• Energy Research

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 optimise 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.

The Architecture, Engineering, and Construction (AEC) industry is transitioning toward using cloud-based Common Data Environments (CDEs) with interlinked BIM models. A CDE that engages all stakeholders of the building's design, construction, and operation phases represents the outset of BIM maturity level 3. This article introduces a CDE called Virtual Commissioning (VC), capable of commissioning an HVAC system before the physical commissioning of the HVAC system. The FSC diagram is introduced, to represent an HVAC BIM model within the VC CDE, and the Revit to FSC exporter, to serialize an HVAC object model from Revit to the FSC diagram. Three microservices were developed to exemplify the ease of developing independently scalable solutions for the VC CDE. Furthermore, the article proves that Modelica simulations can be run, using the microservice architecture of the CDE. To test the robustness of the system architecture for the CDE, two example models were introduced, one simple and one with a high level of complexity. Transferring the example models from Revit to the VC CDE was successful. Finally, in the roadmap for future development, it is proposed that future work should focus on using the CDE for advanced hydraulic simulations, using Modelica and Spawn-of-EnergyPlus.

With increasing focus on the performance of district heating systems, a concept is developed to obtain low district heating return temperatures from domestic hot water systems with a high share of circulation loss. For these systems, it is challenging to realize a low district heating return temperature by direct heat exchange only, due to the high flow of circulation return water at 50 °C. The concept is termed Circulation Booster. The purpose of the Circulation Booster is to boost the domestic hot water circulation temperature and at the same time secure a low district heating return temperature from this part of the service. The domestic hot water circulation temperature is heated in two steps: direct heat exchange and a heat pump. The heat source for the Circulation Booster is district heating, and the heat pump itself is driven by electricity. The paper includes the field experiences from a 1-year test period, concluding that the concept is operating as intended. Further, the performance results regarding electric consumption and district heating return temperatures and an economic feasibility study are presented. The current tariff structure in Denmark related to the district heating return temperature and electric costs gives a feasible economic case for the Circulation Booster concept with a direct payback time of 5,1 years. An increasingly progressive tariff scheme for low district heating return temperature or lower electric costs could further improve the economic feasibility of the Circulation Booster concept.

Abstract 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.

The optimization of control sequences in air handling units (AHUs) presents a significant opportunity for energy savings within HVAC systems. However, many building owners and operators require quantifiable estimates of potential energy savings before committing to retrofitting control systems. Valid estimates of energy savings require system models that consider capacity and limitations of the AHU, but in existing systems, scarce information hinders such modeling efforts. This lack of information complicates AHU modeling and the assessment of alternative control strategies. This paper demonstrates an approach that leverages time-series data from a newly constructed Danish AHU, equipped with multiple sensors for temperature, flow, and pressure, to construct a grey-box model of the unit, including component properties. The estimated parameters for the components are validated against data sheet information, and shows that the estimation procedure is accurate for parameter estimation. To analyze the energy-efficiency of the cooling coil, the model is used to estimate the latent cooling in the cooling coil, as Danish conditions rarely require dehumidification of supply air.

District heating networks increasingly rely on heat pumps, condensing biomass boilers, and excess heat in the transition to sustainable energy systems. Accordingly, district heating operators seek to reduce their networks’ supply and return temperatures to maximise production efficiencies, minimise heat losses from distribution pipes and allow greater utilisation of renewable heat sources and excess heat. Experts have predicted that investing in solutions that reduce heating temperatures in buildings will yield a return on investment of 300% for district heating operators. Therefore, expecting incentives, building operators should identify methods to reduce supply and return temperatures to enable a rapid, widespread transition to low-temperature district heating. Ample research has investigated and documented the feasibility of low-temperature heating in buildings, and this paper presents the first comprehensive review. It synthesises available literature and adds new perspectives to help guide future implementation, research and development of low-temperature heating. The energy and temperature demands of various heating systems provides a background, leading to a review of typical malfunctions and their impacts. The article subsequently reviews the obtainable supply and return temperatures before and after renovating the building envelope and heating systems. It further identifies and summarises vital measures for decreasing heating system temperatures. Ultimately, the authors recommend minimising heating system temperatures using automatic balancing of space heating and ventilation systems, novel solutions for safe domestic hot water supply, and digitally-enabled performance monitoring and optimal control.

Operating district heating systems with low supply and return temperatures improves heat production and distribution efficiency, permitting greater integration of renewable heat sources. Low-temperature district heating is viable without compromising comfort, but faults in end-users’ heating systems constrain temperature reductions. Such faults include malfunctioning valves, improper hydronic balancing, and excessive supply temperature setpoints. Occupants lack the resources to detect and diagnose these faults, so there is a need for automated solutions without requiring additional hardware. This paper proposes a method for improving the operation of an apartment's hydronic floor heating system using data from room thermostats, a heat meter and a circulation pump to identify a grey-box model of the system. The resulting model virtually senses each room loop's heat flux, flow, return temperature, and flow coefficient. The authors tested the model on a low-energy apartment in Denmark, using it to diagnose causes of high return temperatures, including poor hydronic balancing and an excessive supply temperature setpoint and pump setting. The authors also used the model to predict the minimum permissible supply temperature maintaining comfort, yielding a reduction in the energy-weighted supply and return temperatures of 8.6 °C and 6.5 °C, respectively.

On the transition toward low-temperature district heating (DH), generation sectors, distribution networks, and building consumers should all be adapted to low-temperature operation conditions. However, a bottleneck in lowering DH return temperatures is the domestic hot water (DHW) system with a circulation loop in multifamily buildings. Existing systems with a single heat exchanger often led to elevated return temperatures because of the reheating of the circulation loop. This study developed several innovative designs for future-proof DHW substations that decouple the heating of cold water and circulation flows, ensuring lower DH return temperatures in large multifamily buildings. First, a theoretical analysis was performed for benchmarking the return temperature for various proposed design configurations under low-temperature operation conditions; then, the proposed configurations were tested for a Danish multifamily building connected to a medium-low-temperature DH network. In the field tests, compared to a typical DHW substation with a single heat exchanger, the proposed configuration with the circulation loss booster reduced the average DH return temperature from 46.4 °C to 34.1 °C and 27.9 °C for parallel or serial connections, respectively. Economic analysis confirms the viability of the proposed solution, with a payback period ranging from 3.4 to 7.9 years.

The district heating (DH) system is in its transition towards the 4th generation district heating (4GDH), and the high DH return temperatures need to be addressed during the process. In the existing building heating systems, many faults, malfunctions, or sub-optimal operations can lead to high DH return temperatures. However, the field of fault detection and diagnostics (FDD) within heating systems is notably under-researched in contrast to their counterparts in ventilation and air conditioning in building HVAC systems. This divergence can be attributed to several factors, including limited digital integration, a scarcity of data, and the non-obvious nature of faults in heating systems. In this study, we utilized heat cost allocators (HCA) and energy meters to investigate the features and potential impacts of four untraced faults that can lead to high district heating (DH) return temperatures in both space heating and domestic hot water systems in large buildings. We identified component-level faults, including heat exchanger overflow, space heating temperature controller failures, and excessive operating temperatures due to bypass, as well as system-level issues such as non-uniform heat distribution in buildings and its impacts. This exploration provides new, critical insights for advancing FDD research in HVAC and DH systems.