Hot Water Demand Research Articles

Performance analysis of a novel façade-based building integrated photovoltaic-thermal system with phase change material for domestic hot water

Since the current contribution of solar thermal energy to the amount of energy consumed for domestic hot water in buildings worldwide is very low (8.4 %, approximately), the potential of this renewable energy to meet the demand for domestic hot water is quite significant. In this context, this paper presents a new façade-based building integrated photovoltaic-thermal system with phase change material which has been patented recently. It is a modular device for installation on façades and roofs, enabling the production of domestic hot water and electricity using solar energy. The main novelty of the system lies in the use of a photovoltaic glass sheet, which improves the building’s aesthetics and facilitates the architectural integration of this type of devices. Thus, a photovoltaic glass is used for the production of electricity, a phase change material for heat storage and a water-phase change material exchanger for the production of domestic hot water. The design and verification stages of the system were developed for a case study under typical usage conditions. Two dimensional numerical models of five different configurations (of the energy harvesting and storage module and two dimensional and three dimensional numerical models of the finned exchanger were carried out to simulate the charge and discharge processes. The most suitable configuration of the system, characterised by a 12 mm vacuum gap and a low emissivity of the absorber, was able to meet the requirements in terms of water flow rate and supply temperature (45 °C) for a shower time of 5 min.

Enhancing Solar Thermal Efficiency: PCM Integration for Compact Energy Storage Systems

The optimal utilization of solar energy presents a challenge in meeting round-the-clock hot water demands while accommodating limited space in modern architectural designs. Solar collectors, operating solely during daylight hours and weather-dependent, necessitate efficient energy storage to meet evening and morning peak hot water consumption in households. Current architectural trends, marked by smaller boiler rooms, further compound the challenge of integrating conventional solar tanks, emphasizing the critical need for compact energy storage solutions. Addressing this challenge, this paper proposes a ground-breaking solution: a solar tank infused with phase change material (PCM). This innovative tank design not only boasts smaller dimensions but also significantly higher heat capacity compared to conventional counterparts. Crucially, the PCM solar tank enables solar collectors to operate at lower temperatures, potentially elevating the overall efficiency of the solar collector system. The proposed storage system comprises two synergistic heat-absorbing units: a solar water heater and a PCM-based heat storage unit. While the solar water heater fulfills the daytime hot water supply, the PCM storage unit accumulates and retains heat throughout the day, ensuring continuous hot water availability during night hours and overcast periods. Leveraging small cylinders filled with paraffin, serving as the PCM, integrated with solar collectors maximizes solar energy absorption and storage. This paper meticulously compares the performance of the PCM-based thermal energy storage system with conventional sensible heat storage systems, presenting insightful conclusions derived from the comparative analysis. The findings underscore the remarkable efficiency and efficacy of the PCM-based solution in meeting the challenges posed by solar energy intermittency and limited space constraints, offering a promising avenue for sustainable and efficient solar energy utilization in residential settings.

Least-cost solutions to household energy supply decarbonisation in temperate and sub-tropical climates

Decarbonisation of building energy supply is key to achieving current targets of greenhouse gas emission reduction. However, the least-cost, net-zero supply of electrical and thermal loads in residential buildings is affected by building type, local climate and grid emissions intensity and is subject to uncertain future energy prices. This work investigated a typical detached house and an apartment in two climates – temperate and sub-tropical – in Australia. Least-cost optimisation of the technology mixes required to serve building energy loads was undertaken, including a range of appliances powered by electricity, natural gas and hydrogen and the use of distributed energy resourced, notably rooftop solar and residential battery energy storage. Solutions were investigated with increasingly stringent greenhouse gas emission abatement constraints and a range of future prices for electricity and hydrogen, demonstrating: i) progressive electrification is the likely cheapest pathway to net-zero emissions for the two building types studied; ii) should grid decarbonisation lag the abatement constraint applied to the homes, a net-zero dwelling is likely unachievable and the deepest achievable abatement is very expensive; and iii) network-delivered hydrogen may be optimal in a subset of buildings with limited distributed energy resources potential and more peaky heating and hot water demands. In such cases, bio-methane or even synthetic methane may also be preferred. Through significant adoption of distributed energy resources, results suggest an affordable route to deep (i.e. > 50 %) abatement for homeowners that relies less on grid emissions and, beyond the Australian case study, can be extended to a significant proportion of dwellings in other countries.

Development of Dual Intake Port Technology in ORC-Based Power Unit Driven by Solar-Assisted Reservoir

The ORC-based micro-cogeneration systems exploiting a solar source to generate electricity and domestic hot water (DHW) simultaneously are a promising solution to reduce CO2 emissions in the residential sector. In recent years, a huge amount of attention was focused on the development of a technological solution allowing improved performance of solar ORC-based systems frequently working under off-design conditions due to the intermittence of the solar source availability and to the variability in domestic hot water demand. The optimization efforts are focused on the improvement of component technology and plant architecture. The expander is retained as the key component of such micro-cogeneration units. Generally, volumetric machines are adopted thanks to their better capability to deal with severe off-design conditions. Among the volumetric expanders, scroll machines are one of the best candidates thanks to their reliability and to their flexibility in managing two-phase working fluid. Their good efficiency adds further interest to place them among the best candidate machines to be considered. Nevertheless, similarly to other volumetric expanders, an additional research effort is needed toward efficiency improvement. The fixed built-in volume ratio, in fact, could produce an unsteady under- or over-expansion during vane filling and emptying, mainly when the operating conditions depart from the designed ones. To overcome this phenomenon, a dual intake port (DIP) technology was also introduced for the scroll expander. Such technology allows widening the angular extension of the intake phase, thus adapting the ratio between the intake and exhaust volume (so called built-in volume ratio) to the operating condition. Moreover, DIP technology allows increasing the permeability of the machine, ensuring a resulting higher mass flow rate for a given pressure difference at the expander side. On the other hand, for a given mass flow rate, the expander intake pressure diminishes with a positive benefit on scroll efficiency. DIP benefits were already proven experimentally and theoretically in previous works by the authors for Sliding Rotary Vane Expanders (SVRE). In the present paper, the impact of the DIP technology was assessed in a solar-assisted ORC-based micro-cogeneration system operating with scroll expanders and being characterized by reduced power (hundreds of W). It was found that the DIP Scroll allows elaboration of a 32% higher mass flow rate for a given pressure difference between intake and expander sides for the application at hand. This leads to an average power increase of 10% and to an improvement of up to 5% of the expander mechanical efficiency. Such results are particularly interesting for micro-cogeneration ORC-based units that are solar-assisted. Indeed, the high variability of hot source and DHW demand makes the operation of the DIP expander at a wide range of operating conditions. The experimental activity conducted confirms the suitability of the DIP expander to exploit as much as possible the thermal power available from a hot source even when at variable temperatures during operation.

Solar-Assisted Heat Pump with Electric and Thermal Storage: The Role of Appropriate Control Strategies for the Exploitation of the Solar Source

In the EU, the building sector is responsible for 40% of the global energy consumption for final uses and 36% of the carbon dioxide (CO2) emissions. Heat pumps allow for the replacement of conventional systems based on fossil fuels with the perspective of combining PV and solar thermal collectors. In order to rationalize the use of the solar source, this paper examined the self-consumption electricity share, the CO2 equivalent emissions, and the domestic hot water demand covered by renewable sources which were determined in two opposite climatic conditions. These involved both electric and thermal storage systems and considered two different control strategies. The first is commonly used for the management of air-conditioning systems, the second was specifically conceived to maximize the exploitation of the solar source. Results showed that the latter significantly reduced grid dependence in both locations, determining the direct satisfaction of 76% of the thermal and electric loads through the PV self-consumption, determined by 18 kWp of installed PV and a battery capacity of 24 kWh. In terms of equivalent CO2 emissions, when the two control strategies were compared, a remarkable reduction in emissions was registered for the latter, with percentages ranging between 8% and 36% as a function of PV surface and battery capacity. The analysis of domestic hot water supplies revealed disparities between the two localities: the colder first, relied more on heat pumps for water heating, while the warmer second, benefitted from the large availability of solar radiation.

The PEMFC-MHP-CHP system for independent energy supply and peak shaving: Performance analysis under load fluctuations

The Proton Exchange Membrane Fuel Cell - Mechanical Heat Pump - Combined Heat and Power (PEMFC-MHP-CHP) system is a reliable energy supply solution, which can operate independently of the grid during emergencies or be used for peak shaving and load leveling in renewable energy networks. This paper proposes two system structures, namely the separated and integrated systems and establishes MATLAB/Simulink models. Then, a sensitivity analysis is conducted, examining system design and operation. The results show that the separated system can satisfy basic heating and electricity demands without an external power supply at −14.5 °C. Increasing the number of cells by 20, reducing one room's heat load (36 m2), or increasing the current density by 0.1 A/cm2 results in reductions in compressor power consumption ranging from 1.76 % to 2.91 %, 7.42 %–10.57 %, and 6.52 %–16.88 %, respectively. On the other hand, the integrated system has higher power consumption but can operate smoothly at lower temperatures than the separated system. It is suitable for integration with auxiliary power devices in low-temperature environments. The PEMFC's heat output minimally affects system efficiency. Reducing the number of rooms improves system performance, with each reduction of one room decreasing compressor power consumption by 5.43 %–8.55 %. Meanwhile, two design options are identified based on the power supply and hot water demands. Further research and development can enhance the efficiency and performance of these systems, promoting their wider use in the future.

Simulation of a Solar Domestic Water Heating System with Different Collector Efficiences and Different Volumen Storage Tanks.

This paper shows the modeling and dynamic simulation, of a domestic solar water heating installation. The results of simulations performed on an annual basis for a solar system, operated in Santander (Spain), which provides hot water for a family (four persons) are presented. The installation consists in a solar flat collector, a water storage tank, a source of auxiliary energy, and a device for blending water. The mathematical model is used to evaluate the annual variation of the solar fraction respect to the volume of the storage tank, demand hot water temperature required, difference of this temperature and preset storage tank water temperature, and consumption profile of the domestic hot water demand. The results for a number of designs with different storage tank volumes, show that the systems with greater volume yield higher solar fraction values. However, when a larger storage tank volume is used, the solar fraction is less sensitive to a variation of these operation parameters. The results of this simulation may be used to design a solar collector system.

Management of domestic hot water system based on the example of a gas-fired instantaneous water heater

Purpose: The aim of the study was to present the actual efficiency of a gas-fired instantaneous water heater for the production of domestic hot water. Design/methodology/approach: The object of the study was a gas-fired instantaneous domestic hot water heater located in a flat in Krakow. Measurements with a suitable device were carried out for three basic variants of hot water preparation, defined as showering, washing hands and washing dishes. Findings: The tests and analyses carried out have revealed that the current efficiency (average efficiency based on 3 measurements – 77.61%) differs radically from that indicated on the water heater data sheet. Such a drop in efficiency may be caused e.g. by fouling of the surface of the heat exchanger on the flue gas side and scaling of the exchanger on the heated side. Research limitations/implications: The tests proved that the heater did not reach its nominal efficiency during the tests, but on the other hand, simulations of different intensities and durations of water consumption did not drastically change the efficiency of hot water preparation. Practical implications: Regular cleaning of the exchanger would probably reduce the difference between the efficiency declared by the manufacturer and the one achieved during the tests. A clean heater will also ensure a sufficiently low level of carbon monoxide in the flue gas, which has a real impact on the safety of the appliance. Social implications: The values obtained from the measurements for each operating mode can help occupants understand how to prepare water most efficiently with this particular appliance, which will have a real impact on the bills they pay. The user will be provided with information on what percentage of energy they use from the volume of gas burned. Originality/value: Gas-fired instantaneous water heaters provide an efficient and energy-saving solution, especially in situations where the demand for hot water is low, and immediate access to it is crucial. Keywords: hot water, hot water management, gas-fired instantaneous water heater. Category of the paper: Research paper.

Techno-economic analysis of solar thermal collector for sustainable built environment

India has an abundance of solar energy resources. Nearly 90% of the urban dwellers in different parts of India used electricity for heating water. Using electricity for resident water heating not only contributes to negative environment impact via the emission of a substantial quantity of carbon dioxide but also places a financial burden on households. The examination of solar irradiance, economic feasibility, and hot water demands is crucial in assessing the potential of solar thermal collector (STC) to fulfill hot water requirement. In order to enhance the value of the STC, it is essential for the collector to effectively manage operations with a focus on minimizing cost. To get an optimal heating effect, the present STC is fastened with an obstruction inside the tube. The collector is examined in three distinct seasons to observe temperature variations and the appropriate use of the device during non-operational hours. The recommended average temperatures for summer, rainy season, and winter are 68, 57, and 47 °C, respectively. Additionally, a thorough examination of the economic analysis and payback period for STC was also conducted.

Performance Study of a Hybrid Solar-Assisted Ground-Source Heat Pump System Used for Building Heating and Hot Water Demands

Performance Study of a Hybrid Solar-Assisted Ground-Source Heat Pump System Used for Building Heating and Hot Water Demands

Theoretical and experimental analysis of the impact of a recuperative stage on the performance of an ORC-based solar microcogeneration unit

Microgeneration ORC-based units driven by solar energy, which enable combined heat and power generation (CHP), are a promising solution for decarbonizing the domestic sector. However, the intermittent availability of solar energy, coupled with the variability in user demand for domestic hot water (DHW), can lead the system to frequent off-design conditions and a less reliable energy supply. Consequently, increasing attention has recently been focused on the technological and design solutions for improving plant performance and ensuring its continuous operation. This paper presents the results of an experimental campaign carried out to investigate the possible advantages – in terms of efficiency and savings of thermal energy in the upper source – of introducing a recovery heat exchanger (RHX) in the basic configuration of a solar ORC-based system. Tests were conducted on a fully instrumented ORC-based plant with two 12 kW electric heaters providing the thermal power recovered through the solar collectors. The RHX is introduced into a recuperative branch that can be bypassed by closing a dedicated three-way valve. The study aims to investigate the behavior of the ORC unit in the absence of solar radiation (with the electric heaters switched off) when the recovery unit is powered only by the hot water stored in the Thermal Energy Storage (TES) tank. Another purpose of the present work is to evaluate the benefits introduced by the RHX in reducing the temperature decrease of the TES hot water and, consequently, maximizing the operating time of the ORC-based unit. In order to support the experimental analysis, a comprehensive theoretical model of the unit was developed and validated against experimental data. The model was used as a software platform to optimize the plant design and recuperative branch configuration. The theoretical model was developed in the GT-Suite™ environment thus integrating a mono and zero-thermo-fluid-dynamic approach. In this way, a physical representation of the entire ORC-based unit is performed allowing also to define an optimal control strategy for maximizing plant performance under severe off-design and transient conditions.

Code generation for embedded predictive control of gas water heaters

Conventional control strategies usually employed in tankless gas water heaters present difficulty in controlling the hot water temperature when subjected to sudden changes in water flow rate. Inadequate control leads to temperature overshoots and undershoots with long settling times that severely affect the user comfort, increasing water and energy wastage and associated gas emissions. A strategy based on model predictive control is presented to reduce the impact of changes in hot water demand. A semi-empirical model, parameterized with experimental data and compatible with real-time simulation, is used for the heat cell. A tailored state observer is proposed, considering time-varying delays characterizing this thermal process. An automatic code generation software tool was developed for the embedded implementation of gas water heater predictive controllers. Numerical simulations and hardware-in-the-loop experiments were established to evaluate conventional and predictive control strategies. It was demonstrated that embedded model predictive control could be successfully implemented on computationally limited microcontrollers, even for thermal systems with extensive varying time delays. Predictive control has shown significant performance improvements, with decreased temperature fluctuations, a gain in comfort index from 36% to 75% and a reduction of up to 32 s in the settling time.

PREVIS 1.0: Easy software for simulation and sizing of solar water heating system

In this paper we are interested in developing new and easy software for the sizing of a solar installation of hot water production. Such installation is successful only if its determination, its conception and its realization are made with care. This software that we baptized PREVIS 1.0 supplies, for fixed operating conditions, the dimensions of each component of the installation without having to use tables of data or diagrams. PREVIS 1.0 will allow the engineers and the fitters of solar installations of hot water production to make a good sizing in a simple and precise way. Moreover, PREVIS could be coupled to TRNSYS code and this is one of its highlights. Typical university campus for a 240 students was considered as case study. The hot-water demand temperature (45°C) is controlled by a conventional fuel auxiliary heater and a tempering valve. The fluids circulate by pumps activated by electricity. Annual energy performance, in terms of solar fraction, was calculated for Tangier.

An open-loop hybrid photovoltaic solar thermal evacuated tube energy system: A new configuration to enhance techno economic of conventional photovoltaic solar thermal system

One inherent problem of the conventional hybrid photovoltaic solar thermal system is the gradual loss of module power efficiency with low energy harvesting due to steady increase in module temperature with repeated circulation of the cooling water as a result of its closed loop configuration. This poor energy harvesting couple with high initial investment has made investment in this technology unattractive. This study proposes an enhanced hybridisation of the Photovoltaic Solar Thermal System collector with an evacuated tube based on an open-loop cooling configuration and unique flow control strategy. The aim is to improve cooling of solar panel leading to higher energy harvesting and utilisation that improves economic viability that will promote faster adoption. A numerical model is formulated using TRNSYS 18 software to assess the techno economics of the hybrid system. The model is validated using the verification and experimental methods. Overall, the findings show that the enhanced hybridisation and flow control strategy has resulted in higher annual renewable power generation and utilisation by 10 % than the basic hybrid Photovoltaic Solar Thermal system. This has resulted in significant economic savings, cutting operating expenses by 48 %. The ability to tailor the intended use of the system to meet with hot water and electrical power demand of different residential building is another attribute of proposed system that improve its energy harvesting reducing it payback period and doubling it economic benefit hence promoting it fast adaptability.

Optihood – a multi-objective analysis and optimization framework for building energy systems at neighborhood scale

Strategic planning of future energy systems in a district is essential to fully exploit the benefits from sector coupling. In particular, new solutions may arise when a group of buildings is considered rather than individual buildings. Therefore, in order to identify optimal production and storage technologies within a district, new tools are needed. In this paper we present the open-source software framework optihood, which allows for multi-objective optimization and analysis of energy systems for neighborhoods. The tool assists researchers and, in a future stage, energy planners in the design of energy systems as well as in optimizing their operation strategy. After introducing the tool, its applicability is demonstrated with a case study, wherein we investigated the supply of a small neighborhood of four residential buildings with space heat, domestic hot water and electricity demands. The optimizer was allowed to consider heat pumps (geothermal/air-source), PV, batteries and hot water storage tanks as available technologies. We show comparisons between optimal solutions in terms of total annualized cost for the buildings considered (a) together as a group and (b) individually.

Integration of sewage source heat pump and micro-cogeneration system based on domestic hot water demand characteristics: A feasibility study and economic analysis

The reduction of carbon emissions from building space heating and cooling technology has become a crucial approach in achieving global carbon neutrality. Among these technologies, heat pumps and cogeneration systems are effective methods for carbon reduction in buildings. However, in numerous studies of coupled system operation, few scholars have considered the energy use of domestic hot water. This research investigates the operational feasibility of a sewage source heat pump coupled with a micro-cogeneration system, using a large-scale hotel building as the basis for the study. An economic operation strategy, based on the load characteristics of domestic hot water, is proposed and validated through simulation using TRNSYS software. The results showed that the proposed system can reduce peak power usage by 37 % annually, leading to lower operating costs. The calculations indicate that the proposed system can decrease the annual operating cost by 402,300 CNY compared to a single sewage source heat pump system. The payback period for the proposed system is estimated to be 6 years at the current time-of-use electricity price. Under various energy policy conditions, 71 % of the proposed system’s payback period is less than 20 years. Moreover, the emissions of SO2, NOx, and CO2 are reduced by 73.5 %, 73 %, and 64.8 % respectively. The system offers substantial economic and environmental benefits, and this study can serve as a reference for energy supply systems in cold regions.

Hydrogen production of flat plate solar collectors integrated with photovoltaic thermal panels

This study investigates an integrated system's energetic and exergetic evaluations and hydrogen generation efficiency. The proposed system comprises flat plate solar collectors (FPSCs) and photovoltaic thermal panels (PV-Ts) having a total area of 112.32 m2 and 108 m2, an organic Rankine cycle (ORC) containing n-butane as working fluid, a proton exchange membrane (PEM) electrolyzer, a building for fungus production and a storage tank for hot water demand a house. The system analyses are conducted by Engineering Equation Solver (EES) software for a daily solar radiance in July. While the PV-T is refrigerated by the water in the city network with a heat exchanger, warm water is obtained from the PV-T as well as increasing the electricity generation of the PV-T. Thus, a thermal gain is provided by sending the preheated water from PV-Ts to FPSCs. Consequently, when both collectors worked together, fluid was obtained at a minimum and maximum temperature of 100 °C and 132.3 °C during the day. Thus, thanks to both collectors, a higher temperature fluid was sent to the ORC. In addition to the electricity generated by the PV-Ts in the system, electricity is also generated by the ORC from pre-heated hot water by two collectors. Thus, ORC produced a total of 173.96 MJ of electricity during the day, while PV-Ts produced a total of 341.04 MJ throughout the day. Thus, the entire system produced a total of 515 MJ of electricity throughout the day. The total generated electricity is operated in the PEM electrolysis for generating hydrogen via the water electrolysis process in the system. Only the electricity generated by ORC produces 784.18 g of hydrogen per day, while only the electricity generated by PV-Ts produces 1618.4 g of hydrogen. Thus, a total of 2402.58 g of hydrogen is generated by the system. The overall energetic performance of PEM was varied between 67.16% and 69% during the day. The maximum energetic and exergetic performance of the system is 37.02% and 7.32%, while the minimum energetic and exergetic performance is 31.70% and 6.88%, respectively.

Thermal comfort analysis of a trombe wall integrated multi-energy nanogrid building

This research will examine the energy performance, thermal comfort and simple payback for a multi-energy nanogrid house integrated with trombe wall. The aim of the study is to improve annual comfort level percentage of the nanogrid by maximally catering to a 5-person family's thermal energy consumption 1,200 kWh and electric consumption 800 kWh annually. A single zone building model is employed to be conditioned to a temperature range 16–26 °C. Two space heating mechanisms include hydronic hot water network and trombe wall storage that work in day-night synergy. The study demonstrates sustainable heating and cooling temperatures with minimal dependance on auxiliary boiler and cooling energy use. TRNSYS software is selected for the yearly dynamic energy simulations of the proposed nanogrid. Load analysis is performed by establishing hourly and peak zone heating/cooling loads. Simple payback is evaluated based on construction type, build and energy prices and is in range from 1.1 to 4.76 years. Thermal wall accumulated heated air up to 55 °C (Summers) and 40 °C (Winters). The annual thermal energy yield with solar collectors is 2828 kW and the thermal wall is 8677 kWh. Annual comfort level percentage (CLP) reflects an increase from 49% to 87.5%. On average subtropical days the system was able to meet majority 85–90% of the thermal demands, in peak winters the architecture was able to cater 45% space heating and domestic hot water demand, other 55% energy is imported from grid.

Thermodynamic modeling of a residential heat pump for combined space and water heating using binary CO2-based mixtures

The residential sector answers for a considerable share of the energy consumption worldwide. An alternative to meet technical and environmental requirements of space heating and hot water demanded by this sector uses CO2 mixtures as a refrigerant in compression-based cycles. However, while CO2 is nonflammable and has low GWP and ODP, it performs poorly due to its low specific refrigerating effect and large throttling losses. Differently, mixtures with a low-GWP refrigerant and CO2 could enhance the performance of these cycles while attending the environmental requirements. Therefore, this study considered the use of a combined heat pump for space and water heating using binary mixtures of CO2 and several refrigerants. Through a computational model, three configurations were tested, along with different ratios of space heating to hot water demands, and different CO2 mass fractions on the mixture. The heat pump configuration with the best performance range was the one with three gas coolers, where the low temperature gas cooler preheats the water, the intermediate equipment heats the space, and the high temperature gas cooler heats the already preheated water to the operating temperature. For this arrangement, heat pumps primarily dedicated to space heating benefit from mixtures with low mass fractions of CO2, approximately 10%. In contrast, heat pumps primarily attending water heating perform better for high mass fractions of CO2 (∼ 90%). Furthermore, in this combined configuration, about 87% of the mixtures analyzed could have higher coefficient of performance than pure fluids, indicating an improvement of CO2 heat pumps.

Reliability and availability modelling of a retrofitted Diesel-based cogeneration system for heat and hot water demand of an isolated Antarctic base

The reduction of greenhouse gas emissions is a relevant challenge for a sustainable development. Waste heat could be used to produce hot water by using a recovery system. This article studies the availability of a combined heat and power systems (CHP) in extreme area (Antarctic) through the integration of a waste heat recovery system with a diesel generator to produce hot water. The reliability and availability principles are incorporated to explore how the profile of hot water consumption and the hot water storage tank size affect system availability. Different combined heat and power systems are thus classified, and their availability indexes modelled by adopting the continuous Markov approach and the state space model. The results indicate that the CHP systems availability is strongly influenced by the daily hot water demand profile. As a useful recommendation, one of the considerations for increasing availability, reducing costs and greenhouse gas emissions with the CHP system is to include a hot water tank in the analysis.