In Europe, heating and cooling represent half of the consumption of the total energy demand. However, integration of renewable energy sources (RES) in DHC-networks is still limited today, by several barriers mainly related to the unpredictability and lack of dispatchability on the monthly or seasonal base of thermal energy and electricity. Since the EU is committed to be climate-neutral by 2050, this is a matter of concern, in socio-economical and in environmental terms. The share of DHC in the EU heat market is today 12%, 578 TWh/year. Conventional DHC networks energy needs are mostly based on fossil heat-only boilers and combined heat and power (CHP) plants based on fossil fuels (natural gas: 46%, coal: 15%, fuel oil: 10%) and nuclear energy (7%). Several scenarios, however, expect a dynamic growth of RES in DHC, contributing to the EU binding target of 32% RES of the gross final energy consumption in 2030. Estimations for 2050 show several benefits of the decarbonization in the energy sector, see Figure 1.

The idea on which this project is based is to install thermochemical storages in ovens for heat recuperation and control. For the temperature range of this application, thermochemical material for the low-temperature range is used. Due to the high storage density of thermochemical storage materials, very compact systems can be built that can be easily integrated into ovens.

Thermal storage concepts for an oven are currently mostly based either on the conversion into latent heat or on the conversion into electrical energy. Latent heat conversion is designed for short-term use during the cooking process. In particular, the energy present in the (possibly water-laden) exhaust air should be transferred to the supply air. The conversion into electrical energy by means of a thermal generator requires a battery or something similar to store the energy.

To heat the food in the oven, not only the air in the oven but also the oven is heated. This requires a corresponding amount of energy, which is given off as heat to the environment, especially after the cooking process has ended.

The aim of this research project is to develop a functional model for thermochemical heat recuperation in an oven using different thermochemical substances and to carry out extensive investigations into the limitations of the system, the optimal process control, the applicability for the end customer, the practical potential and the challenges, in the real system to perform.

The Institute of Energy Systems and Thermodynamics (IET) has been working on the development of particle based high temperature heat storage systems (Thermal Energy Storage – TES). By 2020 this work has produced four (4) patents, ~15 publications, 6 laboratory scale test rigs, two (2) pilot plants and one (1) license agreement.

The original idea targeted the thermal storage in adiabatic compressed air energy storages (ACAES). Very soon, it became evident that the concept is also applicable in Concentrated Solar Power (CSP), Electro-thermal Energy Storage (ETES) in conjunction with steam and sCO₂ cycles (also named Carnot batteries or PTES - Pumped Thermal Energy Storage) and for industrial heat storage.

All mentioned applications need an indirect particle/fluid heat exchanger, which is optimized for (a) maximized overall thermal performance, hence counter-current characteristic; (b) minimized auxiliary power; (c) minimized costs, hence maximized heat transfer and heat transmission coefficients.

ETES cycles have the additional requirement (d) that the particle suspension flow has to be reversible in order to allow a fast switch from charge to discharge operation and that suspension plug flow is of utmost importance.

IET has developed two basic heat exchanger designs. The original concept, also named sandTES_1.0, was based on longitudinal flow of particles along the tubes. A more recent development called sandTES_2.0, is based on transversal flow across the tubes.

Both concepts use the patented approaches of a 2-stage fluidization grid (for stable and even distribution of fluidization air) and the use of valve-controlled air cushions downstream of the freeboard. The air cushions are obligatory for efficient reversal of particle flow in ETES applications. They are also essential for installing a plug-flow flow behavior on particle side.

sandTES_1 with longitudinal particle flow has the advantage of constant cross section in particle flow direction and the absence of 180° tube bends. It is well suited for applications such as ACAES where fluid side heat transfer is limited and where a high cross section in the tubes is needed due to moderate fluid pressures.

sandTES_2 with transversal flow of particles has the advantages of maximum design flexibility for optimizing both particle and fluid mass flux densities. The transversal flow also allows for the use of transversal helicoid fins, which allow multiplying the outer-diameter based (equivalent) heat transfer coefficient by a factor between 4 to 6 (compared to plain tube). Given that the auxiliary power of a sandTES heat exchanger is directly proportional to the bed volume, high heat transmission coefficients have a high impact on both performance and cost.

Most of the test rigs are dedicated to the analysis of heat transfer, flow characteristic and of system characteristic. One test rig was designed to perform accelerated erosion testing which has the objective to proof that in HTX-applications with no chemical reaction and small particles operated at low fluidization grade (multiple of minimum fluidization velocity) finned tubes are compatible with an operational life of more than 20 years. The experimental work on heat transfer and suspension flow behavior was supported by numerical analysis based on the CPFD code Barracuda. These simulations aimed on the system level for understanding of what is needed for establishing plug flow behavior in horizontal flow FB HTX’s.

Acknowledgments:

Our work was partly supported by

• several projects supported by FFG, the Austrian Research Funding Agency (the first generation of research and test rigs),
• work supported by ARPA-e, in program DAYS project DE-AR0000996 with prime awardee Echogen Power Systems,
• work supported by the DOE Solar Technologies Office, in  project award CPS 38476 “Compact Counterflow Fluidized Bed Particle Heat Exchanger” with prime awardee Sandia National Laboratories,
• work supported by the Department of Energy under Award Number DE-FE0032024 with prime awardee Electric Power Research Institute EPRI.

 

At TU Wien, the main contributors were Karl Schwaiger, Peter Steiner and Stefan Thanheiser, who themselves were supported by numerous Master- and Bachelor students in their final theses.

During the last years, several concepts for thermodynamic power storage have been published. This so-called Electro-thermal energy storage (ETES) also has the titles “pumped thermal energy storage” (PTES) and “Carnot-Battery”.

The Institute of Energy Systems and Thermodynamics (IET) is participating in two projects with partners from the USA.

ETES technologies have the following in common:

• electricity is stored as thermal energy (TES),
• the technology is site-independent,
• depending on the temperature levels, one or two thermal reservoirs are needed,
• in general, two reverse thermodynamic cycles are needed (heat pump cycle for charge and power cycle for discharge).

The thermal storage temperature levels may be above or below ambient temperature. In the case that we choose ambient temperature for the lower temperature, only one thermal storage for high temperature is needed.

In a simple set-up, electrical resistance heating (instead of a heat pump cycle) charges the high-temperature storage.

The combination of a water-steam based Rankine cycle with electric heating and thermal energy storage (TES) yields the special case of a thermal storage power plant (TSPP).

The attached figures show the concepts for both CO₂ and H₂O based concepts.

Acknowledgments:

This publication is partly based upon

• work supported by ARPA-e, in program DAYS project DE-AR0000996 with prime awardee Echogen Power Systems,
• work supported by the Department of Energy under Award Number DE-FE0032024 with prime awardee Electric Power Research Institute EPRI,
• several projects supported by FFG, the Austrian Research Funding Agency (the first generation of research and test rigs).

Nowadays, energy suppliers and process industries are confronted with challenging social and legal requirements to decarbonize, increase energy efficiency and enhance sustainability measures. This results in a growing expansion of renewable energy technologies like solar thermal, photovoltaic or wind power plants. As a consequence, a temporal mismatch between energy supply and demand has to be addressed with flexible solutions, for example energy storages. Currently only a few storage technologies take advantage of the synergies of combined thermal and electrical energy storage.

In the project SyndETES a transient open water-steam cycle is proposed as a system that exploits these synergies. Starting point is the Carnot-battery concept, which stores electrical energy as thermal exergy. In SyndETES this concept is further evolved. Thus, aside from electrical energy, industrial waste heat is utilized as energy source. During high demand periods the stored energy is flexibly released either directly as process heat or as electrical energy, e.g. with a steam turbine.

The overarching goal of SyndETES is an economic and technical evaluation of the proposed system designs. Frameworks and frame conditions are developed in cooperation with a stakeholder pool of experts for the integration of SyndETES-systems into industrial processes. Economic feasibility and operational characteristics shall be assessed. Also, a comparison with a reference system is conducted and shall provide further insight. Finally, further steps to increase the technology readiness level are derived.

The objective of this R&D project was to develop and engineer a 10 MWth high temperature heat recovery from a cement plant and to allow the transport of the heat to industrial customers, which are more than 1.5 km away from the heat source. Crossing public terrains with a heat transport piping at this temperature level has never been implemented before in Austria. The context calls for maximized standards of reliability and safety. As cement plants typically shut down for several weeks in winter, the question of heat storage is of key importance. Environmental compatibility is important in terms of emissions but also to water protection, as the site is situated in a touristic area next to a lake. Only environmentally benign fluids such as H₂O or CO₂ are acceptable as heat transport medium.

For the Overall concept, almost 30 interconnections of the subsystems heat extraction fluid, storage system and district heating system were analyzed thermodynamically and with regard to the technical and economic optimum. From these, we technically designed four (4) concepts in the sense of a basic design and compared analyzed them economically. (K0, K5, K9, K10).

Figure 1 shows the temperature/duty diagram.

Figure 2 shows the process flow diagram of the basic concept K0 without storage.

For the heat extraction, the comparison between the approaches “dust-loaded smooth tube heat exchanger” and “ceramic hot gas filter + finned tube heat exchanger” has shown financial advantages for the finned tube variant. For the techno-economic project concept, the smooth tube variant was nevertheless chosen, because on the one hand, the technical risk is lower and on the other hand, the internal material flows can be better organized.

For the heat transport over 1.5 km (heat link), a district heating system based on steam has clearly emerged as the techno-economically most advantageous solution (Figure 3). This was opposite to the original assessment at the start of the project. The project team was able to work out a technically feasible route between the waste heat source and the potential industrial customers.

We analyzed several types of heat storage. The aim of a heat storage system is, on the one hand, to optimize operation and, on the other hand, to maximize the use of waste heat and thus avoid CO₂ emissions with a temporal decoupling of generation and consumption. The load profiles of the waste heat and the heat demand vary greatly and are not in the same direction. A storage allows supply and demand to be matched.

In the project, a distinction was made between operational storage (6 MWh), day storage (330 MWh) and long-term storage (> 4 GWh). The number of storage cycles increases with decreasing storage size. For the operational storage variant, Ruths steam storage (K5) and pressurized water storage (K10) were evaluated techno-economically. For day storage or larger high-temperature storage (K9), a gravel storage was developed, simulated, tested in the laboratory of the TU Wien and evaluated techno-economically. For long-term storage with use up to seasonal storage, gravel storage and pit water storage were evaluated.

In terms of CO₂ savings and economy, the available waste heat of 70 to 90 GWh (depending on the concept) would have a theoretical CO₂ emission avoidance potential of up to 22,000 tons (22 kT) of CO₂ per year. The analyzed variants with operational storage (K5 and K10), with day storage (K9), or without storage (K0) allow waste heat to be used in the range of 42 GWh annually to 65 GWh annually (47 to 72% of the maximum potential).

The following key data are the essential input variables for the profitability of the project:

• investment costs,
• running costs (operating costs),
• economic observation period (useful life),
• interest rate,
• specific fuel costs,
• substituted amount of primary energy,
• funding (especially invest funding),
• other costs avoided (e.g. taxes per kWh or per tonne of CO₂) based on the amount of primary energy and emissions avoided.

Based on the key data applicable to the project in 2021, it was unfortunately not possible to demonstrate an economic feasibility for any of the concepts examined. For even larger heat storage systems, the economic viability deteriorated under the prevailing framework conditions. The project team worked out which changes in the framework conditions would make implementation possible.

Acknowledgments:

The Austrian Climate and Energy Fund funded this project.

The aim of the SANBA project is to develop a so-called anergy or low-temperature heating and cooling system (<30°C) for the future use of a former military camp and to answer open research questions based on this task. Key elements are the use of industrial low-temperature waste heat from processes in a neighbouring dairy plant as well as the development of refurbishment and conversion concepts for the listed monument buildings. In a first step and as a core content of this industrial research project, after a comprehensive investigation of the site, new and communicating simulation tools have been developed to cope with the complex situation of the area, consisting of different heat sources, protected and potential new buildings, different temperature levels and times of energy demand, different uses of the buildings, etc. The concept of the anergy grid comprises (I) the heat recovery from the wastewater of the neighbouring dairy plant, (II) integration of locally available renewable energy sources, (III) energy storage aspects, (IV) the special challenge of different building standards of the old protected buildings vs. newly built buildings with different usages (living, commercial, education), and therefore different supply temperatures and demand characteristics, and (V) moderate cooling via Free Cooling.

The project SANBA is part of the NEFI thematic model region that positions energy intensive and manufacturing industries and their decarbonization in the center of a long-term innovation process to boost technological development. SANBA contributes to the NEFI-innovation fields Energy Efficiency & New Processes and Renewable Energy & Storage & DSM.

 

The project „Zweifeldspeicher“ (“Two-Field Storage”) deals with the innovative design and operation of seasonal Borehole Thermal Energy Storages (BTES). State-of-the-art BTES are operated in a cyclic way, where the heating and cooling demand is covered from the same BTES field. The “Two-Field Storage” approach splits the BTES in two fields with different temperature levels: one field covers the heating demand, while the cooling demand is covered by the second field. The project team is confident that this approach will yield in a higher efficiency both for heating and cooling. The “Two-Field Storage” approach also makes additional heatsources and –sinks accessible for the thermal storage concept. This one-year project (“Sondierung”) will elaborate the necessary innovations in the fields of BTES prediction and control engineering as well as the technical-economical basics for the conceptual design of a full-scale test and demo site. This demo site will be accompanied by a follow-up F&E&I project.

The project aim was to increase the flexibility of heating networks with the help of a thermal energy storage device.

The investigation was done for the thermal centre Dürnrohr of the company EVN. This thermal centre provides heat for different industrial companies as well as for district heating of the regions Tulln and St. Pölten with the help of an incineration plant and the plant unit of the coal fired power plant Dürnrohr which was still in operation (at the end of the project the power plant was decommissioned). During this project four different thermal energy storage technologies are analysed as thermal energy storage units.

In particular the daily morning peak which was compensated by fossil fuels (coal and natural gas) should be managed in the future in a CO₂-neutral and sustainable way by the integration of a thermal energy storage device.

The aim of the project was a detailed techno-economic design of the investigated thermal energy storage technologies to get a solid technical and economic basis for a targeted selection of an optimal thermal energy storage concept for the heating centre Dürnrohr.