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Condensing boilers in buildings and plants refurbishment
Abstract
Condensing boilers allow better performances when return temperatures from the plant are lower and above all when these temperatures are below the dew temperature of the smoke. That is the reason why many believe that condensing boiler is not suitable for the traditional heating plant terminals: the radiators.Instead the classic control system with weather compensation curve produces relatively low temperatures and an appreciable condensing operation during March, April, October and November. Moreover, condensing boilers allow a better performance than traditional boiler even when they do not perform condensing operation.However for the optimum exploitation of condensing boilers in a radiator plant, variable-flow-rate plants should be selected that permit every outside condition to maintain a high difference between delivery and return temperature. This control selection, which is fully consistent with the use of thermostatic valves, needs variable-flow-rate pumps and, if possible, with proportional head.
... Absorption heat pumps can operate in open or closed cycle. Heat pumps with mechanical vapour compression and the closed cycle absorption system can use for flue gas cooling a direct-contact heat exchanger to ensure a higher heat transfer coefficient between the coolant and flue gas [11]. In addition, the flue gas quenchers (direct-contact exchangers) have the following advantages: the corrosion process is less intense than classic condensers because the walls are always wet, which allows the use of cheaper materials (low alloy stainless steel); through the quencher circulates a certain water flow which does not change quickly when fluctuations in flue gas humidity or boiler load occur, which leads to a steady state operation of heat pump and allows the use of water-to-water heat pumps that are commercially available [9]. ...
... In a system which combines absorption heat pump with direct-contact heat exchanger, the cooled water ( 20°C) in evaporator is pumped into the heat exchanger with direct contact to cool the flue gas below the dew temperature taking the physical and latent heat and then transfers it to evaporator. About 70% of the water vapour is condensed, which means that almost all the condensation latent heat is recovered [11]. Boiler return water is heated in the heat pump receiving the heat generated by fuel combustion to drive the heat pump and also the heat recovered from the exhaust gas from boiler and heat pump. ...
... Boiler return water is heated in the heat pump receiving the heat generated by fuel combustion to drive the heat pump and also the heat recovered from the exhaust gas from boiler and heat pump. Since the cooling water becomes acidic after an operation period, it requires treatment by using alkaline material, and in addition, the heat exchanger should be made of stainless steel [11]. ...
... Although the author concluded a marginal economy for the complete boiler, an attractive pay-back period results for the over-cost, in spite of the low value assumed for the price of NG (0.398$/m 3 ). In a more recent work, Lazzarin [5] presented history and detailed data of a relevant case in the city of Treviso (2378K-days), and calculated for the complete condensing boiler retrofit a payback period of 8 years. Bonaros [6] found for the cities of Athens and Thessaloniki, Greece, pay-back periods of 4.5 and 3.5 years, respectively, when the over-cost against a high-efficiency conventional boiler is considered. ...
... Two similar combi boilers of 24kW capacity were selected for comparison, a condensing and a high efficiency conventional unit, because most units marketed in Greece are of this size. Besides, instantaneous DHW service requires at least 24kW nominal capacity [5]. A boiler with a similar size (26kW) was also examined in [3] but also in [6] (20kW). ...
... In spite of the high instantaneous efficiency of hot water production, small flow rates lower the efficiency of combi boilers (there is no storage) due to cycling. Hence, in the place of the nominal efficiencies of 100% for the condensing and 93% for the conventional unit, seasonal efficiencies of 88% [5] and 80% [4] are respectively assumed. ...
Condensing boilers have been in use for space and domestic water heating in many countries for some years now, and in the present work their potential for using them in Greece is evaluated from both the technical and economic viewpoints. There is a significant market penetration of these devices in Greece, and the economics of their operation vary according to the climate zone in which they are located. The technology proved to be cost effective for the areas of Greece where natural gas is available, with payback periods ranging between 3 and 5 years. Critical factors affecting the economy of condensing boilers proved to be the total heat loss coefficient of the dwelling, the heating media applied, the cost of equipment, the energy prices (for gas and electricity) and last but not least the hours of operation of the heating system. These may vary if the use of the boiler is combined with other systems such as solar water heating or a heat pump, and the latter system may offer a flexible alternative which can produce optimum running costs depending on the demand and the price of gas and electricity applying at the time.
... Outside air has a well known unfavourable characteristic: in the coldest period of the year, when the building heating load is at maximum, the heat pump capacity is reduced [1]. Moreover, being the peak load required for a small fraction of the heating period [2], if the heat pump unit is sized to cover the peak load when the air temperature is at the minimum value, the annual average on-off frequency of the unit might be really high, lowering the seasonal coefficient of performance (SCOP) due to cycling losses [3]. ...
... The thermal efficiency of the condensing boiler was considered to be a function of the water temperature at the boiler inlet. In particular, the thermal efficiency profile suggested by Lazzarin [2] was used. ...
Air source heat pumps have a well known unfavourable characteristic: in the coldest period of the year, when the building heating load is at maximum, the heat pump capacity is reduced. A possible solution is to size the heat pump to cover only a fraction of the peak load using a second heat generator in "hybrid heat pump system". In this study, a dynamic model of a hybrid system was built, and several seasonal simulations were carried out (considering two different climates) to study how the choice of the cut off temperature can influence the annual efficiency of the system and to understand if a bivalent parallel plant can lead to energy savings compared to a bivalent alternative system. With both the considered climates, when a large size heat pump is selected (low bivalent temperature), there might be no benefits by setting a cut off temperature lower than the bivalent temperature, and the differences between the energy performances of the bivalent alternative system and those of the bivalent parallel system are negligible. On the other hand, when a low size heat pump is preferred (high bivalent temperature), the bivalent parallel system can lead to appreciable energy savings compared to a bivalent alternative plant. The main economic advantages of a hybrid system, with respect to the monovalent heat-pump plant, comes from the lower annual electric and gas energy needs.
... In particular, the condensing boiler technology appears attractive in virtue of its potential to go beyond 100% efficiencies, measured on the lower heating value, exploiting condensation heat from the fuel combustion. Condensing boilers reach optimal performances with low return temperatures, below the dew point; however, seasonal efficiencies higher than the ones of traditional boilers can be obtained also with high temperature heating equipment such as radiators [15]. In the near future, due to externalities such as global warming, acid rain and urban smog [16], renewable energy sources (e.g. ...
... This is based on the heat supply as well as on lower internal costs (due to energy consumption for space heating and DHW) and external costs (related to airborne pollutants and greenhouse gas emissions), and the technology is defined "almost sustainable" according to a multi criteria decision analysis -mainly due to fuel costs, energy carrier reserves. Over the last years, a series of advancements on the fuel combustion and on the condensation technique of these boilers were made [15]: condensing boilers with modulation ratios between 25% and 100% of their outputs and small capacity boilers (up to 4 kW) can be installed in buildings with very low energy demand for space heating (providing DHW production by means of a storage tank). A description of the variations in expected performance of this type of boilers, in relation to different loads and uses can be found on [19]. ...
The integration of different energy sources to cover with the maximum efficiency the building energy demand is one of the principles for achieving the target of the nearly net Zero Energy Buildings (nNZEBs), as defined by the EPBD recast (2010/31/EU). Following this principle, there has been a shift from a conventional mono-carrier/mono-converter based logic (one energy source is used in one energy converter to meet each load) to a multi-carrier/multi-converter logic (a mix of energy sources feeds two or more energy converters to cover the energy loads).
This paper reviews the technology, performance and the parameters of the latest multi-energy systems for residential ZEB on the market. The scope of the work is to provide the necessary information in order to design and operate integrated HVAC and domestic hot water (DHW) production systems. After characterising the building energy demand (forms of energy, thermal levels, peak loads and so on), the review focuses on integrated energy systems providing: heating energy for space heating and DHW production; heating and cooling energy for air conditioning and DHW production; heating energy for space heating, DHW production and electricity; heating and cooling energy for air conditioning, DHW production and mechanical ventilation. A schematic of each technology and a table contrasting the strengths, advantages, weaknesses and drawbacks of the various technologies are also provided.
... NGB. For the NGB, we consider a constant efficiency of 95 % [59]. ...
Air-source heat pumps (ASHP) have a significant potential for decarbonizing the heating sector. In this article, we compare the environmental impacts (climate change, particulate matter formation, human toxicity, and ozone depletion) of an ASHP and a natural gas boiler (NGB). The main originality is that we perform the life-cycle analysis (LCA) of the ASHP and the NGB for 18 European countries while sizing the ASHP according to the dwelling thermal demand. We highlight that using refrigerant R290 instead of R32 decreases the ASHP impact on climate change and ozone depletion. Moreover, the building stock is found to greatly influence the potential benefits of ASHP in several countries (e.g. the Czech Republic, Greece). In recent dwellings, ASHP reduces climate change in 17 out of 18 countries, with a 54 % average reduction. However, it often increases particulate matter formation mainly due to the electricity mix, and the use of copper for ASHP manufacturing. Our results can be helpful to European policy makers since they assess in which country ASHP should be installed to yield the highest reduction of environmental impacts. Countrywide, our results can help to deploy ASHP as they indicate which dwelling type should be given priority for ASHP installation.
... In conventional cast-iron and steel boilers the flue gases are maintained at high temperatures in gaseous state to avoid condensation inside the boiler (around 140 ) [6]. As the exhaust gases contain carbon dioxide and water vapor, when condensation forms in a conventional boiler, these two elements combine to form carbonic acid. ...
... Natural gas condensing boiler with a heat duty of 2.8 MW is an essential type of special equipment widely used in industrial production and heating field [20,21]. Compared with conventional gas-fired boilers, condensing boilers have higher thermal efficiency [22] and lower pollutant emissions [23]. While the HENG is fueled in boilers, the volume proportion of water vapor is improved, and then the water vapor dew point (WVDP) is enhanced accordingly [24,25], which is an efficient approach to boost condensing boiler efficiency [26] because of recovering additional latent heat from flue gas. ...
... In recent years, a number of standards and regulations that aim to promote sustainable In addition, energy-savings could also be achieved by substituting conventional energy equipment systems with energy efficient equipment. Such as the use of condensation boilers for heating [99], solar thermal panels for the production of the domestic hot water [100,101], dual flow ventilation systems with high-efficiency heat recovery system [102] or using the Canadian wells. ...
The building sector is one of the largest energy end-use sectors in the world and reducing energy consumption has been the foundation of numerous research works. However, the primary objective of buildings must be to provide a comfortable environment for its occupants. Thus, it is necessary to design energy-efficient buildings so that a trade-off between energy-savings and occupants’ thermal comfort is fulfilled. Recently, glass façades have gained popularity due to their aesthetic appearance. However, they often cause occupants thermal discomfort, in addition to consuming considerable amounts of energy. In light of these conflict characteristics, the main purpose of this thesis is to understand the relationship between design parameters, thermal comfort and heating energy, in order to integrate thermal comfort in the design of energy efficient buildings. Consequently, we adopted a methodology based on the combined use of numerical simulations, Design of Experiments (DoE) technique and an optimization method. This allowed the development of meta-models of thermal comfort and heating energy. These models are then used to integrate thermal comfort in the process of building design. A desirability function was considered in order to simultaneously optimize both thermal comfort and heating energy. This trade-off helps in developing an optimal design of buildings at both energy consumption and thermal comfort levels. The proposed method is applied in a real case study. The obtained results show the added value of integrating thermal comfort in building design.
... For the combustion of methane, the dew point temperature of the formed gases is usually at around 55°C. The condensation process was preferably avoided in traditional, atmospheric boilers for mainly three reasons: the appearance of corrosion provoked by carbonic acid on the metal surfaces of the boiler, cavity formation on the masonry chimneys and inability to provide sufficient natural draft of the exhaust gases under the flue temperature of 140°C [20]. The use of stainless steel and aluminium coils as well as integration of fans for the combustion air, made the condensing process technically possible for the modern condensing boiler units. ...
Despite being seen as an extinguishing technology, gas-fired heating boiler units are still one of the most common sources for meeting the space heating energy demand of residential dwellings. Current energy performance assessment calculations for residential buildings are often based on insufficient details with thermodynamically ambiguous assumptions and lacking of a consistent approach. However, more detailed methodologies usually require a large set of scarcely available input data which vastly limit the (re)usability of these more refined methods. Following the limited availability of design parameters provided by manufacturers, the goal of this research is to determine the necessary level of the modelling detail to reliably predict the performance of gas-fired condensing boilers. A thermodynamically consistent model of a gas-fired condensing boiler unit is developed and discussed through its application in numerical simulations. Calibration is done by using publicly available data and standardised measurement points are used for verification of the results. The values of the exhaust gas temperature and flow rate of the formed condensate are available in the standardised experimental reports of these units. This work suggests that these values should be included in public documents given by the manufactures.
... This process not only recovers the sensible heat of the flue gas but also recovers the latent heat of the moisture content of the flue gas. Condensing economizers are constructed with materials with higher resistance to acid in the acidic condensate [9]. Energy and exergy analyses have been applied to conventional hot water and steam boilers in several studies. ...
Condensing boilers are used in commercial and residential buildings extensively. In this paper, a loss method is proposed to estimate the energy and exergy efficiencies of condensing hot water boilers. The presented method is based on the development of the method presented in ASME PTC 4.1. Energy loss terms consist of exhaust flue gas, carbon monoxide formation, radiation, and condensate outflow sensible heat. Exergy loss terms also include radiation losses, physical exergy of the exhaust flue gas, chemical exergy of the exhaust flue gas, increase in the chemical exergy of the flue gas due to carbon monoxide formation, condensate outflow exergy, boiler exergy destruction, and economizer exergy destruction. Energy and exergy efficiencies are calculated by estimation of these loss terms. To depict the method's capability and compare results with the direct method, an experimental setup was designed and constructed. Results of energy and exergy audition of the boiler by applying the loss method are compared with the direct method. The results show that, although the condensing economizer improves energy efficiency, it does not improve the exergy efficiency significantly. The energy and exergy efficiencies were calculated 98.65 and 5.14 percent, respectively.
... We will focus on a condensing boiler which, whenever the return temperatures from the heating system is below the dew temperature of the flue gas, can recover the latent heat of water vapor in the flue gas so as to achieve higher efficiency than traditional boilers. Above the dew temperature, no latent heat is recovered and the boiler will operate in a non-condensing mode [53]. We assume that the boiler has no dynamics, which amounts to assuming being in steady-state operation. ...
In smart buildings, the models used for energy management and those used for maintenance scheduling differ in scope and structure: while the models for energy management describe continuous states (energy, temperature), the models used for maintenance scheduling describe only a few discrete states (healthy/faulty equipment, and fault typology). In addition, models for energy management typically assume the Heating, Ventilation, and Air Conditioning (HVAC) equipment to be healthy, whereas the models for maintenance scheduling are rarely human-centric, i.e. they do not take possible human factors (e.g. discomfort) into account. As a result, it is very difficult to integrate energy management and maintenance scheduling strategies in an efficient way. In this work, a holistic framework for energy-aware and comfort-driven maintenance is proposed: energy management and maintenance scheduling are integrated in the same optimization framework. Continuous and discrete states are embedded as hybrid dynamics of the system, while considering both continuous controls (for energy management) and discrete controls (for maintenance scheduling). To account for the need to estimate the equipment efficiency online, the solution to the problem is addressed via an adaptive dual control formulation. We show, via a zone-boiler-radiator simulator, that the best economic cost of the system is achieved by active learning strategies, in which control interacts with estimation (dual control design).
... They found 7.04% higher efficiency in their work comparing to the existing condensing boilers. Condensing boilers in buildings and plants refurbishment are the title of a research work studied by Lazzarin [12]. Chen et al. [13] investigated on the condensing boiler applications in the process industry. ...
In the present study, a comparative investigation has been performed on the impact of the hot gas passages on a sectional cast-iron boiler (CIB) performance. To this end, various shapes of the hot gas passages such as the circle (case-1), hexagon (case-2), square (case-3), vertical ellipse with constant aspect ratio (AR) of 2 (case-4), and horizontal ellipse with AR=2 (case-5), have been considered and compared with each other in order to find the best geometry among the cases under consideration. Computations are carried out at two water flow rates of 0.015 and 0.030 kg/s and a three-dimensional flow domain is designed based on the dimensions of a real CIB section. It is found that case-5 has the maximum heat transfer rate and goodness factor with the highest sensitivity to the water flow rate among all cases under consideration. It was hoped that the obtained results arouse interest among the boiler designers.
... Up to now, the most widely-used technologies for waste heat recovery from industrial boilers are based on the condensing heat exchanger, including the conventional condensing boiler [7][8][9][10][11][12][13] and the integrated condensing system [14,15]. The conventional condensing boiler recovers waste heat by equipping a condensing heat exchanger directly between the exhaust flue gas and the cooling medium. ...
The open absorption system can recover water and latent heat from industrial flue gases with a higher return water temperature than conventional condensing boilers. In spite of the high efficiency, there are still a lot of unsolved problems in real applications, including the steady operation control and the optimal parameter selection. In this paper, we established a whole-cycle simulation model of a flue-gas-driven open absorption system, which was validated by various experiments to be capable of predicting the operating states of every working fluid for all kinds of actual conditions. Based on this model, we analyzed the actual performance of the system with varying temperatures, flow rates and humidity in a large range. We found that the balance between the vapor absorption and vapor generation is crucial for the optimal operation of the system. The real-time flow rate of the circulating solution should be adjusted with the varying working conditions to achieve the optimal system performance. We further proposed an optimized system based on the theoretical analysis, which can enhance the overall performance by redistributing the circulating solution within the system. The optimized scheme was well demonstrated by experiments with different flue gas humidity ratios. Compared with the original system, the heat recovery and the water recovery of the optimized one were increased by 3.0–23.8% and 5.1–41.4%, respectively, when the split ratio (flow rate ratio of the regenerating solution and the spraying solution) was varied between 0.8 and 0.2. The whole-cycle simulation results and the proposed optimization scheme will evidently contribute to the efficiency improvement of industrial waste heat recovery.
... They found 7.04% higher efficiency in their work comparing to the existing condensing boilers. Condensing boilers in buildings and plants refurbishment are the title of a research work studied by Lazzarin [12]. Chen et al. [13] investigated on the condensing boiler applications in the process industry. ...
This study is a three-dimensional research on the heat and fluid flow within the domestic cast-iron sectional boilers (CIBs). Considerations are focused on the shape effects of the hot gas passages on the boiler performance. For this purpose, various hot gas passage shapes such as the circle (Case-1), hexagon (Case-2), square (Case-3), and ellipse (with a constant aspect ratio of 2, Case-4) have been examined and compared with each other. Computations are performed under two practical water flow rates of 0.015 and 0.030 kg/s. For enhanced visualization, several results have been demonstrated in the present study. It is found that the shape of the hot gas passage in a domestic CIB affects the heat and fluid flow within the section, strongly. On the other hand, it is revealed that among the cases under consideration, the square shape (Case-3) for the hot gas passage has the best thermal performance with minimum pressure drop in a sectional CIB. It was hoped the obtained results arouse interest among the boiler designers.
... In addition, nameplate information with respect to efficiency will have manufacturing tolerance associated. In our study, we capture both effects as uncertainty in the efficiency coefficient (assuming a normal distribution with mean 0.904 and standard deviation 0.011) and a performance curve that correlates efficiency with the return water temperature [30]. • Lighting consumption: The National Lighting Product Information Program (NLPIP) identifies that the nameplate power of many lighting fixtures generally deviates from field-tested values, so we introduce an uncertainty distribution of the percentage difference between monitored and nameplate power by fitting to multiple test samples reported by NLPIP [31] to reflect the variability. ...
Previous market analyses have projected a huge potential for reduction of building energy consumption through retrofits. Large-scale application of currently available technologies could save at least 20% of the total energy consumed by buildings in the next decade. It is expected that such savings will not be realized unless involved parties, and in particular the clients and investors, trust the predicted savings and can engage in a transparent contracting process. The authors argue that this requires a comprehensive and innovative investment valuation method. Conventional investment decisions in energy efficiency rely on deterministic analysis of net present value and internal rate of return, both of which suffer from two critical drawbacks. First, the assumption that decisions are made with certain, accurate information is unrealistic. The other drawback is that traditional investment valuation does not take the risk preferences of each decision maker into account, hence it does not recognize that a certain decision maker will weigh the risk of future loss more heavily than a potential gain, or vice versa. This paper aims to address these drawbacks by applying axiomatic utility theory as a normative basis for decision-making. We analyze optimal decisions involved in drawing up a performance contract that accounts for uncertainty and uses explicit risk preferences of the decision maker. How this can work in practice is demonstrated in a real-life case study. It is discussed how the proposed methodology manages uncertainty in energy saving predictions while incorporating a decision maker's risk preferences. Results show that the methodology results in a decision that will rationally maximize the decision maker's value. As energy performance becomes an important factor in awarding construction projects, the availability of risk-transparent information will increase the willingness of all parties to follow through with a contract and agree on the budget. Moreover, the downstream automation of the decision process that maximizes all decision makers' values will lead to a higher realization rate, thus benefitting the construction market as a whole.
... Undersizing an on-off heat pump with respect to the peak load lowers control requirements, however without eliminating them. Of course, undersizing heat pump capacity increases the number of operation hours of the auxiliary system (Lazzarin, 2012). As mentioned above, capacity control of a fixed speed heat pump gives rise to a sequence of on-off cycles that lower the efficiency of the unit. ...
In recent years the use of high efficient variable speed heat pumps has spread widely. Though several studies were carried out in the past comparing on-off and modulating strategies in heat pump units, only few works considered the whole heat pump system. Moreover, because of lack of knowledge about cycling losses, most of comparisons between modulating and on-off strategies did not consider the effects of the reduced coefficient of performance of a system at start-up. In this work simulations were carried out considering both on-off and modulating air-to-water heat pump systems in a single family house located in Italy. The aim of this study is to evaluate how the heat pump sizing, the thermal storage sizing and the cycling losses phenomenon can influence the annual energy performance of both the systems (on-off and modulating) and to investigate when modulating heat pump plants are actually more efficient than on-off. © 2016 Elsevier Ltd and International Institute of Refrigeration. All rights reserved.
This review presents a comprehensive examination of recent advancements and findings related to return-temperature reduction in District Heating (DH) systems, with a focus on enhancing overall system efficiency at end-user sites. The review categorizes and clarifies various return-temperature reduction techniques, emphasizing aspects such as building energy performance, heat emitters, thermostatic radiator valves, and substation units. One shall note that return temperature is not a parameter that can be directly controlled within a DH system; instead, it is influenced indirectly by adjusting various system parameters throughout the design, commissioning, operation, and control phases. Key insights include the direct impact of heat demand on return temperatures; the pivotal role of indoor heating systems in optimizing thermal energy use in relation to heat demand; the significance of thermostatic radiator valves in regulating heat output and maintaining low return temperatures; the advantages of ventilation radiators and add-on fans in enhancing radiator efficiency; the necessity for effective substation operation to improve system cooling capacity; and the critical role of operational control strategies in achieving optimal system performance. These findings underscore the need for integrated approaches in DH system design and operation to achieve lower return temperatures and improve overall system efficiency.
According to the official statistical reports, gas-fired boiler units still remain to be one of the main equipment types for meeting the space heating and daily hot water demand of the residential dwellings across the European Union. Due to the prevalence of the natural gas grid and performance stability, gas-fired boilers are considered to remain as one of the standard energy sources. On the other hand, even though gas-fired water heating technology is a well-known concept, existing numerical models found in the literature are often case-specific with poor reusability mostly reflected in fitted efficiencies. Algorithms behind these models usually require the input of large amount of hardly attainable design characteristics of the units. In this paper, a modelling method for acquiring the performance of a heating gas-fired condensing boiler unit will be shown. The model is based on the limited input data available in the official characteristics of the units issued by the relevant manufacturers. The simulations are programmed by using the programming language Modelica and the software tool Dymola. The model is based on the fixed natural gas intake which combusts into a stable mixture of the combustion gases that further heat the circulating water. During the heat transfer process inside the condensing boilers there is a possibility for condensate formation out of the water vapour of the combustion gases which increases the efficiency of the unit. The formation of condensate, however, is depending on the return water temperature of the unit which has to be lower than the dew point temperature of the combustion gasses. The goal of this research is to determine how accurate can performance indicators of gas-fired boilers be attained with the use of a limited amount of available input data together with clearly defined assumptions that follow the modelling methodology.
The combi boilers are commonly used for the heating purposes in residences since the natural gas has started to be used prevalently as fuel. Depending on this common usage, the investigations on the development of these kinds of boilers accelerated in the last decades. In this aim, it is very important to determine the improvement limits from the thermodynamic point of view. In this study, a 24 kW commercial condensed combi boiler was experimentally performed under the real operating conditions to determine the improvement paths from the thermodynamics point of view. In this aim, the advanced exergy analysis was conducted to determine the improvement potential of the combi boiler by defining ideal and unavoidable conditions including the combustion process. The most improvable components were determined as the combustion unit and recuperator. In the combustion unit, it was determined that the gas radiation has a considerable rate with 40.16% in the heat transfer phenomena for the real operating conditions. As conclusion, an improvement potential of approximately 14.32% and 7.09% was determined for the combustion unit and recuperator, respectively. The overall improvement potential of the combi boiler system was determined as 22.17%.
During the last decade, the application of electronics to the different components of HVAC systems has offered new and sophisticated control systems able to adapt the behaviour of each single device and of the whole plant to design specifications. These sophisticated finely-tuned control systems are, in principle, able to play an im-portant role in reducing energy consumption and im-proving thermal comfort. At the same time, there has been a marked increase in the complexity of the HVAC plant layout. Combining the complexity of plants with that of the control systems and taking into account the possibility of equipping each component of the plant with its own control system, the result is a new genera-tion of energy systems with many possible interactions between controllers. Hence it becomes impossible for a designer to evaluate, in an easy way, the effects of such interactions. As a consequence, it is difficult to know, at the design-stage, how the overall control system will op-erate. Nowadays, dynamic simulation of the complete HVAC system makes it possible to emulate the system in which the controllers operate. In this paper, the dynamic model of a basic HVAC system involving a condensing boiler, radiators with thermostatic valves and an inverter driven hydraulic pump is presented. Each element of the circuit is equipped with its own control. The model of the system has been built by using a custom-made library of Simulink blocks specifically created for the dynamic sim-ulation of controlled HVAC systems. The dynamic model will be used in order to underline the strong influence of the control system on the HVAC energy efficiency and thermal comfort conditions. Specific design rules limiting the negative interaction among the activated control sys-tems are inferred by the results shown in this paper.
In recent years, circulating fluidized bed boiler (CFB) combustion technology, which is used as a purification technology in coal combustion, has been widely studied. Some studies have conducted simplified investigations into calculating the standard boiler thermal efficiency. However, these studies are mainly aimed at simplified calculations of the boiler’s heat balance and data correction, and an online prediction model based on the core influential parameters have not been developed.
To solve this problem, this study first calculates the boiler heat balance and thermal efficiency calculation to analyze thermal efficiency calculation factors. The exhaust heat loss q2 and the incomplete combustion heat loss of gas q3 decrease as the slag carbon content increases. For every 1% increase in the slag carbon content, the thermal efficiency is reduced by 1.95%; for every 5 °C increase in the exhaust gas temperature, the thermal efficiency is reduced by 0.26%.
Then a simplified boiler thermal efficiency model, based on the slag carbon content and exhaust gas temperature determined to be the core parameters, is derived and compared with the measured results. The functional relationship is derived as follows: η=98.012–0.053tpy-1.944Clz (tpy < 200 °C, Clz < 2%).
In district heating, conventional flue gas heat recovery systems do not perform well anymore when the return water temperature is high. In this paper, a hybrid heat recovery system based on vapor compression refrigeration and liquid desiccant dehumidification is then proposed, with proper refrigerants and desiccants recommended. A simulation model of this system is established and a typical case is studied based on the model with a return water temperature of 60 °C. Then, an equivalent thermal efficiency is introduced to evaluate the system performance. Parametric analyses are conducted by changing the liquid-gas flowrate ratio and the inlet solution temperature in the second stage of the absorber in the system. According to the parametric analyses, a liquid-gas mass flowrate ratio of 3.2–3.4 and an inlet solution temperature of 45 °C are recommended as the best working conditions considering the economic and ecological effects. Under these conditions, the equivalent thermal efficiency of a boiler can be increased from 90% to 104%. For a boiler with a rated heating capacity of 7 MW, about CNY 236,000 (USD 34,000)can be saved per month. Besides, the flue gas is finally discharged at a relative humidity of only 33%. The formation of white smoke can thus be avoided and the contaminant attachment in the atmosphere around can be alleviated. The proposed system is efficient and environmentally friendly and the simulation results can offer references to real applications under variable working conditions.
Gas-driven sorption heat pumps (GDSHP) show significant potential to reduce primary energy use, associated emissions and energy costs for space heating and domestic hot water production in residential applications. This study considered a bivalent heating system consisting of a sorption heat pump and a condensing boiler, and focuses on the optimal heating capacity of each of these components relative to each other. Two bivalent systems were considered: one based on a solid chemisorption cycle (GDSHPA), and one based on a resorption cycle (GDSHPB). Simulations of year-round space heating loads for two single-family houses, one in New York and the other Minnesota, were carried out and the seasonal gas coefficient of performance (SGCOP) calculated. The sorption heat pump's design heating capacity as a fraction of the bivalent system's total heating capacity was varied from 0 to 100%. Results show that SGCOP was effectively constant for sorption heat pump design capacity greater than 41% of the peak bivalent GDSHPA design capacity in Minnesota, and 32% for GDSHPB. In New York, these values were 42% and 34% for GDSHPA and GDSHPB respectively. The payback period was also evaluated based on postulated sorption heat pump component costs. The fastest payback was achieved with sorption heat pump design capacity between 22 and 44%.
Compared with indirect-contact heat exchanger, gas-water direct-contact heat exchanger has superiority of decreasing metal heat-exchange surface, small temperature difference and volume, less investment and good antiseptic effect. This paper studies the droplets movement characteristics and performance of gas-water heat and mass transfer. The gas-water direct-contact heat transfer differential equation has been set up to obtain temperature distribution of flue gas and water. The main factors are also analyzed. In this paper, a direct-contact heat transfer model is established for a project, and the results of theoretical calculation and engineering operation are compared. The model has been verified.
This paper studies the energy and economic impacts from the replacement of a high-temperature hydronic heating system, consisting of an oil-fired conventional boiler and constant-speed circulator, by a low-temperature system with gas-fired condensing boiler and electronically controlled variable speed pump, in a residential building. Five case studies, which are categorized by the overall thermal insulation of the building and by the installed heating system, are examined in each of the two largest towns in Greece, namely Athens and Thessaloniki. In each study, the central heating system is designed and the thermal and electrical energy consumption is estimated by simulating the system according to the Bin method. Additionally, the operating cost, the cost of replacing the heating system, and the payback period of the investment are calculated. In the cases analysed, without any thermal insulation upgrade, the annual reduction of the thermal energy requirements varies from 26 to 29%. By upgrading the thermal insulation, thermal energy savings vary from 40 to 84%, depending on the previous insulation level of the building. With the use of variable speed circulators, electrical energy savings exceed 87% in all case studies. The fuel and electricity cost savings (prices of 2016) vary from 37 to 51%.
The residential sector is responsible for approximately 40% of final energy consumption in the EU, being a potential target to increase energy efficiency and to improve urban air quality. New high efficiency heating oil boilers have been developed requiring fuel adaption by diminishing the sulphur content in the fuel. The present work has led to conclude that further fuel development, including the use of alternative fuels, may be directed to reducing fossil fuel consumption and decreasing the formation of pollutants. Specific fuel properties such as distillation curve and nitrogen content show an influence on pollutant emissions, especially NOx, which can be reduced below the lowest limit required for gas domestic boilers, which until now set the state of the art in terms of NOx. emissions. The addition of renewable fuels in heating oil formulations, contributing to global CO2 decrease, does not affect the reduction in NOx emission as far as it does not affect nitrogen content and distillation curve of the fuel. Besides, a map of NOx emissions, useful to predict NOx emissions from latest generation heating oil boilers, independently on the type of fuel, has been inferred and an equation has been proposed to guarantee low NOx emissions depending on fuel properties.
A series of experiments were performed in a semi-virtual environment to investigate the performance of a gas boiler under dynamic operation conditions. The real condensing boiler was placed in the laboratory, and connected to thermal benches which emulate the thermal loads generated by a virtual building model. The results revealed first that the boiler efficiency drops when the Domestic Hot Water (DHW) needs prevail over space heating, due to the start-up losses provoked by a more frequent switching. The heating curve control enables to save energy and to reach higher efficiency levels in spring season, but this was not entirely verified in winter (full load heating). The impact of an increased thermal mass and insulation level was also experimented, as a preliminary step towards investigating energy flexibility in a nZEB-type building.
Condensing boilers are efficient for the utilization of natural gas. However, condensed droplets in flue gas are supercooled after condensation heat transfer, and direct contact devices cannot accurately measure the temperature of flue gas containing such droplets. Therefore, an in situ heating interlayer pumping device was designed. Experimental results showed that in situ heating interlayer pumping device can effectively filtered out the influence of supercooled droplets, the temperature of flue gas measured by the proposed device was about 3 °C higher than that measured by direct contact temperature measuring device; the device optimum flow rate was 0.35 L/min, optimum heat tracing temperature was in the range 100–140 °C; the effective reliability of the device was verified experimentally using the positive balance efficiency and the counter balance efficiency of 30-kW gas-fired condensing heat-transfer test platform.
In order to maintain the competitiveness and improve the environmental performance of district heating in the future, it is essential to transition to lower operating temperatures than those of current third-generation district heating systems. Heat demands are expected to decrease as buildings become more energy efficient, and new types of heat sources are likely to be integrated. Therefore, future supply and return temperatures of 50–55 °C and 20–30 °C, have been suggested for the fourth-generation district heating systems.
Previous studies on radiator temperatures are limited to the radiator's design conditions for modeling purposes, and there are no published studies that have established what the actual supply and return temperatures in radiator systems are. The purpose of this study is therefore to investigate what the temperatures are and to provide statistics on the radiator temperatures on the secondary side of district heating systems.
The results, incorporating data from 109 radiator systems located in Gothenburg's district heating system, showed that the average supply and return temperatures for design outdoor temperature −16 °C are 64 °C and 42 °C, respectively. For an outdoor temperature of 5 °C or more all radiator systems had supply temperatures of 55 °C or less. The results also showed that radiator systems with a large radiator heat transfer area had the largest potential for lower radiator operating temperatures, as indicated by the normalized radiator constant and the specific space heating demand.
Thermal performance of a simplified cast iron sectional boiler was numerically investigated in the present study. Computations were performed using the water mass flow rates in the range of 0.2 kg/s ≤ m. ≤ 0.5 kg/s. Eight different scenarios, including the circle, square, horizontal ellipse (HE) and vertical ellipse (VE) with aspect ratios (Ar) of 1.2, 1.4, and 1.6 have been considered for the cross section of hot gas passage. Flow patterns and thermal performance of all multi-pass cases were studied and compared with those of the single-pass sectional boiler. Several results have been obtained in this study. For example, depending on the hot gas passage shape, different vortical structures were realized within the section in which the section with square passage showed more complex flow behavior in comparison to the other cases. On the other hand, it was revealed that among the under consideration cases, section with square passage has the highest heat transfer, pressure drop, and thermal performance. Beyond the case with square passage, section equipped with vertical elliptic (Ar=1.6) passage was identified as the highest thermal performance. It was hoped the obtained results from this research arouse interest among the thermal engineers.
Condensing boilers achieve higher efficiency than traditional boilers by using waste heat in flue gases to preheat cold return water entering the boiler. Water vapor produced during combustion is condensed into liquid form, thus recovering its latent heat of vaporization, leading to around 10–12% increased efficiency. Many countries have encouraged the use of condensing boilers with financial incentives. It is thus important to develop software tools to assess the correct functioning of the boiler and eventually detect problems. Current monitoring tools are based on boiler static maps and on large sets of historical data, and are unable to assess timely loss of performance due to degradation of the efficiency curve or water leakages. This work develops a set of fault detection and diagnosis tools for dynamic energy efficiency monitoring and assessment in condensing boilers, i.e. performance degradation and faults can be detected using real-time measurements: this real-time feature is particularly relevant because of the limited amount of data that can be stored by state-of-the-art building energy management systems. The monitoring tools are organized as follows: a bimodal parameter estimator to detect deviations of the efficiency of the boiler from nominal values in both condensing and noncondensing mode; a virtual sensor for the estimation of the water mass flow rate; filters to detect actuator and sensor faults, possibly due to control and sensing problems. Most importantly, structural properties for detection and isolation of actuators and sensing faults are given: these properties are crucial to understand which faults can be diagnosed given the available measurements. The effectiveness of these tools is verified via extensive simulations.
Gas-engine driven heat pump could avoid energy loss in the process of generation, transmission and distribution, and effectively reduce the electric power load at the peak time in summer. In the present study, a novel stand-alone gas engine heat pump system with energy storage is put forward, which combined with the energy storage system and the autonomous power supply system. The annual performance optimization model of the gas engine heat pump system with energy storage system is proposed, which is calculated based on APSO algorithm. The simulation results were well agreed with experimental data, the average difference is 8.9%, 9.63% and 21.7% for the annual primary energy ratio of heating, cooling, transitional season, respectively. Based on the simulation results, the autonomous generating capacity E and the speed of engine n have high effect on the objective function, and the ratio of electricity to heat/cool/domestic hot water should be no greater than 0.1. The annual primary energy ratio of the gas engine heat pump system with energy storage system in heating, cooling and transitional season was increased by 6.11, 19.11% and 34.83% compared with the gas-engine driven heat pump system, respectively. The gas engine heat pump system with energy storage system can be used indifferent climate regions and buildings, which could provide a more stable work mode and higher annual primary energy ratio than the gas-engine driven heat pump system with a little energy loss.
Building energy consumption counts for almost a third of the global energy consumption and most of that is used for building heating and cooling. A comparative study was performed on heating systems regarding the number of cascading boilers and the influence on heating efficiency. Results indicate a 15% increase in efficiency in case of multiple boiler systems compared to single boiler installations.
The feasibility of individual natural gas fired boiler-based heating systems in the retrofitting of buildings constructed in the 50–60 s in Bilbao (northern Spain) is evaluated in this paper. A holistic approach through dynamic simulations using TRNSYS is employed for the purpose. An existing dwelling previously monitored and used to validate the model applied is selected as a case study. 54 different scenarios are evaluated, which arise from the combination of 3 different envelope options, 2 types of heat production units, 3 heat production temperatures and 3 comfort temperature set-points. The cases are evaluated in terms of energy results, economic aspects, and the influence of user behaviour. Regarding the latter, the influence of the potential rebound effect is also evaluated. The results show energy savings nearby 10% when condensing boilers are compared with high efficiency boilers. In relation to hot water production temperature, energy savings between 5 and 10% are found when the temperature is lowered from 60 to 50 °C. The greatest impact on energy consumption is related to the occupants’ behaviour: reductions up to 89% are achieved if the indoor temperature set-point is lowered 2 °C. This is reinforced with the results related to the rebound effect, which show significant differences on energy consumption values. These evidences demonstrate that the user behaviour is an essential feature to be considered in studies regarding buildings energy performance. As a consequence, the holistic approach herein employed emerges as a key tool to be applied in further works related with the topic.
An empirical analysis involving 130 Italian industrial firms showed that the economic viability of investments in energy efficiency technologies is mostly evaluated through indicators such as Pay-Back Time (PBT) and Internal Rate of Return (IRR), whose acceptability thresholds are affected by decision makers’ risk propensity and other contingencies, such as the firm’s financial health.
Our analysis suggests that these evaluation approaches hinder the adoption of several energy efficiency technologies, such as combined heat and power (CHP) plants, electric motors, variable speed drives (VSD), uninterruptible power supply (UPS), which are in fact economically viable if analyzed from a life cycle cost perspective, but appear to be unsustainable if analyzed through PBT or IRR indicators.
This paper addresses this issue by introducing a new evaluation perspective for investments in industrial energy efficiency technologies. Inspired by the life cycle economic assessment methodology for energy production plants – the so-called Levelized Cost Of Electricity (LCOE) – our indicator, called Levelized Energy Efficiency Cost (LEEC), correlates the energy savings that can be achieved through the implementation of an energy efficiency technology and the total costs incurred throughout the entire life cycle of the technology, e.g., initial investments, Operation & Maintenance (O&M), disposal costs. Accordingly, a technology can be considered as economically viable if the LEEC is lower than the energy price incurred by the firm, because in this case the economic benefits resulting from the energy saving due to the adoption of the technology is higher than the cost paid to obtain and operate it during its entire life cycle.
The application of such methodology in different Italian energy-intensive industrial sectors (e.g., automotive, cement, iron & steel and pulp & paper) shows that most of the considered technologies are economically viable, from the life cycle perspective on which this methodology is grounded. Therefore we suggest that the LEEC is a clear and simple tool for companies’ decision makers to evaluate energy efficiency projects, to be used in combination with more traditional PBT or IRR indicators to gain a better understanding of the real economic viability of energy efficiency technologies.
Heating boilers loses much of the heat developed by the combustion evacuating flue gas at high temperatures. To recover as much of this heat, flue gas has to be cooled below the dew temperature using different systems. Selection of the heat recovery systems requires a thermodynamic and economic analysis. The most important parameters influencing the choice are: type of fuel, temperature of exhaust gases and boiler operating time. Heat recovery from the flue gas of a 2000 kW boiler using a heat pump with mechanical vapour compression increases the energy efficiency from 87% to 94% that means reduction of fuel consumption by 8.28%. The payback period is about 3.82 years.
The legislation of various European countries imposes limits on the demand for building heating and cooling in order to reduce the primary energy consumptions. Moreover, the legislation prescribes that a fraction of the demand for building cooling, heating and power must be met through renewable energy sources. Among renewable energy systems, wind power, solar photovoltaic, solar thermal energy, solar cooling and heat pumps (though only "partially" renewable) have to be mentioned. In this framework combined heat and power (CHP) systems can provide a further solution to reduce the primary energy consumption. Due to the availability of different technologies, a key factor is the choice of the allocation strategy which allows the division of the energy demands among the various technologies in order to minimize the primary energy consumption. Since the cost of the technologies and the actual tariff and incentive scenarios depend on the specific country and may lead to not optimal allocation strategies in terms of primary energy consumption, these economic parameters are not taken into consideration in the analysis. Therefore, the obtained solutions represent a target which the policies should aim to achieve. This paper aims to develop and apply a methodology for the optimal allocation of the demand among CHP and renewable energy systems, with the aim of minimizing the primary energy consumption, by accounting for legislative constraints. The methodology is then applied to different climatic scenarios to evaluate the effects of a variation of the demand and technology characteristics on the allocation of the loads. Moreover, an analysis on the combined effects is presented. Finally, some guidelines are obtained.
Seasonal input–output efficiencies of two heating plants with condensing boilers in cold climate institutional buildings (3300 m ² school and 12,000 m ² university building) were evaluated. Plant efficiency remained about the same or declined with load, contrary to typical lab boiler ratings, which show efficiency increasing at lower loads. These results occurred when the boilers were operated with return water temperatures largely in the condensing range. Heating plant load was often 25% or less of the rated load of a single boiler, resulting in heating plant input–output efficiencies well below rated boiler efficiencies.
Practical application: In-situ boiler plant input–output efficiencies can differ widely from manufacturer's boiler efficiency curves. For applications such as simulation for energy-efficient design, effective decision-making depends on accurately estimating real-world performance.
This paper presents a new technology which combines the absorption heat pump and the direct-contact heat exchanger. Based on the analysis of natural gas features and thermal efficiency of the gas boilers, it analyzes existing and new techniques for heat recovery, focusing upon removing heat from the flue gas to a point where it can be below the dew point as significant to realizing the total heat recovery for the gas boiler. Through field test and experimental analysis of an actual case in a district heating system operating at various system load, this paper strives to identify challenges related to high return water temperature, achieving a low-temperature cold source, and enhancing the heat transfer effect; while observing if the system performance is satisfactory, and whether the system can achieve very stable performance under different working conditions and realize the total design heat capacity improvement of 12%. Data will be examined for boiler load and its influences on the condensing rate with suggestions for further improvements.
The refurbishment of a building with better insulation particularly in the walls and in the roof with the windows replacement gives rise to a relevant reduction of the heating load. Then the existing boiler is surely oversized and it must be replaced. The selection of the model is connected to the new design load, but it is also bound to the nominal capacity of the equipment on sale, that is never below about 10 kW. Then it is of paramount importance the modulating ratio that is till which capacity the boiler can operate continuously without working in the inefficient ON–OFF control.
For a typical detached building, first of all the load reductions with common refurbishment is considered for three climates: cold, temperate and mild in Italy. Condensing boiler seasonal performances are then compared with modulating ratio that from the simple ON–OFF arrives till 1:10 for two different nominal capacity.
Finally a further performance improvement can be obtained with variable flow rate pumps, operating with a temperature drop relatively constant between supply and return. Whereas the already high boiler efficiency is not appreciably increased, the electricity saving to drive the pumps can be really outstanding.
The behaviour of the condensing boilers is strictly bound to the temperatures of the water coming back from the plant. This temperature depends on the control modes and on the meteorological conditions. The seasonal performance has been computed for a heating plant of a building equipped with a condensing boiler simulating the load at short time intervals through a suitable modification of TRNSYS. The study has been carried out in the climate of Padova. For other towns a rough estimate is proposed.
An experimental study was carried out on a central plant for the heating of domestic hot water for a block of 143 flats and 15 offices. The behaviour of the condensing boilers serving the plant was examined and the energy costs of recirculation and distribution were analysed. Since the losses due to recirculation are of the same order as the useful energy, the influence of the various parameters on the losses has been studied, and some methods of lowering them are proposed.
Performance analysis of different applications for condensing boiler on residential buildings in several climatic areas
- R Lazzarin
- F Busato
- F Minchio
R. Lazzarin, F. Busato, F. Minchio, Performance analysis of different applications for condensing boiler on residential buildings in several climatic areas,
in: Proceedings of the 3rd International Conference on Sustainable Energy Technologies, Nottingham, UK, 28-30 June 2004, 2005.
Le caldaie a condensazione: dalla teoria agli impianti (Condensing boilers: from theory to practice), PEG Editrice
- R Lazzarin
R. Lazzarin, Le caldaie a condensazione: dalla teoria agli impianti (Condensing
boilers: from theory to practice), PEG Editrice, Milano, 1986, ISBN: 88-7067-052-X.
Prestazioni delle caldaie a condensazione Misure sperimentali per il riscaldamento centralizzato (Condensing boiler performance. Experimental survey of a centralised plant
- F Busato
- R Lazzarin
- R Magnaguagno
- F Minchio
- F Talamini
F. Busato, R. Lazzarin, R. Magnaguagno, F. Minchio, F. Talamini, Prestazioni delle caldaie a condensazione. Misure sperimentali per il riscaldamento centralizzato (Condensing boiler performance. Experimental survey of a centralised plant), vol. 50, CDA, 2006, pp. 26–31, ISSN: 0373-7772.
Misure sperimentali per il riscaldamento centralizzato (Condensing boiler performance
- F Busato
- R Lazzarin
- R Magnaguagno
- F Minchio
- F Talamini
F. Busato, R. Lazzarin, R. Magnaguagno, F. Minchio, F. Talamini, Prestazioni delle
caldaie a condensazione. Misure sperimentali per il riscaldamento centralizzato
(Condensing boiler performance. Experimental survey of a centralised plant),
vol. 50, CDA, 2006, pp. 26-31, ISSN: 0373-7772.