1,720,993 research outputs found
Experimental characterization of variable-speed packaged rooftop units in presence of evaporator fouling
No full textVariable-speed packaged rooftop units have been increasingly adopted in the commercial sector. If not properly maintained, the fouling of heat exchangers and the improper refrigerant charge could increase energy consumption. In this work, the response of variable-speed rooftop units to evaporator fouling is investigated based on experiments on a variable-speed 17.5 kW rooftop unit. A novel testing procedure is proposed, aimed at covering a broad range of operating conditions by a reduced number of tests. Two opposite cases were investigated: (i) even in presence of fouling, the controller can restore the required cooling capacity, and (ii) the intensity of the fouling does not allow the controller to restore the cooling capacity. While in the former case an increase in the indoor fan rotating speed is observed, with no variations in the evaporating pressure and the inlet temperature of cooled air, in the latter one the mismatch between the cooling load and the capacity induces a deviation of the indoor air condition from the setpoint. A regression model is proposed to quantify these deviations. Experimental results showed that in a faulty scenario, the maximum cooling capacity deliverable could decrease up to 15-20 %, and the indoor fan consumption increased up to 40%. In case of heavy faults involving cooling capacity shortage, a 2.3 °C deviation from the indoor setpoint temperature was estimated by the proposed regression model
On the reliability of thermoeconomic diagnosis of fouled evaporators: assessing the influence of geometries, operating conditions and reference state
No full textThermoeconomic diagnosis of refrigeration systems is a pioneeristic approach, which has been proven to achieve good performances (under favorable conditions) for the detection of specific faults such as condenser and evaporator fouling and compressor valve leakage. In this paper the sensitivity of performance in detecting fouled Direct Expansion (DX) coils is investigated; the examined parameters are the temperature and relative humidity of coil inlet air and the sensible/latent heat ratio of the coil (varied by considering different numbers of rows). As the thermoeconomic model requires splitting the specific exergy of cooling air into “mechanical”, “thermal” and “chemical” fractions, and since under fouled conditions the evaporator strongly modifies its exergetic performance, it is worthwhile discussing the possible existence of “optimal performance” regions for the use of this diagnostic technique
Coupling salinity gradient heat engines with power generation systems and industrial processes
No full textThe rising concern about global warming and the increasing trend in energy consumption have been pushing the need for efficient use of energy and intensive adoption of renewable energy sources. In general terms, energy efficiency means using less energy to perform the same task. Action aimed at improving the energy conversion efficiency of processes are the cheapest and the most immediate ways to reduce the consumption of fossil fuels and contain greenhouse gas emissions
An agent-based model to support the preliminary design and operation of heating and power grids with cogeneration units and photovoltaic panels in densely populated areas
No full textThe increase in the energy demands of residential neighborhoods constitutes a huge challenge to be faced. A viable option lies in the insertion of both renewable energy production systems and cogeneration units within urban territories. On-site production allows for the satisfaction of the heating and electrical demands of end-users
and favors the distribution of produced energy within the neighborhood, especially under the regulatory framework of energy communities. This paper proposes a tool to model heating and electricity networks integrated with cogeneration units and PV panels installed in urban areas. The tool accounts for the following possibilities: (i) energy flow management and power sizes of both heating and electric networks, and (ii) evaluation of the electricity sharing configurations arising within energy communities. To better clarify the capability of the tool in supporting energy analysts and decision-makers, a densely populated area in Southern Italy is proposed as a case study. Regarding thermal distribution, results demonstrated that it is better to plan pipelines extended up to 300 m to take full advantage of the insertion of the CHP in the areas. Different considerations apply to the electrical distribution; here, an energy community extended over a geographical area with a radius of around 100 m is sufficient to guarantee an almost fully connected grid and obtain in any case exceeding production to be released to the centralized network
Thermoeconomic Analysis Under Dynamic Operating Conditions for Space Heating and Cooling Systems in Small Residential Buildings
No full textDue to the social concern for energy savings and ecological improvement necessity in the performance of buildings, new analysis tools emerged.
Thermoeconomics connects the physics and economics of energy conversion processes through the second law of thermodynamics. A limited number of thermoeconomic applications in buildings have been proposed, due to the difficulties in dealing with very irregular energy load profiles and unsteady plant operating conditions, which require the use of dynamic approaches that increase the complexity of the method. The present study highlights the potential of thermoeconomics as a support for decision making, due to the capability to identify trade-offs between cost and efficiency. Dynamic thermoeconomic analysis is performed for a school dwelling, supplied with space heating and cooling integrated with a mechanical ventilation Air Handling Unit.
The different sources of irreversibility are evaluated and the main targets for system improvement are identified, calculating exergetic, exergoeconomic and exergoenvironmental costs
Improving energy efficiency of commercial buildings by Combined Heat Cooling and Power plants
Full text linkCommercial buildings play a key-role in the energy consumption of the building sectors. Recent statistics have shown that as the number of commercial buildings is continuously increasing, their effects on energy consumption are expected to grow. These buildings are characterized by high energy demand mainly due to lighting and HVAC requirements. Rooms of energy saving exist by considering that: (i) electricity demands and HVAC requirements occur simultaneously during the day and (ii) both demands are currently satisfied by using separate energy systems. It is apparent that the adoption of polygeneration systems could represent a valid solution to achieve energy savings. To this aim, the paper investigated the profitability of a trigeneration system for commercial buildings, considering a big Do It Yourself shop located in the northern part of Italy, as case study. The analysis was based on (i) energy consumption data collected by energy-audits and (ii) a profit-oriented management strategy for the trigeneration systems proposed in literature. Results showed that trigeneration represents a profitable energy conversion system thanks to revenues achieved by selling surplus electricity and the support of financial mechanism for “High-Efficient” eligibility. In comparison with the currently adopted energy conversion systems, important reductions in energy consumption and CO2 emissions are observed
Exergoeconomic analysis as support in decision-making for the design and operation of multiple chiller systems in air conditioning applications
No full textMultiple-chillers systems represent viable solutions for medium/large-scale air conditioning applications characterized by variable cooling demand. The energy efficiency of such systems is influenced by the number of chillers, the combination of cooling capacities, and the load-sharing among the units. Large efforts have been devoted to developing efficient operation strategies for these systems, but rules of thumb are usually adopted for selecting cooling capacities thus leaving room for energy and economic savings. In this paper, exergoeconomic analysis is proposed as a promising method to identify near-optimal design and operation strategies, due to the capability of exergoeconomic indicators to account simultaneously for capital and operating costs. The potential of the method is illustrated for a hydronic system supplying an air-handling unit installed in an office building. Design alternatives are compared, with chillers of equal or different capacities operated in a parallel or series configuration, and the cost-effectiveness of different load sharing strategies is also investigated. A thermoeconomic model for multiple-chillers systems is formulated, considering the actual performance of chillers under full- and part-load conditions derived by a plant simulator. Results show that the exergoeconomic cost of chilled water reduced by about 7% and 30% when passing from evenly to unevenly sized systems in both series and parallel configurations. It is also found that the symmetric load sharing strategy leads to a 14–18% reduction in the cost of chiller water compared to the sequential one. The study confirms that this method may represent systematic and thermodynamically-sound support for engineers in this field
Electrical hybrid heat pumps assisted by natural gas boilers: a review
No full textIn this paper, a comprehensive review is performed on hybrid systems using electrical Heat Pumps for Heating, Ventilation & Air Conditioning systems and Domestic Hot Water production using case studies from 2016 to 2022. This work particularly focuses on studies that investigated systems with electrical Heat Pumps assisted by a gas boiler. Some studies that considered the exploitation of renewable energy sources, but not as a whole sub-stitute for fossil sources, have also been included. In particular, the analysis investigated the layouts of the systems by considering the type and the size of the components, the control methods applied (by distinguishing rule-based control methods and predictive control methods), and the main performance indices used to calculate energy, exergy, environmental and economic performance. Most of the analysed studies demonstrated that the hybrid systems could be very advantageous if the switch or the integration of the two generators is well managed. Nevertheless, the energy and economic performances are very different for each system studied. They are highly influenced by the control strategies implemented and by the objective functions adopted. Furthermore, climate can influence performance while affecting the Coefficient Of Performance of the heat pump and the availability of energy sources. Regarding economic issues, the systems' performances vary according to specific Country tariff structures. The review reported a promising research activity also pursued by manufacturers of components and equipment. Nevertheless, the paper highlights new possible research and development directions
Dynamic simulation of a 4th generation district heating network with the presence of prosumers
Full text linkDistrict Heating Network is identified as a promising technology for decarbonizing urban areas. Thanks to the surplus of heat available from distributed renewable energy plants, a typical heat consumer of the network could become an energy producer during the day (typically referred to as a “prosumer”). Most of the models for thermal grids developed during past years usually assumed a centralized production of the consumed heat. The increasing presence of prosumers will require accurate dynamic modelling to monitor the changes induced in the thermohydraulic parameters of the network. To fill this knowledge gap, this paper aims at developing a model of a thermal grid with prosumers in the TRNSYS environment. The model allows for the dynamic monitoring of the main thermohydraulic parameters of the network. To show these capabilities, a ring-shaped network serving a cluster of 10 residential users located in Palermo (Italy) was assumed as the case study. Different scenarios are investigated based on the presence of solar collectors, prosumers along the network, and cooling by an absorption chiller. The achievable energy and emissions savings are calculated. The results of the study show that even only decreasing the operating temperature can significantly reduce heat losses via the network pipes. In particular, a temperature drop from 100 °C to 80 °C can reduce heat losses by 27.1%. Furthermore, the heat losses can be decreased by up to 52.8% when the network temperature is lowered from 100 °C to 60 °C. Additionally, the presence of prosumers and the solar field could lead to a 31.3% reduction in the energy produced by the centralized plant and a 17.6% reduction in energy consumed for pumping
Promoting the Flexibility of Thermal Prosumers Equipped with Heat Pumps to Support Power Grid Management
Full text linkThe increasing share of renewable energy sources in energy systems will lead to unpredictable moments of surplus/deficit in energy production. To address this issue, users with heat pumps can provide support to power grid operators through flexible unit operation achieved via Demand Response programs. For buildings connected to low-temperature heating networks with ensured third-party access, further room for flexibility can be explored by investigating the production of surplus heat that can be sold to the network. A key aspect lies in the identification of the energy pricing options that could encourage such flexible operation of a heat pump by “thermal prosumers”. To this aim, the present study investigates the impact of ad hoc variations in the electricity purchasing price through discounts or penalties included in the “network cost” component of the price on cost-effective operation of a heat pump connected to the thermal network. To discuss the effects of different pricing options in terms of increased flexibility, an office building located in Italy and equipped with a high-temperature heat pump is adopted as the case study. A heuristic profit-oriented management strategy of the heat pump is assumed, and dynamic simulations are performed. The results indicate that at current electricity prices, the heat pump operation is profitable both when supplying the heat to meet the building’s requirements and when producing surplus heat for sale to the thermal network. In addition, it is revealed that the penalties applied to the electricity purchasing price are effective in encouraging changes in the heat pump operation strategy, reducing its average production (the building increasingly relying on buying heat from the network) and the associated electricity consumption by 46.0% and 79.7% in the “light” and “severe” local power deficit scenarios, respectively
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