Ultra-low Temperature District Heating Research Articles

Enhancing Ultra Low Temperature District Heating Systems with Neighborhood-Scale Heat Pumps and Triple-Pipe Distribution: A Techno-Economic Analysis

Ultra-Low Temperature District Heating (ULTDH) systems are advanced District Heating (DH) systems enabling the efficient utilization of low-temperature heat sources for heating systems. A significant challenge in ULTDH systems is the economic inefficiency associated with use of booster Heat Pump (HP) in buildings to achieve the desired Domestic Hot Water (DHW) temperature. To address this issue, this study proposes an innovative ULTDH system incorporating neighborhood-scale water-source booster HPs and distribution triple-pipes. Triple pipe’s dual supply line capabilities facilitate the neighborhood-scale water-source booster HPs to be brought out at the neighborhood scale, demonstrating an intelligent approach. This approach reduces the capital investment and enhances the Coefficient of Performance (COP) by utilizing supply line of DH system as the heat source. A comprehensive case study is conducted to demonstrate the superior techno-economic performance of the proposed ULTDH system compared to Low Temperature District Heating (LTDH) as the benchmark solution. All components of both systems are sized by determining the DHW and Space Heating (SH) demands for each neighborhood. Thermodynamic analysis is conducted at a 1-minute time resolution for an entire year. Six different heat production scenarios are developed to determine the Levelized Cost of Heat (LCOH). The results indicate that while the energy performance of the ULTDH surpasses LTDH, the rise in electricity prices favors the ULTDH system.

Booster heat pump with drop-in zeotropic mixtures applied in ultra-low temperature district heating system

The pursuit of sustainable district heating solutions has driven a growing interest in ultra-low temperature district heating (ULTDH) systems, where booster heat pumps (BHPs) play a pivotal role despite challenges posed by their efficiency limitations under large temperature glide conditions. This paper investigates the potential of drop-in R-1234yf/R-32 zeotropic mixtures in BHPs compared to a baseline R-134a system, within the context of a ULTDH framework. This study focused on the viability of the mixtures of R-1234yf/R-32 with the composition ratio of 80 %/20 % and 90 %/10 %. The investigation reveals disparities in compressor efficiency and heat exchanger pressure drop at the component level. Device-level analysis unveils increased COP for R-1234yf/R-32 mixtures, alongside with maximum second-law efficiencies reaching 0.32. A remarkable enhancement in heating capacity up to 58 % was found. System-level analysis demonstrated exergetic efficiencies and identified preferable district heating temperatures. Exergetic efficiencies of 0.47, 0.55, and 0.59 were achieved for domestic hot water preparation at district heating supply temperatures of 30 °C, 35 °C, and 40 °C, with a subsequent shift in optimal district heating temperatures as central heating station efficiency decreased. Temperature profile analysis underscored challenges stemming from excessive subcooling, highlighting the need for configuration refinements.

Energy and economic simulation of a renewable energy community applied to a new generation ultra-low temperature district heating and cooling network

The work describes the energy and economic simulation of a renewable energy community with a social purpose applied to a residential and commercial new district in Milan, the eight buildings of which are connected to a new generation low temperature district heating and cooling network. The system is designed considering two substations for each building, each one providing heating and cooling load profiles by means of heat pumps, and an energy centre that exploits groundwater to extract and dissipate compensating heat at low temperature. Some roof mounted photovoltaic panels, owned by the district residents, cover the electricity needs resulting from the net metering of the renewable energy community. The members of the energy communities are in fact the multifamily buildings of the district acting as prosumers and some fragile families from the surroundings as simple consumers. The economic profits, represented by the subsidies coming from the diffuse self-consumed shared energy and from sold overproduced electricity, are distributed among the members to guarantee, first of all, an economic help against energy poverty to fragile families, and, secondly, a short pay-back-time for photovoltaics. Therefore, the operational strategy of the district network is optimized to maximize the shared electricity and the relative economic benefit by shifting, when possible, the electricity demand when the solar production is available. Finally, three different profit distribution mechanisms are analysed. The added value of this work is the evaluation, by means of a specific case study analysis, of the feasibility of an electric energy community from an economic as well as a regulatory point of view under current legislation.

Corrigendum to: Energy and economic simulation of a renewable energy community applied to a new generation ultra-low temperature district heating and cooling network

Corrigendum to: Energy and economic simulation of a renewable energy community applied to a new generation ultra-low temperature district heating and cooling network

Demand side flexibility for a Heat Booster Substation with ultra low temperature district heating

Ultra-low temperature district heating (ULTDH) systems improve the heat distribution efficiency by reducing thermal network losses and provide an easier access to Renewable Energy Sources (RES) by utilizing them as heat sources through Power-to-Heat solutions. In this study, Domestic Hot Water (DHW) preparation at a Heat Booster Substation (HBS) supplied with ULTDH is investigated for generating demand side flexibility. The main components of the HBS are a Hot Water Storage Tank (HWST) and an electric-driven heat pump. A genetic algorithm based control strategy is developed to optimally utilize the thermal energy storage capability of the HBS by intelligently scheduling the charging of the HWST based on time-varying electricity price signals. A multi-objective control strategy is designed to reduce energy costs while generating flexibility on the demand side. The components of the HBS are numerically modeled and experimentally validated to form a dynamic non-linear model of the HBS. Further, the data-driven seasonal ARIMA model, ARIMA(0,1,1)x(0,1,1)168, is developed to forecast the DHW consumption based on a statistical analysis of the historic consumption profiles over a two-year period. The forecast RMSE of the DHW prediction model is less than 0.03kgs−1 over the two-year test period, indicating that the prediction model is able to accurately forecast the DHW consumption of the residents. With the control-oriented HBS numerical model and the DHW prediction model serving as decision support tools, the developed control strategy is implemented at the HBS. Results show that in comparison to a rule-based control strategy, the developed control strategy demonstrates significant demand side flexibility while attaining energy cost savings of more than 91% and 84% at time resolutions of 15 and 30 min for the control interval respectively. With increased volatility in electrical prices due to higher penetration of RES, greater energy cost savings of more than 150% are observed.

Integration of booster heat pumps in ultra-low temperature district heating network: Prototype demonstration and refrigerant charge investigation

Decentral booster heat pumps (BHPs) integrated in ultra-low temperature district heating (ULTDH) systems are deemed a sustainable solution for low-carbon building systems. The performance of BHPs is critical for the implementation of ULTDH systems. In this study, we experimentally investigate the effect of refrigerant charge on temperature, pressure, system coefficient of performance (COP), and heating capacity to evaluate the operability and sensitivity of ULTDH. A performance-guaranteed range of refrigerant charge is proposed to ensure the feasibility of the system. We also analyse the component-level exergy destruction and the system-level exergetic efficiency of a BHP prototype. The exergetic efficiencies range from 24.9 % to 33.4 % under operational conditions for ULTDH. The condenser and evaporator's exergy destruction is influenced by the temperature profiles on both sides, suggesting the potential to improve BHP performance by tailoring operating parameters. Additionally, the return water temperature significantly affects the system COP when the domestic hot water supply temperature is fixed. Within the allowable range of return water temperature, BHP operation with high evaporator outlet water temperature can improve the overall efficiency of the ULTDH system.

Roadmaps for heating and cooling system transitions seen through uncertainty and sensitivity analysis

Future district heating systems should enable efficient and economic energy supply, which can be achieved by lowering the system temperatures and boosting it at demand-side. Current solutions include the ultra-low-temperature district heating (ULTDH) and fifth generation district heating and cooling (5GDHC) systems. The transition towards these systems is subject to multiple future uncertainties such as the energy price, investment cost, and demand changes, which were missing in previous works. To investigate the effects of these uncertainties on conclusions brought by established design roadmaps for future DHCs, a five-step framework, which combines the energy system optimization with stochastic simulations, uncertainty analysis and sensitivity assessment, is developed in this study. The framework is applied on a hypothetical 0.25 km2 square district with varying uncertain parameters. Based on stochastic cases, the index named cost-saving probability (CSP) is utilized to reflect the potential of being economic attractive when comparing the energy systems. For the transition towards the ULTDHC, 5GDHC, and individual systems, the most sensitive factors for the CSP are the area demand density, overlapping heating and cooling demand, and linear demand density, respectively. The investment in thermal energy storage (TES) becomes important only when the integration of a larger share of renewable energy is targeted. A roadmap summarizing the promoting and hindering factors for the system transition is provided, pointing out the future focus area for DHC design. The results from the sensitivity analysis also revealed the limited role of TES in integrating variable renewable energy in high-efficiency DHC systems.

Energy analysis of an ultra-low temperature district heating and cooling system with coaxial borehole heat exchangers

The decarbonisation of society requires developing new environmentally friendly solutions for heating and cooling. Ultra-low temperature district heating and cooling (ULTDHC) allows operation with energy sources and systems hard to integrate into previous generations of district networks, making this technology more efficient and sustainable. This work analyses a novel integration of coaxial borehole heat exchangers (CBHEs) to control the temperature of the network, drastically reducing operating costs and emissions compared to existing systems. The concept is analysed based on single and double-loop networks. As a case analysis, it is studied to a ULTDH located in the south of Spain with annual heating and cooling demands of 1045 MWh and 416 MWh, respectively. The single-loop configuration requires more 100 m depth CBHEs connected, 47, whereas the double-loop layout, with a more efficient thermal balance, requires 42. In contrast, it has higher values of annualised costs. The sensitivity analysis of the main parameters proves the system is robust and effective in compensating for external variations. It operates more efficiently in warm/hot regions, increasing the possible applications and implantation expansion. The proposal is useful for new systems and for adapting the existing ones.

Simulation research of a dual-loop booster heat pump system on district heating under ultra-low temperature

ULTDH (ultra-low temperature district heating) system improves the shortcoming of significant loss of heating network in traditional district heating systems. This work proposes a dual-loop booster heat pump for ULTDH to improve the heating capacity and COP. The system is modeled by MATLAB, and the water temperature between the two loops is solved by the NSGA-II algorithm (Non-dominated Sorting Genetic Algorithm-II algorithm). The simulation results show that when the heat source temperature changes from 20 °C to 40 °C, the COP and heating capacity of the dual-loop system increase by 43.79% and 90.30%, respectively, which are higher than those of the single-loop system. When the temperature difference between the inlet and outlet of the evaporator is increased, the heating capacity of the dual-loop system is still 7.31–33.47% higher than that of the single-loop system. When the hot water supply temperature is 60 °C, the COP of the dual-loop system is 3.34, which is 23.70% higher than that of the single-loop system. During the whole heating period in Beijing, the dual-loop system can save 17.67 % of the electric energy than the single-loop system. The dual-loop system shows excellent advantages in heating capacity and COP.

Assessment of district heating and cooling systems transition with respect to future changes in demand profiles and renewable energy supplies

District energy systems are about to shift towards closer temperature configurations, i.e. low-temperature district heating and high-temperature district cooling. Challenges and benefits of these transitions are mostly analyzed from a perspective of current energy demand and supply scenarios while the influence from future changes in these domains remains unknown. Based on a representative residential community in the Nordic district heating context, centralized district heating and cooling (DHC), ultra-low temperature district heating (ULTDH), and bi-directional fifth generation 5GDHC systems were assessed from technical, economical, and environmental aspects. Moreover, the applications of thermal energy storage (TES) and their roles in the future DHC systems were also investigated. The assessment was done by a generalized methodology framework, integrating the future changes, multiple operation scenarios modellings and system design optimizations. Results suggest that in the future low-energy building stock, the increased cooling demand makes the 5GDHC system the most economically attractive choice. In the supply side, with a 50% share of wind power in the future national grid, the electricity prices can make 5GDHC and ULTDHC either cost-saving or more expensive compared to the central DHC system dependent on if nuclear plants are decommissioned or not. Besides, with increasing power production from VRE, the limited application of TES for active shift of electricity demand is found when a system’s heat-to-power ratio is high. The methodology framework can be applied to similar systems to increase the understandings on system transitions.

Analysis of an ultra-low temperature district heating and cooling as a storage system for renewable integration

Sector coupling is necessary for efficient renewable integration since almost all renewable energy sources depend on environmental parameter variations. This paper follows a research line that studies the application of ultra-low temperature district heating and cooling systems, with working temperatures between 6 and 40 °C, to integrate renewable sources with a storage strategy, using the distribution network as a storage system. This work analyses the impact on the annual operation of the water volume, insulation characteristics, demand patterns, photovoltaic generation, design temperature limits and European climates. The optimal design of the district heating and cooling as a storage system will differ depending on the objective, to integrate the maximum amount of renewables excess or obtain maximum electricity savings. For the system located in Seville, hot Mediterranean climate, network insulation is almost negligible with water volumes below 30 m3; for greater values, the self-regulation temperature of the district heating and cooling system is relevant. Moreover, the maximum temperature increment in the distribution network is positive to minimise operational costs. Within the analyses performed in different European regions, the better results of grid consumption savings were obtained in hot Mediterranean areas, 33 %, meanwhile better renewable integration into the district system was obtained in the warm Mediterranean, with 65 % of the photovoltaic excess integrated.

A solar-assisted low-temperature district heating and cooling network coupled with a ground-source heat pump

Towards the development of highly integrated and energy efficient heating and cooling systems, with an energy community perspective, the present paper proposes a novel technical solution for the provision of air-conditioning, domestic hot water and electricity to a small residential district in heating-dominant regions. Three reference climates have been considered: Helsinki, Berlin and Strasbourg. Detailed dynamic models have been created using TRNSYS and NeMo, and long term operations of the energy system, including a new-generation ultra-low temperature district heating and cooling network have been performed. The core of the energy system is the network supplied by a high-efficiency ground source heat pump and used as the source and sink by booster heat pumps installed in the substations. Rooftop photovoltaic thermal panels partially meet the electrical demand of the district, as well as the thermal load for domestic hot water production. Moreover, the panels are cooled by the network, obtaining a reduction in the thermal unbalance to the ground and enhancing their electrical efficiency. This solution allows obtaining high coefficient of performance for the heat pumps in the substations and supply stations, reaching values of 5.4 and 4.0, respectively, for heating provision in the coldest locality. The proposed multi-energy district reaches an electrical self-consumption of 71% in the coldest locality and efficiently combines different renewable energy sources at district level in cold climates.

Low and Ultra-Low Temperature District Heating Equipped by Heat Pumps—An Analysis of the Best Operative Conditions for a Swiss Case Study

The manuscript analyses the management of low and ultra-low-temperature district heating systems (DHS) coupled with centralised and decentralised heat pumps. Operative conditions are defined in order to satisfy the heating needs without overloading the electric grid. The results are achieved by dynamic simulations, based on a real DHS located in southern Switzerland. At the building level, the heating needs are estimated considering real data and simultaneous energy simulations. Two DHS configurations, alternatives to the existing one, are simulated and suitable parameters for the management of the DHS are selected. The global performance of the two DHS is evaluated by KPIs also including the flexibility and the impact on the electric peak due to heat pumps. The achieved results are discussed providing suggestions for the stakeholders involved in DHS management for an optimal matching of the electric grid and thermal networks towards a reduction of the peak power. The rule-based control strategies defined allow the expected electric peak shaving and load levelling, conversely, the yearly energy consumptions are lightly increased and have to be further investigated. The outcomes demonstrate a global better performance of the ultra-low temperature DHS in terms of response to the applied control strategies and of energy savings.

Energetic and economic analysis of decoupled strategy for heating and cooling production with CO2 booster heat pumps for ultra-low temperature district network

The ultra-low temperature district heating and cooling (ULTDHC) systems are among the most interesting solutions for the decarbonisation of heating and cooling sectors. It is a technology where different optimised integrations and operations are still under development. The booster heat pumps (BHPs) or substations associated with this new generation take on a more critical role because the energy supplied by the main network is not enough for covering the user's requirements. Reviewing the literature, it has been seen necessary to make a short and long-term energetic and economic analysis of BHPs operation (considering heating, cooling and SHW requirements) under variable ULTDHC conditions. On this line, this work focuses on the analysis of the operation of two of the most promising BHP types: one with a single CO2 heat pump, type 1, and the other with two CO2 heat pumps with a decoupled operation, type 2. Both BHPs are connected to an ultra-low temperature District Heating and Cooling network with variable parameters throughout the year. The simulation is carried out in a representative building located in Toledo (Spain), in an area with considerable heating and cooling demands throughout the year. The objectives of the work are the following: analyse the daily and annual operations of the two BHPs types, study the effect of outdoor temperature on their optimal operation, point out their main parameter influence and variation and make an economic evaluation of these systems. The BHP type 2 investment is 8.3% more expensive than type 1 but the economic analysis shows an annual saving in heating and cooling operation of the building of 7.7% for BHP type 2 regarding type 1. The size of the BHP, total demand and buildings supplied affects the profitability of the BHP type. For this analysis, type 2 shows more profitable than type 1 when the annual heating demand is greater than a certain threshold.

Model predictive control for a heat booster substation in ultra low temperature district heating systems

District heating systems may support an increased penetration of stochastic renewable energy technologies and a reduction in centralized combined heat and power plants to reduce carbon dioxide emissions. Ultra low temperature district heating minimizes transport heat losses while enabling the utilization of low-grade surplus heat. Local heat booster substations can heat water to useable temperatures using a heat pump and a hot water tank for storage and flexible operation. This paper proposes a hybrid model predictive control strategy in which an existing heat booster substation is modelled and its charging schedule optimized in real-time over a 24-h forecasted prediction horizon. This enables load shifting whereby scheduling of the heat pump minimizes operation costs. The realisation of energy flexibility can support greater utilization of renewable energy sources and surplus heat in energy supply systems to reduce primary energy consumption. The linear hybrid model predictive controller was successfully implemented in a real 22-flat multifamily building in Copenhagen to verify the control strategy. A comparison of the proposed model predictive control scheduling to the standard rule-based control showed average savings of 23 % on the electricity costs.

Novel District Heating Systems: Methods and Simulation Results

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.

Steady state behavior of a booster heat pump for hot water supply in ultra-low temperature district heating network

Employing booster heat pumps (BHP) in district heating systems can provide a way to use low temperature heat sources for district heating as they allow locally increasing the water temperature on the user side where necessary. Thereby, it can provide decreased heat loss of a district heating system. This experimental study investigated steady state thermal behavior of a BHP for cases with different forward temperature and different flow rates on the user side. The unit used R134a as working fluid.The results revealed that the Coefficient of Performance (COP) of the BHP system has an optimal value at the studied range. At a forward temperature of 45 °C the heat pump reached maximum COP of 4.95, heating capacity of 15.36 kW, and exergy efficiency of 33.4 %, respectively. Lorenz COP and Lorenz efficiency is ranged from 13 to 27 and 0.16 to 0.26, respectively. Exergy destruction of each component as well as exergy efficiency of BHP used in the proposed system have also been analyzed, which showed that the compressor take the highest the exergy destruction among the components. The exergy efficiency ranged from 24.9% to 33.4% in the considered conditions. The share of exergy destruction of condenser and evaporator were influenced by the heat transfer condition on both sides, which reveals that the BHP can achieve potential performance improvement by better matching heat transfer profile of condenser and evaporator.

Heat pump efficiency in fifth generation ultra-low temperature district heating networks using a wastewater heat source

Ultra-low district heating (ULTDH) networks are operated at temperatures below 30 °C. However, the temperatures supplied are still too low for direct heating of the consumers. Heat pumps use the heating network as a source and can supply the consumers with space heating and domestic hot water as needed. Although individual manufacturers provide performance figures for the source temperature of 20 °C, these are usually derived from extrapolation from standard conditions with source temperatures of 0 °C and 10 °C. Up to now practical measurements are missing how efficiently heat pumps work in an ULTDH network environment and how the network temperature spread affects the performance. In this paper test rigs are used to investigate heat pumps under the conditions in ULTDH networks at a network supply temperature of 20 °C. It is shown that heat pumps can operate up to twice as efficient as with a geothermal probe. Furthermore, the network temperature spread has a significant influence on the performance of the heat pump. Heat pumps operate significantly better with a low source temperature spread.

The clean energy transition of heating and cooling in touristic infrastructures using shallow geothermal energy in the Canary Islands

The Canary Island archipelago holds 2.2 million inhabitants and received more than 14 million tourist visitors in 2019. Its volcanic origin and the presence of recent volcanic activity highlight its potential geothermal interest for heat production, therefore offering an attractive pathway for a renewable energy supply, not only for power generation and deep geothermal exploitation but also for low-enthalpy heating and cooling production. This work investigates nine touristic infrastructures in the Canary Islands which experienced a transition from conventional heat production systems to shallow geothermal energy (SGE) systems. The SGE systems established consisted in ultra-low temperature district heating and cooling (DHC) micro-networks involving, in most cases, small apartment blocks fed by shallow geothermal plants with an average cooling capacity of 602 kW and a heating capacity of 614 kW. The examination of this transition has exhibited an average saving of 374 MWh, 69,235 € and 280 tCO2 for each installation every year. This work provides evidence of the economical, energetical and environmental advantages of SGE technology in volcanic islands facing both and enormous heating and cooling demand due to tourism, in an energy-dependent system that also faces the challenge of the decarbonization of the heating production sector.

Influence of the Thermal Energy Storage Strategy on the Performance of a Booster Heat Pump for Domestic Hot Water Production System Based on the Use of Low Temperature Heat Source

Energy recovery from a low temperature heat source using heat pump technology is becoming a popular application. The domestic hot water demand has the characteristic of being very irregular along the day, with periods in which the demand is very intensive and long periods in which it is quite small. In order to use heat pumps for this kind of applications efficiently, the proper sizing and design of the water storage tank is critical. In this work, the optimal sizing of the two possible tank alternatives, closed stratified tank and variable-water-volume tank, is presented, and their respective performance compared, for domestic hot water production based on low temperature energy recovery in two potential applications (grey water and ultra-low temperature district heating). The results show that the efficiency of these kind of systems is very high and that variable-water-volume tanks allow a better use of the energy source, with an 8% higher exergy efficiency and around 3% better seasonal performance factor (SPF), being able to provide similar comfort levels with a smaller system size.