District Heating Supply Temperature Research Articles

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.

Potential for supply temperature reduction of existing district heating substations

Reducing operating temperatures is a crucial goal for transforming district heating networks into sustainable systems as it allows the integration of low exergy heat (e.g. renewable, waste heat). Many criticalities arise when reducing operating temperatures in existing networks that are not designed to operate in these conditions. The criticalities occur at different levels: in the building, in the thermal substations, in the pipelines and in the production plants. In this paper, the reduction of supply temperature in existing district heating substations is analyzed. A methodology including a model of the thermal substation and a data analysis software is developed to estimate the potential temperature reduction that could be applied to existing substations. The application of the model to an existing large-scale district heating network in Northern Italy shows that all the analyzed substations are currently able to shift their operation from 120 °C to 104 °C, and that district heating supply temperatures around 90 °C can be realistically achieved with few improvements on the system.

Staged control of domestic hot water storage tanks to support district heating efficiency

Storage tanks are commonly used for domestic hot water (DHW) preparation in large buildings supplied by district heating (DH), especially to cope with peak demand. The charging control of DHW tank systems is often suboptimal, increasing return temperatures and harming the overall DH operation efficiency. This paper presents two novel control concepts to optimize DHW tank charging, satisfying comfort and hygienic requirements without leading to excessive DH flows. The first, more complex control concept employs the smart energy meter sometimes used for DHW billing. It inspired the development of a second, broadly implementable control concept employing a staged proportional gain with an added temperature sensor. The authors tested and refined this staged-gain concept using a validated Modelica model of a real DHW system in a Danish multistory residential building. The authors subsequently implemented the staged-gain control concept in the field, successfully reducing the energy-weighted DH return temperature by 7 °C and the total DH flow by 23.6% compared to the conventional thermostatic control. This analysis accounted for the variation in DHW tapping, DHW temperature, DH supply temperature, and cold water temperature. Furthermore, the performance was robust to relaxed settings of the valve constraints, demonstrating minimal configuration requirements for new implementations.

A decision support model for waste heat recovery systems design in Data Center and High-Performance Computing clusters utilizing liquid cooling and Phase Change Materials

The Data Center (DC) and High-Performance Computing (HPC) sector is increasingly becoming one of the single largest energy consuming sectors in the energy system and the demand for new DCs is growing with the rising use of cloud-based services, social media and internet usage in general. DCs and HPC clusters employ energy intensive cooling in order to dissipate the generated heat from the Information Technology (IT) equipment. Conventionally, the cooling is almost exclusively done by air-based systems; however, the use of liquid based cooling has shown potential to increase computational efficiency of the systems and decrease cooling needs by operation above the free cooling limit. Furthermore, the liquid coolant can operate at higher temperatures allowing high temperature waste heat recovery. In addition, load shifting through Latent Thermal Energy Storage (LTES) will allow the non-controllable waste heat resource to be stored seasonally, thereby granting the ability to cover larger District Heating (DH) loads. In this work, a decision support model is developed that will take basic information regarding a HPC cluster or DC as inputs. The decision support model will provide a parameterized output that shows different configurations and design parameters that can be utilized for the system. The main outputs includes yearly energy savings, yearly cost savings and efficiency gains through the Power Usage Efficiency and the Energy Reuse Efficiency. The decision support model is demonstrated in a Danish case study. Electricity savings between 8.14 % and 10.8 % of the total cluster electricity consumption and a waste heat recovery potential of 85 MWh/year to 576 MWh/year are obtained. It is shown that if the DH covered is needed to be self-sufficient the configuration would require an LTES with PCM mass of 500 kg, but system configurations that operate as an addition to an existing local heating source shows increases in yearly energy savings of 332 % compared to self-sufficient configurations. The goal of the decision support model is to assist the design of future waste heat recovery applications through selection of system parameters including coolant temperatures, energy storage design parameters, DH supply temperatures and DH load coverage from the DC or HPC cluster.

Overview of Solutions for the Low-Temperature Operation of Domestic Hot-Water Systems with a Circulation Loop

The operation of typical domestic hot water (DHW) systems with a storage tank and circulation loop, according to the regulations for hygiene and comfort, results in a significant heat demand at high operating temperatures that leads to high return temperatures to the district heating system. This article presents the potential for the low-temperature operation of new DHW solutions based on energy balance calculations and some tests in real buildings. The main results are three recommended solutions depending on combinations of the following three criteria: district heating supply temperature, relative circulation heat loss due to the use of hot water, and the existence of a low-temperature space heating system. The first solution, based on a heating power limitation in DHW tanks, with a safety functionality, may secure the required DHW temperature at all times, resulting in the limited heating power of the tank, extended reheating periods, and a DH return temperature of below 30 °C. The second solution, based on the redirection of the return flow from the DHW system to the low-temperature space heating system, can cool the return temperature to the level of the space heating system return temperature below 35 °C. The third solution, based on the use of a micro-booster heat pump system, can deliver circulation heat loss and result in a low return temperature below 35 °C. These solutions can help in the transition to low-temperature district heating.

Dynamic Approach to Evaluate the Effect of Reducing District Heating Temperature on Indoor Thermal Comfort

To reduce energy consumption for space heating, a coordinated action on energy supply, building fabric and occupant behavior is required to realize sustainable improvements. A reduction in district heating supply temperature is an interesting option to allow the incorporation of renewable energy sources and reduce distribution losses, but its impact on the final users must be considered. This aspect is especially critical as most European countries feature an old building stock, with poor insulation and heating systems designed for high-temperature operation. In this study, a complete methodology is devised to evaluate the effect of district heating temperature reduction on the end users by modeling all the stages of the system, from the primary heat exchanger to the indoor environment. A dynamic energy performance engine, based on EN ISO 52016-1:2017 standard and completed with a heat exchanger model, is implemented, and its outputs are used to calculate thermal comfort indicators throughout the heating season. As a practical application, the method is used to evaluate different scenarios resulting from the reduction of primary supply temperature of a second-generation district heating network in Northern Italy. Several building typologies dating back to different periods are considered, in the conservative assumption of radiator heating. The results of the simulations show that the most severe discomfort situations are experienced in buildings built before 1990, but in recent buildings the amount of discomfort occurrences can be high because of the poor output of radiators when working at very low temperatures. Among the possible measures that could help the transition, actions on the primary side, on the installed power and on the building fabric are considered. The investigation method requires a limited amount of input data and is applicable to different scales, from the individual building to entire urban areas lined up for renovation.

System effects of lowered district heating supply temperatures

Lowering temperature levels of a district heating (DH) system may offer several advantages such as reduced distribution losses, increased efficiency of flue gas condensation equipment and increased electricity generation in combined heat and power plants. In a broader perspective this can result in more efficient use of natural resources as well as reduced climate-impacting emissions. This study examines how decreased DH supply temperatures influence the power-to-heat ratio and thereby electricity production and fuel use in a combined heat and power plant. Carbon dioxide equivalent (CO2-eqv.) emissions and primary energy use were calculated with three different marginal electricity perspectives. A regional DH system situated in mid-Sweden was used as a case study and the energy system cost optimization modelling tool MODEST (Model for Optimization of Dynamic Energy Systems with Time-Dependent Components and Boundary Conditions) was used. The results show that decreasing the DH supply temperature results in increased electricity production as well as increased fuel use within the system. Further, there is a significant difference in CO2-eqv. emissions and primary energy use for the studied marginal electricity perspectives.

Comparison of COP estimation methods for large-scale heat pumps used in energy planning

This paper compares estimation of the coefficient of performance (COP) of a large-scale heat pump (HP) for district heating based on four methods to the COP obtained using a detailed thermodynamic HP model. Four heat sources and varying district heating supply temperatures were considered. The COP estimation methods are based on constant COP, Lorenz efficiency, exergy efficiency and a method presented by Jensen et al. (2018). They were implemented in an energy planning tool and further analysed. The planning tool was used to assess HP implementation in a new district in Copenhagen, Denmark. The change in seasonal COP of the HPs, the economic results and optimal HP capacities were compared.The results show that the Jensen et al. (2018) method provides good approximations and that the planning tool identifies a similar solution compared to the use of the thermodynamic HP model. Assuming a constant Lorenz efficiency, exergy efficiency or COP over the year resulted in large deviations in COP, especially for operations very different from the design conditions. Consequently, other heat sources were found for the most economical solution. The accuracy of the three methods decreased when the initial assumptions of constant COP and efficiencies differed from the ones at design conditions.

A method for technical assessment of power-to-heat use cases to couple local district heating and electrical distribution grids

The transition of district heating and electrical distribution grids from traditionally independent to actively coupled and operated networks is seen as an important step on the way to smart energy networks. This work presents a method that enables a detailed technical assessment of the operation of such coupled heat and power networks. It is based on a sequential coupling approach of a dynamic thermal-hydraulic model for the district heating network and a quasi-static model for the electrical distribution network. Different use cases are highlighted where a local coupling of the networks with power-to-heat is supporting the transition to smart energy networks, i.e., lowering district heating supply temperatures, accommodating renewable energy sources in the power network and integrating low-temperature heat sources into the district heating network. All three use cases are implemented in example applications to showcase the versatility of the method. The results underline the presented method's ability to perform detailed technical assessments of coupled heat and power networks.

A multipurpose test rig for district heating substations: domestic hot water preparation and keep-warm function comparison

A well performing District Heating Substation (DHS) is crucial for the efficiency of the District Heating (DH), especially with the shift towards low temperature 4th generation DH systems. For this reason, testing and characterization of commercially available DHSs becomes important to estimate their effect on the DH network. Within the thermo-technical laboratory of EnergyVille, a multipurpose test rig has been built for testing DHSs. In this setup, different DH conditions and heat demand profiles for space heating and for Domestic Hot Water (DHW) can be emulated. Independent tests have been performed on 4 DHSs from three different manufacturers, focused on the DHW preparation for low DH supply temperature and on the stand-by/keep-warm operation of the substations. The latter maintains a certain temperature within the heat exchanger to avoid delays in the delivery of DHW. The results showed that improvements are needed on DHW production for lower DH supply temperatures. Also, enhancements are needed to reduce losses from the keep-warm function. Given that DH systems can have thousands of substations, this will reduce the overall losses and improve the performance of the DH network.

Field experience with ULTDH substation for multifamily building

Ultra-low temperature district heating (ULTDH) substations may be an important enabler for integrating higher shares of renewables and waste heat in the district heating (DH) network. In the paper, we describe a concept and first experimental results for producing domestic hot water (DHW) at DH supply temperatures of 45°C. The substation utilises a heat pump for boosting the DH temperature up to approximately 60°C to an accumulation tank, after which the DHW can be produced on demand for temperatures up to 55°C. Additionally, the system included a separate heat pump to supply the DHW circulation heat demand. The DH accumulator tank provides load shift opportunities, which is important going towards the integrated, flexible and renewable based future energy system. The two heat pumps operate with a COP of approximately 5, which results in representative share of approximately 11-13 % electricity to supply the DHW including circulation.

Ultra-low temperature district heating system with central heat pump and local boosters for low-heat-density area: Analyses on a real case in Denmark

Low temperature district heating (DH) system gives easier access to the renewable energy as heat sources and improves the heat distribution efficiency. From the exergy point of view, low DH supply temperature also better matches the exergy demand of space heating and domestic hot water. It is more beneficial to operate district heating system under lower temperature level for the heat-sparse area where the distribution losses accounts for a large proportion in the total heat supply. In this study, the actual performance of a case ultra-low temperature district heating (ULTDH) system in Denmark was investigated based on long-term measurements. The system combines the central heat pump and local boosters, while the impact of such configuration on the overall system performance was analysed. The energy, exergy and economy performances of the case system were compared to medium temperature district heating system (MTDH) and low-temperature district heating system (LTDH). The results show that the LTDH system without supplementary heating has the highest energy and exergy efficiency. While the ULTDH system has better performance compared to the MTDH system in energy, exergy and economy due to substantial savings from the distribution heat loss.

Cost Analysis of In-House Heat Substations in Next Generation Heat Networks

Lowering the district heating supply temperature is crucial for improving the efficiency of the heat network. A developed concept of fourth generation low temperature district heating with supply temperature just above the required end user conditions allows for heat loss reduction, increases the plant room efficiency and enables the integration of alternative energy source for heat production. Due to the lowered temperature approach the cost of the in-house substation increases. This paper presents analysis of the cost increase of the in-house heat substation to enable the cost-benefit analysis of next generation district heating systems while considering new and refurbished systems. Heat transfer calculation software tool has been used to analyse the cost increased of the substation due to required high performance of the plate heat exchanger while considering that the rest of the components are the same model/size, including heat meter, control valve and pipework. It is concluded in this paper that by an accurate design of the in-house systems it is feasible to provide a cost effective solution of in-house heat substations operating in next generation heat networks.

Analysis of the Impact of Decreasing District Heating Supply Temperature on Combined Heat and Power Plant Operation

Abstract District heating systems are widely used to supply heat to different groups of heat consumers. The district heating system offers great opportunities for combined heat and power production. In this paper decreasing district heating supply temperature is analysed in the context of combined heat and power plant operation. A mathematical model of a CHP plant is developed using both empirical and theoretical equations. The model is used for analysis of modified CHP plant operation modes with reduced district heating supply temperature. Conclusions on the benefits of new operation modes are introduced.

Renewable-based low-temperature district heating for existing buildings in various stages of refurbishment

Denmark is aiming for a fossil-free heating sector for buildings by 2035. Judging by the national heating plan, this will be achieved mainly by a further spread of DH (district heating) based on the renewable heat sources. To make the most cost-effective use of these sources, the DH supply temperature should be as low as possible. We used IDA–ICE software to simulate a typical Danish single-family house from the 1970s connected to DH at three different stages of envelope and space heating system refurbishment. We wanted to investigate how low the DH supply temperature can be without reducing the current high level of thermal comfort for occupants or the good efficiency of the DH network. Our results show that, for a typical single-family house from the 1970s, even a small refurbishment measure such as replacing the windows allows the reduction of the maximum DH supply temperature from 78 to 67 °C and, for 98% of the year, to below 60 °C. However for the temperatures below 60 °C a low-temperature DH substation is required for DHW (domestic hot water) heating. This research shows that renewable sources of heat can be integrated into the DH system without problems and contribute to the fossil-free heating sector already today.

99/02107 Performance of solar still under Jordanian climate: Hamdan, M. A. et al. Energy Convers. Manage., 1999, 40, (5), 495–503

Denmark is aiming for a fossil-free heating sector for buildings by 2035. Judging by the national heating plan, this will be achieved mainly by a further spread of DH (district heating) based on the renewable heat sources. To make the most cost-effective use of these sources, the DH supply temperature should be as low as possible. We used IDA–ICE software to simulate a typical Danish single-family house from the 1970s connected to DH at three different stages of envelope and space heating system refurbishment. We wanted to investigate how low the DH supply temperature can be without reducing the current high level of thermal comfort for occupants or the good efficiency of the DH network. Our results show that, for a typical single-family house from the 1970s, even a small refurbishment measure such as replacing the windows allows the reduction of the maximum DH supply temperature from 78 to 67 °C and, for 98% of the year, to below 60 °C. However for the temperatures below 60 °C a low-temperature DH substation is required for DHW (domestic hot water) heating. This research shows that renewable sources of heat can be integrated into the DH system without problems and contribute to the fossil-free heating sector already today.