one phase subcritical, two-phase and supercritical water/steam conditions. So far, no design principles have been published for macro-sized vertical headers and horizontal tube bundles. This paper investigates the unbalance in flow distribution and other flow instabilities of such an unusual heat exchanger and its scale-up using APROS simulations. The steady-state simulations of the tallest investigated heat exchanger show a strong unbalance in the mass flows between individual heat exchanger tubes: the smallest mass flow was only 55% of the largest one when generating steam. Smaller heat exchangers experience less pronounced unbalance factors in the order of 88%. When steam is condensed, the unbalance is insignificant. Dynamic simulations of start-up and shutdown revealed instabilities and that the flow maldistributions even lead to flow reversals in most investigated cases although neither last for stationary operation. Part-load cases show higher unbalances than their respective full-load case by 9 to 17 percentage points. Header design modifications - secondary headers with submodules and orifices - improve the unbalance factor from 55% to 88%.

This work summarizes the methodology developed within the H2020 EU project SCARABEUS for the analysis of innovative CO2-based mixtures used as working fluid in transcritical cycles for CSP applications. By adding a specific quantity of carefully selected dopant to CO2 it is possible to reach a high mixture critical temperature, suitable for air cooled cycles at very high minimum temperature in hot environments with high efficiencies. Along the methodology, some results related to the design of the turbine and the heat exchangers for the innovative mixtures are presented, including a focus on the system integration between the power block and the solar plant: as such, these results can be considered as interesting research outcomes to widen the knowledge on innovative solutions for efficient power cycles. The new power cycles are foreseen to drastically increase the profitability and cut the specific cost of CSP systems with respect to the state-of-the-art

Hydrogen blending into natural gas grids can support early renewable hydrogen deployment, but admissible injection depends on local gas flow, blending limits, and upstream hydrogen concentrations. This paper analyses these effects for a regional high-pressure gas-grid section in Styria, Austria, with two hydrogen injection points. A transient gas-network model derives time- and location-dependent injection limits, which are integrated into an electrolyser dispatch optimisation with fixed trailer demand and annual gas-grid injection demand. Three cases are compared: unrestricted injection, a “CH₄-based” limit without upstream hydrogen, and an “H₂-aware” case representing potential upstream hydrogen injection. For the analysed configuration, blending constraints shift operation away from favourable electricity-price periods, particularly when low prices coincide with reduced gas demand. In the 2025 reference case, the “H₂-aware” constraint increases the electricity-cost contribution from 4.22 to 5.45 EUR/kgH₂. A robustness analysis using electricity-price series for 2020, 2022, and 2025 shows that the “H₂-aware” constraint increases the electricity-cost contribution by 15.8–29.1% relative to unrestricted injection. The results demonstrate that dynamic gas-grid constraints should be considered when assessing blending-based electrolyser projects, while the quantitative findings remain specific to the analysed network and assumptions.

The dry-cooled Rankine cycle working with a zeotropic mixture of CO2+C6F6 is influenced by the ambient temperature as air is used as the heat sink. Varying heat sink temperatures allow for operating the cycle under sliding condensation pressure which may benefit a hybrid PV-CSP plant. The study demonstrates the effect of this operation mode on composition shift and condensation pressure and investigates the cycle performance. The results show that defining the turbine design conditions significantly impact whether the system´s thermodynamic performance behaves acceptably in off-design conditions. Operating the turbine which was designed for a big pressure ratio in part-load especially if both, inlet and outlet pressure are at off-design conditions, is not favorable and leads to deteriorated efficiencies. Under some constraints for turbine and heat exchanger design, the proposed cycle enhances the hybrid PV-CSP system.