District cooling substation design and control to achieve high return temperatures

Low return temperatures are a prevailing issue in district cooling systems negatively affecting operating costs and energy efficiency. In this study, three aspects of district cooling substation design and control were investigated with the aim to increase the return temperatures: 1) secondary supply temperature setpoint, 2) primary flow rate and 3) the flow rate relation between the primary and secondary flows. Two different control strategies limiting the secondary setpoint and the primary flow were tested in four buildings supplied by district cooling. Also, the secondary flow was measured along with an NTU analysis and predictions with a heat balance and a support vector regression model. The results showed the control strategies successfully increased the primary return temperature with 0.6–1.6 \ub0C and eliminated flow in the saturation zone. The primary and secondary flows were shown to be unbalanced in fourteen of sixteen substations causing a low heat exchanger temperature effectiveness. The preferred method for predicting the secondary flow was support vector regression. The novelties of this paper are the conducted field tests and measurements with associated analyses, contributing with knowledge about the actual operation of district cooling substations and outcomes when implementing improvement measures to increase the primary return temperature

Combining direct ground cooling with ground-source heat pumps and district heating: Energy and economic analysis

Direct ground cooling (DGC) is a method used in cold climates to provide cooling to buildings without the use of any mechanical refrigeration. When DGC is utilized for providing cooling, ground-source heat pumps (GSHPs) and district heating (DH) are the two commonly used technologies for providing heating to the buildings. This article investigates the coupling of DGC with GSHPs and DH in terms of purchased energy and lifecycle costs. An office building equipped with active chilled beams for cooling and radiators for heating is used as a reference. Six cases based on different combinations of building envelope characteristics and thus different building heating and cooling loads are considered. The results show that using DGC-DH significantly reduces the amount of purchased electricity. However, the total energy cost is lower when DGC-GSHP is used. In addition, the DGC-GSHP can be more viable when the ground loads are well balanced. Investment costs, including borehole installation and equipment costs, are lower for the DGC-DH in the majority of the investigated cases. The lifecycle cost is lower for the DGC-DH in most of the investigated cases due to lower equipment costs

Analysis of Swedish school buildings\u27 energy performance certificates with focus on ventilation systems

Energy performance certificates are valuable sources of information about buildings. They are primarily used to assess the buildings\u27 energy performance, however the data included can also be used for building stock description or analysis from different perspectives. School buildings account for a substantial part of the Swedish public building stock and represent a great opportunity for implementation of energy saving strategies. To improve the energy efficiency, it is first important to analyse and understand the current energy use and identify the key factors responsible for most of the energy use. In Sweden, data used for EPC compilation are in most cases real measured data opposite to other European countries where EPC comprises calculated data practices. Therefore, the energy performance value provides a much more realistic representation of the building energy use. This study analyses certain aspects of school buildings\u27 energy performance using data available in EPCs, such as year of construction, floor area, heat supply systems and ventilation system. Comparison with data from some other European countries is also presented. The data which could be included in the certificate to extend the potential of EPC use in other areas, such as evaluation of indoor environmental quality, is also discussed

Size-resolved simulation of particulate matters and CO2 concentration in passenger vehicle cabins

The main aim of this study is to develop a mathematical size-dependent vehicle cabin model for particulate matter concentration including PM2.5\ua0(particles of aerodynamic diameter less than 2.5\ua0μm) and UFPs (ultrafine particles of aerodynamic diameter less than 100\ua0nm), as well as CO2\ua0concentration. The ventilation airflow rate and cabin volume parameters are defined from a previously developed vehicle model for climate system design. The model simulates different filter statuses, application of pre-ionization, different airflow rates and recirculation degrees. Both particle mass and count concentration within 10–2530\ua0nm are simulated. Parameters in the model are defined from either available component test data (for example filter efficiencies) or assumptions from corresponding studies (for example particle infiltration and deposition rates). To validate the model, road measurements of particle and CO2\ua0concentrations outside two vehicles were used as model inputs. The simulated inside PM2.5, UFP and CO2\ua0concentration were compared with the inside measurements. Generally, the simulation agrees well with measured data (Person’s\ua0r\ua00.89–0.92), and the simulation of aged filter with ionization is showing higher deviation than others. The simulation using medium airflows agrees better than the simulation using other airflows, both lower and higher. The reason for this may be that the filter efficiency data used in the model were obtained at airflows close to the medium airflow. When all size bins are compared, the sizes of 100–300\ua0nm were slightly overestimated. The results indicated that among others, expanded filter efficiency data as a function of filter ageing and airflow rate would possibly enhance the simulation accuracy. An initial application sample study on recirculation degrees presents the model’s possible application in developing advanced climate control strategies

Using data-driven indoor temperature setpoints in energy simulations of existing buildings: A Swedish case study

Building energy analyses of large samples or building stocks commonly use National building stock temperature averages in their calculations. However, such averages may not be representative of the conditions in a specific building type and may mask meaningful information found at building or dwelling level. Analysis of indoor temperature data from the Swedish housing stock showed that 25% out of approximately 1000 dwellings were heated at a temperature ≥23\ub0C in wintertime. If indoor temperature management is considered as a potential energy saving measure for the building stock it may be more effective to explore implementation in these specific dwellings, than considering average temperature reduction across the entire building stock. This however would require more detailed input data on indoor temperatures. Would such an approach be worthwhile? To answer this question, two types of Swedish multifamily buildings were simulated with i) business-as-usual scenarios and ii) setpoints based on indoor temperature data from the last Swedish National Survey. The study shows that using data-driven, dwelling-specific indoor temperatures could lead to more effective decision making on indoor temperature management, targeting buildings and dwellings where temperature reduction would most likely cause the least compromise on comfort. Such a strategy however should be complementary to a wider plan of improved energy efficiency measures across the building stock

Integration of solar thermal systems in existing district heating systems

The integration of large solar heating systems in district heating (DH) networks with large combined heat and power (CHP) plants is rarely considered. This is often due to low costs for heat but also due to subsidies for the electricity by CHP plants. Possible changes in subsidies and requirements in the reduction of fossil fuel based CO2 emissions raise an awareness of improving the operational flexibility of fossil fuelled CHP plants. This paper provides a rather simple but detailed methodology of including large solar heating systems in an existing district heating system, where heat is supplied by a large CHP plant. It uses hourly data of load and temperature patterns as well as radiation data and collector efficiency data to determine collector field size and storage size. The possibility of largely independent operation of sub-networks is analysed, which allows different system temperatures. It is demonstrated that a sub-network can operate without a back-up boiler and that both network parts benefit from the interaction. (C) 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)

Influence of system operation on the design and performance of a direct ground-coupled cooling system

Sizing of borehole heat exchangers (BHEs) for direct ground cooling systems (DGCSs) is a critical part of the overall system design. This study investigates the thermal performance and sizing of a DGCS with two different operation strategies using experimental and simulation approaches. The traditional on/off operation strategy keeps a constant room temperature. The continuous operation strategy has the potential to reduce the building peak cooling loads by precooling the space and having a variable room temperature measures. The experimental results from the laboratory-scale setup show the differences in the hourly room heat extraction rates and the room temperature pattern for the operation strategies applied. The experimental data is also used to develop a simulation model. The simulation results show that applying the continuous strategy reduces the building peak cooling loads and lowers the heat injection rates to the ground. For new BHEs, applying the continuous strategy can result in shorter BHEs, owing to the significantly lower ground heat injection rates. For existing BHEs, applying the continuous strategy can decrease the borehole outlet fluid temperature and thus, increase the cooling capacity of the building cooling system. The findings of this study have implications for developing the widespread use of DGCSs

Energy Renovation Strategies for Office Buildings using Direct Ground Cooling Systems

Direct ground cooling systems (DGCS) can provide comfort cooling to buildings without the use of any refrigeration-based cooling methods. DGCS is an emerging technology, commonly used for new office buildings in cold climates. This study aims at evaluating the energy-saving possibilities of a DGCS compared to a conventional chiller system for an existing office building. A typical Swedish office building with a chiller-based cooling system and in need of an energy renovation is taken as a reference case. A range of possible renovation measures are applied on the building and the cooling system, and the results are evaluated in terms of borehole design and building energy demand. The results show that applying the DGCS substantially reduces the building’s purchased energy, as chiller electricity demand is eliminated. In addition, implementing the renovation measures to reduce the thermal demand of the building could further reduce purchased energy. The results suggest implementing the DGCS after performing the renovation measures. This may lead to a considerable reduction in the required borehole length and hence, the drilling costs. Results from this study provide useful inputs for designing boreholes in ground-coupled systems for new and existing office buildings

Flow rate optimization in run-around heat recovery systems

Heat recovery technologies are used to reduce the energy use and the operating costs for ventilation systems in buildings. Run-around heat recovery systems for ventilation are commonly used in buildings when cross-contamination between the air streams is not acceptable, in buildings with complex ducting and in retrofit projects with space limitations. The design and operation of run-around systems are rather complex, especially in ventilation systems with variable air flow rates since the coupling liquid flow rate must be adjusted with respect to the air flow rate.This paper presents a mathematical model of a run-around heat recovery system. The model is validated with lab measurements and used further in parametric studies to evaluate how the overall thermal effectiveness of a system is influenced by different heat exchanger configurations, coupling liquids and operating conditions. Important findings suggests that the thermal effectiveness is highly sensitive to the coupling liquid flow rate, particularly for systems designed for high thermal effectiveness and for variable air volumes. The optimum liquid flow rate cannot only be determined by the air flow rate as it is influenced by the heat exchanger configuration and the liquid properties and not always found within the turbulent flow regime

A comparative study on borehole heat exchanger size for direct ground coupled cooling systems using active chilled beams and TABS

Direct ground cooling is a method for cooling buildings whereby free cooling is provided by circulating water through borehole heat exchangers (BHEs). Since no refrigeration cooling is involved, supply water temperature to the building’s cooling system is dependent mainly on BHE sizing. This study investigates the sizing of BHEs for direct ground cooling systems, with a particular focus on the influence of terminal unit types and their operating strategies. Experimental results using a direct ground-coupled active chilled beam (ACB) system are used to develop a simulation model for an office building. The model is also modified for thermally activated building systems (TABS). The simulation results show that using TABS instead of ACBs for a similar BHE reduced the ground peak hourly loads, resulting in a lower borehole outlet temperature. Resizing BHE depth to reach similar maximum borehole outlet temperatures according to the actual heat extraction rate from the cooling systems resulted in a significantly shorter BHE depth with TABS compared to ACBs. However, indoor temperature was generally warmer with TABS, due to their slower heat extraction rate from the room. The findings are practical for analysing the design and operation of BHEs for different types of terminal units