• Energy Research

Low-temperature district heating (LTDH) networks can integrate sustainable energy sources and waste industrial heat towards decarbonisation goals by 2050. LTDH networks can be realised through the low-temperature operation of heating systems in buildings. However, the low-temperature operation of heating systems is obstructed by inefficient radiator control by end-users or other technical errors. This study investigated the implementation of a strategy for low-temperature operation of radiator systems by calculating the minimum supply temperature and using an innovative treatment of data from electronic heat cost allocators to identify radiators not in use and locate the critical apartments with higher heat demands. According to the results, the low-temperature operation of radiator systems is possible. Although, the minimum supply temperature should be calculated based on the higher heat demand of the critical apartment identified to avoid complaints regarding poor thermal comfort. An energy weighted average supply temperature of 55 °C can be achieved, resulting in an average energy weighted return temperature of 31.3 °C in the system. Testing of a reduced supply temperature in the building case highlighted the existence of critical apartments. The investigation highlighted that the increased heat loss to the poorly heated neighbouring apartments heavily influences the critical apartments.

In the quest to increase the share of renewable and residual energy sources in our energy system, and to reduce its greenhouse gas emissions, district heating networks and seasonal thermal energy storage have the potential to play a key role. Different studies prove the techno-economic potential of these technologies but, due to the added complexity, it is challenging to design and control such systems. This paper describes an integrated optimal design and control algorithm, which is applied to the design of a district heating network with solar thermal collectors, seasonal thermal energy storage and excess heat injection. The focus is mostly on the choice of the size and location of these technologies and less on the network layout optimisation. The algorithm uses a two-layer program, namely with a design optimisation layer implemented as a genetic algorithm and an optimal control evaluation layer implemented using the Python optimal control problem toolbox called modesto. This optimisation strategy is applied to the fictional district energy system case of the city of Genk in Belgium. We show that this algorithm can find optimal designs with respect to multiple objective functions and that even in the cheaper, less renewable solutions, seasonal thermal energy storage systems are installed in large quantities.

This paper discusses the charging of multiple plug-in hybrid electric vehicles (PHEVs) in an apartment building, equipped with a photovoltaic (PV) system. Different charging strategies and charging power ratings are examined, which are assessed in terms of their grid impact, the self-consumption of local electricity generation, and the electric driving range. The impact of a residential building, which incorporates electric vehicle (EV) charging, on the distribution grid can be significantly reduced by using simple EV charging strategies. These strategies include complementing night-time with day-time charging, peak shaving at vehicle level, and charging the surplus of local generation. Effective results are obtained using only the knowledge of the present battery state of charge, next departure time, and the instantaneous local generation surplus. The simultaneity of the EV charging and the PV production increases. The increase in electric driving range is negligible for three-phase charging.

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.

Abstract A ground-source heat pump system in combination with different low temperature heating systems was installed in a new school building in Belgium. During the first two years of operation, measurement data indicated that the difference between heating and cooling load could affect the long term thermal stability of the ground storage and thus may result in poor energy efficiency of the GSHP system. This paper describes two low-cost measures that can be taken in order to reduce or prevent such unfavourable conditions. The school building and HVAC system were modeled and on-site measurement data was used for validation. First, the HVAC system was optimized by integrating an additional cooling coil in the exhaust airflow of the air handling unit in order to recuperate waste heat from the exhaust ventilation air. Additionally, the effect of passive cooling during summer holidays on the annual energy balance was investigated. Results indicated that heat recuperation of the exhaust ventilation air and passive cooling of the school building during weekends and summer holidays are complementary and contribute to a more stable ground temperature, consequently significant energy savings can be achieved.

Abstract The work presented in this paper relates to a small scale district heating network heated by a gas-fired CHP. In most common situations, such a CHP is heat driven operated, meaning that the CHP will switch on whenever heat is needed, while not taking into account the demand of electricity at that time. In this paper however, an active control strategy is developed, aiming to maximize the profit of the CHP, selling its electricity on the spot market. The CHP will therefore switch on at moments of high electricity prices. Nevertheless, since there never is a perfect match between the demand of heat and the demand of electricity, thermal energy storage is included in the network to overcome this difference. Here, three different storage concepts are compared: (1) a central buffer tank next to the CHP; (2) small storage vessels distributed over the different connected buildings; and (3) the use of the thermal mass of the buildings as storage capacity. Besides the development of the control algorithms based on model predictive control, a simulation model of the network is described to evaluate the performance of the different storage concept during a representative winter week. The results show that the presented control algorithm can significantly influence the heat demand profile of the connected buildings. As a result, active control of the CHP can drastically increase the profit of the CHP. The concept with the distributed buffers gives the best results, whereas the profit for the thermal mass concept is only marginally smaller. Since in this latter case no significant investment costs are needed, the conclusion for this case study is that the use of thermal mass of buildings for demand side management in district heating systems is very promising.

4 th generation district heating and cooling networks (shortly THERNETs) are often coined as a crucial technology to enable the transition towards low-carbon smart energy systems. Most importantly, they open perspectives for integration of low-grade residual heat from industry, renewable energy sources (such as geothermal heat and cold and solar thermal collectors), more efficient energy conversion units (such as collective heat pumps), while thermal energy storage (TES) systems increase system flexibility. In order to optimize design and control of such complex systems, a toolbox modesto (Multi-objective district energy systems toolbox for optimization) is under development. However, the representation of seasonal heat and cold storage systems on an annual basis requires large computational power. In an attempt to decrease computational cost, a technique with representative time slices (inspired by and combining aspects from optimization studies of electrical energy systems, unit commitment problems, thermal systems with short term energy storage and smaller scale industrial thermal systems with longer term energy storage) is developed and tested. The aim of this study is to investigate the applicability of such representative time periods to optimize seasonal TES systems in THERNETs. To this end a full year optimization is compared to one with representative time periods for a realistic case study that uses demand profiles from the city of Genk (Belgium) and energy system parameters from Marstal (Denmark). This comparative study shows that modelling with representative periods is sufficient to mimic the behaviour of a full year optimization. However, when curtailment of solar heat injection occurs, not all representations yield the same results. It was found that for the studied case, a selection of 12 representative weeks performs best, while all reduced optimizations result in a substantial reduction (speed-up of on average x4.8 to x7.7) of the calculation time.

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.