Hot Water Circulation Research Articles

Energy flexibility potential of domestic hot water systems in apartment buildings

Domestic Hot Water (DHW) storage tanks are identified as a main source of flexible energy use in buildings. As a basis for energy management in apartment buildings, this paper describes the aggregated DHW use in a case building, and analyses the potential for DHW energy flexibility by simulating different control options. The case study for the work is an apartment building in Oslo with 56 apartments and a shared DHW system. Energy measurements are available for consumed hot water, hot water circulation, and energy supplied to the DHW tanks. The measurements are presented with minute, hourly and daily values. Aggregated daily energy use for the consumed hot water is in average 362 kWh, while the energy supplied is 555 kWh. The potential for energy flexibility is analysed for a base case and for four different rule-based control options: Power limitation, Spot price savings, Flexibility sale and Solar energy. Economic consequences of the control options are compared. With the Norwegian tariff structure, maximum hourly power use has the main impact on the cost. Control systems that aim to reduce the maximum power use may be combined with spot price savings or to offer end-user flexibility services to the grid.

Further development of the change-point model – Differentiating thermal power characteristics for a residential district in a cold climate

The building and service sector accounts for nearly 40% of total energy use in Sweden. The existing, historic building stock accounts for large part of this energy use and comprises an important part of the national pursuit to increase energy efficiency. One main problem for decreasing energy use in the existing building stock is the lack of data describing thermal performance characteristics. This paper presents a novel development of the change-point model for predicting the thermal performance of buildings using selected time periods based on time-dependent variations in climate and user behavior. The predicted thermal power characteristics include total specific heat losses (Qtotal), energy use for hot water circulation (HWC) and hot tap water (HTW), and balance temperature. A residential district with 73 historic buildings in Linköping, Sweden, has been used as the study object.The developed model is shown to be effective and robust for describing building thermal performance. The average R2 was 0.70 for predictions of specific heat losses. The sensitivity analyses conclude that the selected time steps and months correspond to the highest R2 value. The average variation width for prediction of the balance temperature is 0.9 °C for buildings in the interquartile range based on a three-year comparison of hourly heating power supply data. Moreover, from a property owner perspective, the model is shown to be useful for identifying deviating thermal power characteristics and can easily be used to get an overview of a district.

Performance of air source air conditioning water heater using trombone coil dummy condenser with different diameter and pipe length

The Air Source Air Conditioning Water Heater (ASACWH) performance as an energy source to heat water in the tank using dummy condenser type of trombone coil with different diameter and pipe length without hot water circulation has been investigated. The diameter and length of the dummy condenser pipe are intensely affected by ACWH performance. In this study, cooling capacity, Coefficient of Performance (COP), compressor power and room temperature were evaluated in three types of trombone coil (6.4 mm coil diameter with a length of 7.9 m, 6.4 mm coil diameter with a length of 5.3 m, and 9.5 mm coil diameter with a length of 5.3 m) with different cooling load variation. This study used cooling load with a variation of 0 W, 1000 W, 2000 W, and 3000 W without hot water circulation in the simulation room. It was found that the ASACWH using a pipe with a coil diameter of 6.4 mm and length of 7.9 m performed the highest cooling capacity and COP, and produced more comfortable room temperature than the other two pipes. The results indicated that when the cooling load icreased from 0 W to 3000 W, the compressor power increased by 11.3%, 6.3%, and 9.3%, using the 6.4 mm coil diameter with 7.9 m length, 6.4mm diameter of the coil with 5.3m length and 9.5mm coil diameter with 5.3m length, respectively.

Measured and Simulated Energy Use in a Secondary School Building in Sweden—A Case Study of Validation, Airing, and Occupancy Behaviour

In this case study, the energy performance of a secondary school building from the 1960s in Gävle, Sweden, was modelled in the building energy simulation (BES) tool IDA ICE version 4.8 prior to major renovation planning. The objectives of the study were to validate the BES model during both occupied and unoccupied periods, investigate how to model airing and varying occupancy behaviour, and finally investigate energy use to identify potential energy-efficiency measures. The BES model was validated by using field measurements and evidence-based input. Thermal bridges, infiltration, mechanical ventilation, domestic hot water circulation losses, and space heating power were calculated and measured. A backcasting method was developed to model heat losses due to airing, opening windows and doors, and other occupancy behaviour through regression analysis between daily heat power and outdoor temperature. Validation results show good agreement: 3.4% discrepancy between space heating measurements and simulations during an unoccupied week. Corresponding monthly discrepancy varied between 5.5% and 10.6% during three months with occupants. Annual simulation indicates that the best potential renovation measures are changing to efficient windows, improved envelope airtightness, new controls of the HVAC system, and increased external wall thermal insulation.

Study on solar energy utilization characteristics of a solar building integrated wall

This study proposed a solar building integrated wall in which a solar hot water circulation system was installed. The wall was provided with a solar collector and a thermal radiation plate, which were connected through circulating pipes with a control valve and a water pump. In the cold climate of Xiangtan, experiments and simulations on the solar energy utilization efficiency of the wall were carried out. The results show that the heat transfer amount of the solar building integrated wall increased with the increase of the solar irradiance, whereas the solar thermal utilization efficiency decreased. Increasing the circulating water flow can increase the heat transfer amount of the wall and solar thermal utilization efficiency, thus change the effective thermal resistance of the wall. Under the studied conditions, the heat transfer amount and solar thermal utilization efficiency reached 1530.21 W and 63.8%, respectively. The circulating water flow does not need to be greater than 200 L/(h·m2). The area ratio of the thermal radiation plate to solar collector has a great influence on the heat transfer amount of the wall and the solar thermal utilization efficiency. Increasing the area of thermal radiation plate is beneficial to improving the solar thermal utilization efficiency.

Gas Production from Hot Water Circulation through Hydraulic Fractures in Methane Hydrate-Bearing Sediments: THC-Coupled Simulation of Production Mechanisms

Methane hydrates, widely found in permafrost and deep marine sediments, have great potential as a future energy source. Conventional production schemes perform poorly for challenging hydrate reserv...

Theoretical modeling of a glass-cooled solar still incorporating PCM and coupled to flat plate solar collector

Abstract The performance of a modified solar still was investigated considering the following cases: solar still with glass cooling (SC), SC connected to an external collector (SCC), and SCC with phase change material (SCCP). Three different PCMs were used; sodium acetate trihydrate, sodium thiosulfate pentahydrate, and paraffin wax. The effect of various parameters; controllable (hot water circulation rate, PCM mass, and cooling water flowrate) and uncontrollable parameters (solar irradiation, ambient temperature, and wind speed) were investigated. Increasing the solar irradiation from 200 to 700 W/m2 increased the productivity from 0.9 to 3.4, 2.35 to 10, and 3 to 11.9 ml/min for the SC, SCC, and SCCP (with SAT as PCM) systems, respectively. The addition of external collector and PCM increased the productivity 2.4 times. Increasing the coolant mass flow rate from 0 to 10 kg/s, increased the productivity from 1 to 2.14, 1 to 6.65, and 1 to 7.5 ml/min for the SC, SCC, and SCCP systems, respectively. Additionally, increasing the hot water circulation rate of external collector from 0 to 0.1 kg/s increased the productivity from 2.4 to 6 and from 4 to 7.4 ml/min for SCC and SCCP system, respectively. The optimum mass ratio of basin water: PCM was found to be 2:1, while the best PCM type was found to be the one with the highest melting point and latent heat of fusion. The 24-h operation for a typical May month weather data in Jordan, revealed that the productivity improved 1.8 times for SCC and 2.3 times for SCCP-SAT compared with SC system.

Analytical investigation of gas production from methane hydrates and the associated heat and mass transfer upon thermal stimulation employing a coaxial wellbore

In this study, an analytical approach has been developed to couple the wellbore heating process by hot water circulation and the associated methane hydrate dissociation in the reservoir. A coaxial wellbore is assumed as the heat source where both conduction and convection heat transfers are considered. It consists of an inner tube and an outer structure of casing, gravel, and cement layers. In the reservoir, a similarity solution employing a moving boundary separating the dissociated and undissociated zones is employed to build the analytical solution. Two different operating schemes for water supply into wellbore heat source have been studied: i) from the inner tube; and ii) from the annulus section of the wellbore. Temperature distribution along the wellbore, temperature and pressure distributions in the reservoir, hydrate dissociation rate, and energy efficiency considering various initial and boundary conditions and reservoir properties are evaluated. The two different operating schemes have almost the same results with slightly higher gas production in the case of hot water entry into annulus, which is in direct contact with the reservoir. Increasing the inlet water temperature or decreasing the wellbore pressure increases gas production. Applying them simultaneously results in a greater gas production and energy efficiency. Some of the reservoir’s properties, such as porosity, thermal diffusivity, thermal conductivity, and reservoir thickness, have direct relation with the dissociation rate, but the reservoir’s permeability and gas viscosity have almost no impact on the process. The wellbore parameters, such as flow rate of hot water, inlet temperature, and wellbore radius except the inner tube radius, have direct impact on the wellbore mean temperature and the associated results in the dissociation process.

Mapping of Domestic Hot Water Circulation Losses in Buildings – Preliminary Results from 134 Measurements

The hot water circulation system in a building is a system which helps prevent Legionella problems whilst ensuring that tenants have access to hot water quickly. Poorly designed or implemented systems not only increase the risk to people’s health and thermal comfort, but even result in an increase in the energy needed for this system to function properly. Results from previous studies showed that the total hot water circulation system loss can be as high as 25 kWh/m2 heated floor area per year. The purpose of this project is to measure the total energy use per year of the hot water circulation system in about 200 multifamily dwellings of different ages to verify that a system loss of 4 kWh/m2, year is a realistic assumption for both newer and older/retrofitted buildings. The preliminary results from the first 134 measurements showed that the assumption of 4 kWh/m2, year is rarely fulfilled. An average energy use of more than three times this is more common, even in newer buildings. Whilst some of the total energy lost is used to heat the buildings, it is not desirable because it is an uncontrolled energy flow.

Proposed Method for Probabilistic Energy Simulations for Multi-Family Dwellings

As regulations regarding energy use and emissions of CO2 equivalents in buildings become more stringent, the need for more accurate tools and improved methods for predicting these parameters in building performance simulations increases. In the first part of this project, a probabilistic method was developed and applied to the transient energy calculations and evaluated using a single-family dwelling case study. The method was used to successfully predict the variation of the energy use in 26 houses built in the same residential area and with identical building characteristics and services. This project continues the development and testing of the probabilistic method for energy calculations by applying it to a multi-family building. The complexity of the building model increases as the multi-family model consists of 52 zones, compared to the single-zone model used for the single-family dwelling. The multi-family model also includes additional parameters that are evaluated, such as the domestic hot water circulation losses. This paper presents the probabilistic method applied to the building performance simulations used to predict the energy use for the multi-family building and discusses the differences between the previous and new method used in this study.

Simultaneous fat-referenced proton resonance frequency shift thermometry and MR elastography for the monitoring of thermal ablations.

Simultaneous fat-referenced proton resonance frequency shift (FRPRFS) thermometry combined with MR elastography (MRE) is proposed, to continuously monitor thermal ablations for all types of soft tissues, including fat-containing tissues. Fat-referenced proton resonance frequency shift thermometry makes it possible to measure temperature even in the water fraction of fat-containing tissues while enabling local field-drift correction. Magnetic resonance elastography allows measuring the mechanical properties of tissues that are related to tissue structural damage. A gradient-echo MR sequence framework was proposed that combines the need for multiple TE acquisitions for the water-fat separation of FRPRFS, and the need for multiple MRE phase offsets for elastogram reconstructions. Feasibility was first assessed in a fat-containing gelatin phantom undergoing moderate heating by a hot water circulation system. Subsequently, high intensity focused ultrasound heating was conducted in porcine muscle tissue ex vivo (N = 4; 2 samples, 2 locations/sample). Both FRPRFS temperature maps and elastograms were updated every 4.1 seconds. In the gelatin phantom, FRPRFS was in good agreement with optical fiber thermometry (average difference 1.2 ± 1°C). In ex vivo high-intensity focused ultrasound experiments on muscle tissue, the shear modulus was found to decrease significantly by 34.3% ± 7.7% (experiment 1, sample 1), 17.9% ± 10.0% (experiment 2, sample 1), 55.1% ± 8.7% (experiment 3, sample 2), and 34.7% ± 8.4% (experiment 4, sample 2) as a result of temperature increase (ΔT = 22.5°C ± 4.2°C, 14.0°C ± 2.8°C, 14.7°C ± 3.7°C, and 14.5°C ± 3.0°C, respectively). This study demonstrated the feasibility of monitoring thermal ablations with FRPRFS thermometry together with MRE, even in fat-containing tissues. The acquisition time is similar to non-FRPRFS thermometry combined with MRE.

Reduction of Heat Losses Using Quadruple Heating Pre-Insulated Networks: A Case Study

The paper presents an analysis of heat loss and reductions of annual emissions of air pollutants of a quadruple pre-insulated heating network by comparing this solution with the existing pre-insulated network consisting of four pre-insulated single pipes and the variant consisting of two twin pipe pre-insulated. For calculations, an existing heating network located in central Poland was adopted, where heat is transported for heating purposes of buildings and domestic hot water with circulation of domestic hot water through four separate pre-insulated underground pipes. The idea of the construction of four pre-insulated pipes presented in the paper consists in the location of four steel pipes in a common round thermal insulation, which perform the role of heat transport for heating purposes in multi-family buildings (supply and return) and two pipes transporting hot water (a pipe with domestic hot water with circulation). In Poland, heating pipes used in multi-family housing have a larger diameter compared to domestic hot water pipes, which is why standard twin pipe heating pipes have been used in the construction of four pre-insulated networks, in which the domestic hot water pipe has been added to the thermal insulation and circulation of domestic hot water. In order to determine heat losses, a simplified two-dimensional model of conductive heat transfer was developed using Fortran to create a computer program. The results of numerical simulations show that the use of twin pipes for the construction of pre-insulated quadruple networks has contributed to a significant reduction in heat loss in relation to the existing single pre-insulated network (up to 57.1%), while reducing the thermal insulation field of the cross-section of the pre-insulated pipe by 21.4%.

Exergetic review on passive and active systems for ventilation

The conceptualization of warm/cool and wet/dry exergies for a better understanding of various heating/cooling systems has been found very useful for describing the whole thermodynamic process of various built-environmental systems, from building envelopes to mechanical heating and cooling systems. In parallel to these sub-concepts of exergy, the conceptualization of “high-pressure/low-pressure” exergies, which is associated with mechanical non-equilibria, is also necessary for full exergetic description of flow and circulation within the built-environmental systems, from natural and mechanical ventilation to the circulation of hot or cold water between heat exchangers. This paper describes the essence of “high-pressure/low-pressure” exergies and also demonstrates a couple of example calculations, which clarify how natural ventilation systems function from the exergetic viewpoint. The conclusion reached by the present analyses is that the exergy input and consumption for natural ventilation is much smaller than those for mechanical ventilation: e.g., for the number of air change at the rate of 2 h-1 in a room whose floor area is 36 m2, the exergy inputs for wind-driven and buoyancy-driven ventilation are approximately 0.1% of the electricity input for the same air change rate with mechanical ventilation.

Energy consumption for domestic hot water use in Norwegian hotels and nursing homes

Domestic Hot Water (DHW) production constitutes a significant proportion of the energy demand of modern buildings, and as the building envelope is improved the share increases. This article discusses the results from a measurement campaign in Norwegian hotels and nursing homes. The energy demand for DHW and distribution heat losses for 3 hotels and 3 nursing homes are shown. The results show that number of bedrooms is a better parameter for describing DHW consumption than sqm of heated floor area. There are large variations in the measured distribution losses, mainly due to malfunctioning of the hot water circulation system. For nursing homes, the measured energy consumption is significantly lower than the normative profiles, which can have large impact on the requirements for the design of the building heating system. For hotels, the measured energy consumption is in the range of the normative profiles.

Development of a realistic human respiratory tract cast representing physiological thermal conditions

Toxicological assessment of inhaled aerosols and their effects on respiratory tissue cells requires accurate measurements of particle delivery, properties, evolution, and deposition in the human respiratory tract. These measurements are affected by both anatomical features and physiological factors, such as thermal conditions in the respiratory system. We constructed a segmented cast model, based on a realistic geometry of the human respiratory tract, equipped with features to control the temperature of the air flow. We then evaluated the thermal equilibrium of the air flow through the cast using both experimental measurements and computational fluid dynamics modeling. Uniform temperature distribution in the cast shell was achieved using parallel connection of the cast segments to a thermal bath by flow splitter. Air temperature inside the cast was shown to require <1 min to equilibrate with the temperature of the cast during internal circulation of hot water. Air flow temperature was shown to equilibrate with the cast temperature in the mouth–throat region, and a uniform temperature distribution was achieved in the other segments, resembling the thermal conditions of respiratory flow in the human lung. We showed that the cast successfully represents the physiological thermal conditions of the human respiratory system.

Electricity generation from a three-horizontal-well enhanced geothermal system in the Qiabuqia geothermal field, China: Slickwater fracturing treatments for different reservoir scenarios

Hot dry rock (HDR), as a clean renewable energy source, can help fulfill future energy needs and reduce the greenhouse effect. This energy source in the Qiabuqia geothermal field of Northwest China is expected to generate electricity. In this study, we used the lab parameters and the GR1 well data from the Qiabuqia site to numerically investigate the reservoir stimulation and electricity generation from a three-horizontal-well EGS system created in the granitic reservoir at depth 3500–3700 m, and at an average temperature of 218 °C. Slickwater fracturing and the resulting reservoir scenarios for different expected conditions in the region were analyzed in order to find suitable fracturing strategies. We then proposed three optimized EGS scenarios and evaluated their economic and environmental benefits. Results indicate that EGS power generation significantly depends on the reservoir stimulation results. The potential 2.61–4.90 MW installed capacity and 433.72–913.05 GWh total power generation are expected to be realized if the well spacing of 300–500 m and the hot water circulation rate of 20–40 kg/s in a 20-year period are achieved via the slickwater fracturing treatment. The final production temperature can be maintained at approximately 200 °C, and the corresponding temperature drops less than 10%. Such a case of geothermal engineering is expected to result in a reduction in greenhouse gas (GHG) emissions of 0.15–1.1 Mt, and its levelized cost of energy (LCOE) is estimated at 0.047–0.066 $/kWh.

Comprehensive analysis of the performance and intrinsic energy losses of centralized Domestic Hot Water (DHW) systems in commercial (educational) buildings

The energy demand for supplying Domestic Hot Water (DHW) has an important share in the overall thermal energy consumption of commercial buildings. The aim of this paper is to analyze the performance of DHW production in commercial public service, i.e., educational buildings, to subsequently identify frequent inherent shortcomings of centralised DHW installations and to provide measures for optimization or economically more viable solutions. Several buildings of the University of Natural Resources and Life Sciences in Vienna have been investigated in terms of DHW consumption and performance of the installed centralised systems.Referring to the investigated university buildings, it can be stated, that in cases of low to medium consumption figures, typical centralized DHW installations with a hot water circulation pipework achieve only poor efficiencies in the range of 2–12%. As for one particular centralized DHW system in operation, focusing on demand controlled DHW supply through adapted flow control, e.g., reducing the runtime of the circulating pumps at specific time intervals, and, if applicable and feasible, decreasing hot water flow- and storage capacities can reduce annual energy consumption for DHW by 15–25% and improve the overall system efficiency significantly as could be evaluated in a detailed case study.A seemingly economical yet ecologically controversial option for improving centralised DHW systems is the partial or complete conversion to electric point-of-use water heaters. This reduces thermal energy losses almost completely, hereby significantly increasing the efficiency of a DHW supply system. Nevertheless, existing hot water supply systems, energized by district heating, usually consume less primary energy and most likely generate fewer CO2 emissions compared to electric DHW heaters.

On the possibilities to increase energy efficiency of domestic hot water preparation systems in existing buildings – Long term field research

Energy consumption for domestic hot water (DHW) preparation is one of the main components of the energy balance of the existing and newly designed buildings. However, based on the number of existing buildings and the related opportunities to improve energy efficiency, an important issue is to propose effective and low cost methods that allow to reduce the heat consumption in existing domestic hot water systems and are fast in the implementation.This article presents the results of an experimental research conducted in 12 objects (9 multifamily buildings and 3 thermal substations) assigned to three groups (A, B and C), depending on the analyzed feature of the domestic hot water system. The objects in the group A were analyzed in detail (each 1 h during one full heating season) in order to clearly show the amount of the heat used for circulation of hot water. The group B of objects characterized by the installation of temperature control valves (TCVs) under the risers of circulation installation. The third group consisted of the objects in which the temperature of hot water was decreased during the night hours. The objects from group B and C were analyzed over 8 years.The share of heat losses associated with the circulation of hot water in the total heat consumption supplied for its preparation was in the range from 56.7% to 70.5% for the Group A objects.On the other hand, the energy savings were calculated using different methodologies and for the recommended one they ranged from 8.5% to 49.5% for group B objects and from 6.0% to 14.4% for group C objects.

Determination of Optimum Hot-Water Temperatures for PCM Radiant Floor-Heating Systems Based on the Wet Construction Method

Owing to use of mortar, which demonstrates low heat storage and discharge performance, conventional radiant floor-heating systems, based on the wet construction method and hot-water circulation, consume large amounts of energy. This study proposes a new type of radiant floor-heating system that is capable of reducing energy consumption via use of the latent heat of a phase change material (PCM), whereby the phase change, which occurs within, is induced by the thermal energy supplied by hot water. Simulation analyses revealed that hot-water supply temperatures, required to maintain the floor-surface and indoor-air temperatures at the set point using PCM latent heat, were in the range 40–41 °C. At supply water temperatures measuring less than 39 °C or exceeding 42 °C, the latent-heat effect of the phase change of the PCM tended to fail, and the corresponding floor-surface temperature assumed a value different from that corresponding to the set point. By contrast, supply temperatures in the range 40–41 °C resulted in return temperatures measuring approximately 27.4–27.5 °C, which in turn corresponded to an indoor air temperature of 21.6–22.6 °C that was stably maintained within ±0.6 °C of the 22 °C set-point temperature.

Heat production optimization using bio-inspired algorithms

Energy efficiency of industrial systems is one of key features for optimal use of resources and the lowest costs of energy for users. In the recent time optimization of heating plants and heat distribution systems becomes an important venue for novel methods and innovative constructions. Various proposals can be seen for more efficient performance of heating systems in changing weather conditions.In this article results of using bio-inspired methods for intensification of the district heating plant to work with maximum efficiency at the lowest costs are presented. The research is focused on developing bio-inspired approaches for a mathematical model of a district heating plant in various weather conditions. The research model represents a sample district heating plant, in which circulation of hot water is performed in two heat exchangers supplied by controlled pumps. The system was calibrated with the use of proposed Polar Bear Optimization and the results were compared to one of best known heuristics, Particle Swarm Optimization. An objective function describing the operation of the plant was developed and found applicable for proposed bio-inspired approach. The research results have shown that proposed methodology is efficient for all simulated weather conditions and various boundary conditions. Comparison the obtained results with non-optimal parameters confirms huge profits from applying right settings of the system.