The performances of a hybrid energy system for decentralized heating are investigated. The proposed energy system consists of a solar collector, an air-source heat pump, a gas-fired boiler and a hot water tank. A mathematical model is developed to predict the operating characteristics of the system. The simulation results are compared with experimental data. Such a comparison indicates that the model accuracy is sufficient. The influence of the flat plate solar collector area on the economic and energy efficiency of such system is also evaluated through numerical simulations. Finally, this system is optimized using the method of orthogonal design. The results clearly demonstrate that the solar-heat pump-gas combined system is more convenient and efficient than the simple gas system and the heat pump-gas combined system, whereas it is less convenient but more efficient than the solar-assisted gas system.

Space heating takes a great proportion of global energy consumption [

A large number of investigations have been performed to study the performance of hybrid energy heating system. Among the renewable energy sources that can be used, solar thermal energy is considered to be one of the most efficient and environmental friendly energy form [_{2} emissions and improve air quality [^{2} solar collector in comparison to a traditional solar domestic hot water system. Li et al. [

In view of this, this study proposes a hybrid energy system which combines solar collector, heat pump and gas-fired boiler to satisfy the heating needs of the users. The system performance is predicted and optimized through a mathematical model which is verified by the experimental data. The effects of key parameters on the system performance are studied with model. The study has significance in the energy cascade utilization and energy saving compared with the traditional heating supply systems.

A mathematical model is developed based on the coupled models of different components, including the models of solar collector, heat pump, gas-fired boiler and storage tank. In order to simulate the operation of each unit, the models of heating units are established by several simple models. The heat storage tank is the key component of the system, which adopts hierarchical non-steady state mathematical model to reflect the relationship between the heating unit and heating supply unit.

The heat gains of solar collector can be calculated as follows:

The incident solar radiation on a tilted surface (_{θ}) can be estimated as follows:

while _{b} can be calculated by:

The declination,

where _{th} day of a year.

Air-source heat pump mainly consists of four parts: Evaporator, compressor, condenser and expansion valve. In order to calculate the power consumption of compressor and the heat release of condenser, the model of air-source heat pump is established. State points distribution of refrigerant are shown in

As can be seen in _{6} can be expressed as: _{6} is evaporation pressure, _{1} can be expressed as: _{1} can be expressed as: _{4} can be expressed as: _{4} is the condensing pressure, _{2} can be expressed as: _{5} can be expressed as:

The model of compressor is developed by the method of polytropic exponent. The operation of compressor can be calculated according to the following relations:

Refrigerant mass flow rate of compressor can be calculated as follows:

Theoretical power consumption by compressor can be represented as follows:

Input power of compressor motor can be evaluated as follows:

Discharge temperature of compressor can be estimated as follows:

Thermal energy provided by heat pump can be calculated as follows:

The

The thermodynamic properties calculation of refrigerant runs several times during the simulation, and the Cleland correlation formula is introduced to establish the refrigerant properties.

According to the schematic of heat pump and the thermophysical parameters of each point calculated by Cleland correlation formula, the heat pump model is built according to the following equations:

Evaporator function

Compressor function

Condensator function

Expansion valve function

According to these mathematical formulas, the enthalpy value of each state point can be calculated. The power consumption and heat release of the heat pump can be obtained according to the compressor model. The heat pump model solution flow chart is demonstrated in

The gas-fired boiler is to heat the supply hot water when the water temperature at the upper tank (_{x2}) cannot reach the required temperature _{W} (i.e., 50°C). The gas-fired boiler is also used to increase the supply temperature of heating system from the tank when it cannot reach the required temperature _{C} (i.e., 45°C).

The expression of gas consumption can be evaluated as follows:

where _{31} and _{32} can be estimated as follows:

In practical application, the thermal stratification phenomena of heat storage tank is serious. In order to calculate the temperature in the tank accurately, the model of heat storage tank is established by the distributed parameter method and the heat storage tank is divided into two segments along the vertical direction.

In order to establish the mathematical model conveniently and solve it easily, the following assumptions are made: (1) Ignore the heat loss of heat storage tank; (2) Ignore the heat loss of system pipeline.

Unsteady state thermal equations of the tank are described based on energy balance and solved by the software of Matlab. The schematic of heat storage tank is as follows:

Schematic of the bottom tank is shown in

The energy balance equations for the bottom segment of the tank can be represented as follows:

_{1} is the thermal energy provided by the bottom heat exchange coil and it can be calculated as follows:

Average logarithmic temperature difference _{m1} can be calculated as follows:

And _{1} can also be represented as follows:

The energy balance equations for the bottom heat exchange coil can be estimated as follows:

The schematic of the upper tank is illustrated in _{h} is higher than _{x2}. The energy balance equations for the upper segment are shown as follows:

The thermal energy provided by the upper heat exchange coil _{4} can be represented as follows:

The average logarithmic temperature difference _{m2} can be calculated as follows:

_{4} can also be represented as follows:

The energy balance equations for the upper heat exchange coil can be estimated as follows:

The thermal energy provided to the heating system by gas-fired boiler _{32} can be calculated as follows:

The stratified model solution flow chart is illustrated in

The hybrid energy system model solution flow is demonstrated in

In order to give a comprehensive evaluation of the hybrid energy system, the performance index and economical index are proposed in this paper.

Annual performance factor is the performance index of the system, which refers to the ratio of total heat provided by the system to the primary energy consumption and it can be calculated as follows:

The annual cost is the economical index, which includes the initial investment and operation costs, and it can be calculated as follows:

The initial investment (_{0}) includes the cost of flat plate solar collector (_{1}), air-source heat pump (_{2}), gas-fired boiler (_{3}), heat storage tank (_{4}) and system fixed initial investment (_{m}), including water pumps, valves, piping et al. It can be calculated as follows:

The operation costs considers the electricity consumption of solar system (_{t}) and heat pump (_{h}), gas consumption of gas-fired boiler (_{g}). It can be estimated as follows:

In order to verify the accuracy of the mathematical model, a prototype of the hybrid system is built. The structure of the hybrid energy system is illustrated in

The heat storage tank is the connecting link of the three heating units, which is the core component of the whole system. Therefore, the temperature distribution of the tank has important influence on the thermal performance of the whole system and plays an important role in improving the economic and energy efficiency.

The lay-out of the experimental system is presented in

Component | Characteristics |
---|---|

Solar collector | Product model: P-G/0.6-L/TL-1.8-ZT/G-A1 Flat plate collector Aperture area: 1.79 m^{2} Number of collector: 2 |

Solar cycle working medium | Density: 1.12 kg/L Specific heat capacity: 3.4 kJ/(K·kg) |

Air-source heat pump | Product model: KRF52-F2 Heating capacity: 5.2 KW Cryogen: R22 |

Gas-fired boiler | Product model: L1PB20-H188 Rated heat input: 18 KW |

Heat storage tank | Product model: SXT300-TRB Volume: 300 L Upper immersed coil heat exchange size: Φ25 mm×6107 mm Bottom immersed coil heat exchange:Φ25 mm × 3924 mm |

The solar radiation intensity (with an accuracy of ±2%), the ambient wind speed (with an accuracy of ±(0.3 + 0.03) m/s) and the ambient temperature (with an accuracy of ±0.1°C) were measured by a meteorological station. Temperatures were measured by Pt100 thermocouples with an accuracy of ±0.1°C. Mass flow rate was measured by turbine flow meter and electromagnetic flow meter (with an accuracy of ±1%). Electricity consumption was measured by electricity meter (with an accuracy of ±1%). The type and position of the measuring instruments are presented in

Type | Symbol | Position |
---|---|---|

Temperature sensors (PT100) | T1, T2 | Solar collectors (outlet, inlet) |

Temperature sensors (PT100) | T3, T4 | Air-source heat pump (outlet inlet) |

Temperature sensors (PT100) | T5, T6 | Heat storage tank (upper, bottom) |

Temperature sensors (PT100) | T7 | Heat storage tank (inlet) |

Temperature sensors (PT100) | T8, T9 | Heating temperature of gas-fired heating water heater (inlet, outlet) |

Temperature sensors (PT100) | T10, T11 | Hot water temperature of gas-fired heating water heater (inlet, outlet) |

Temperature sensors (PT100) | T12,T13 | Heating system (inlet, outlet) |

Temperature sensors (PT100) | T14,T15 | Cooling system (inlet, outlet) |

Turbine flowmeter | F1 | Solar collector circuit |

Turbine flowmeter | F2 | Air-source heat pump circuit |

Electromagnetic flowmeter | F3 | Tank circuit |

Electromagnetic flowmeter | F4 | Heating circuit |

Electromagnetic flowmeter | F5 | Cooling circuit |

Gas flow meter | F6 | Gas-fired boiler |

Watt-hour meter | E1 | Air-source heat pump cycle pump |

Watt-hour meter | E2 | Solar cycle pump |

The thermal power provided by the solar collectors (_{tt}), heat pump (_{ht}) and gas-fired boiler (_{rt}) in this experimental system were calculated on the basis of the measured individual quantities, according to the relation as follows:

Experiments were conducted from 7:00 AM to 19:00 PM during the period of August 22nd~September 5th. The heating water and domestic hot water supply temperature are set to be 50°C. The data is recorded per minute automatically. ^{2}. The wind speed fluctuated with time and ranged from 0 to 2.8 m/s.

The hot water daily load of the tested system is 56511 kJ and the heating load is 172553 kJ. According to the test results, the heat transfer coefficient of the upper coil is 870 W/(m^{2}·K) while it is 940 W/(m^{2}·K) for the tank bottom.

Comparison between the measured and simulated upper tank water temperature is conducted and the results are illustrated in

^{3} (3.77%) for gas consumed by gas-fired boiler. Therefore, the deviation is considered to be acceptable.

Measured value | Simulated value | Deviation | % | |
---|---|---|---|---|

The thermal energy provided by |
26.71 | 24.65 | 2.05 | 7.68 |

The thermal energy provided by |
94.28 | 100.09 | 5.81 | 6.16 |

The electrical energy consumed by air-source heat pump/kW·h | 5.76 | 5.38 | 0.38 | 6.60 |

The thermal energy provided |
96.58 | 103.81 | 7.23 | 7.47 |

The gas consumed by gas-fired boiler/m^{3} |
3.75 | 3.61 | 0.14 | 3.77 |

The influence of flat plate solar collector area on the hybrid energy system performance, system optimization and comparison with other systems are investigated based on the model. The annual comprehensive energy efficiency ratio and annual cost are considered to evaluate the hybrid energy system. A residential site for 3 people living with the total area of 105.16 m^{2} is studied to evaluate the performance of the hybrid energy system.

Domestic hot water load and heating load are the key parameters in this study and the domestic hot water load can be calculated as follows:

where _{r} is the water consumption per person and the value is 80 L, _{r} is the temperature of hot water and it is 60°C, _{L} is the temperature of cold water and it is 20°C.

Hourly hot water load can be estimated as follows:

where _{h} is proportional parameter and the results are given in

Heating load of the residential site is calculated by eQuest, and the results are shown in

According to the results, the maximum hot water load is 2.791 kW, the maximum heating load is 2.8 kW and the total load is 10523.74 kJ.

The maximum area of solar collector _{max} can be calculated as follows:

where _{w} and _{h}, solar fraction _{T} is the average daily solar radiation; the average collector efficiency

The maximum heat capacity of heat pump _{h max} can be estimated as follows:

The maximum heat capacity of gas-boiled _{r max} can be calculated as follows:

The volume of tank

According to the heating demand, the specifications of the hybrid energy system are obtained, as shown in

Equipment | Specifications | Unit price (dollar) |
---|---|---|

Air-source heat pump | 3.5 kW | 571 |

5.2 kW | 714 | |

Tank | 200 L | 143 |

300 L | 214 | |

Flat plate solar heat collector | 2 m^{2} |
114 |

Gas-fired boiler | 18 kW | 857 |

^{2}, the black line shows the variation of annual cost under the condition that the heat pump power rating is 5.2 kW. The red line shows the variation of the annual cost under the condition that the heat pump power rating is 3.5 kW.

^{2}. The minimum annual cost is obtained when the solar collector area is 5 m^{2}. The annual comprehensive energy efficiency ratio increases (from 1.16 to 1.73 in blue line, from 1.2 to 1.62 in orange line). It can be attributed to the larger the solar collector area the more energy can be provided by solar, while the hot water and heating load remain constant.

According to ^{2}, the power rating of air source heat pump ranges from 0 to 5.59 kW, the volume of water tank ranges from 0 to 1330 L. Considering the equipment models listed in

Factor A: The area of the solar collector in three levels: 2, 4, 6 m^{2}.

Factor B: The power rating of the air source heat pump in two levels: 5.2, 3.5 kW.

Factor C: The volume of the water tank in two levels: 200, 300 L.

In order to optimize the hybrid energy system, orthogonal experiment method is employed to optimize combinations of each unit. The annual cost of the system under different combinations is simulated by the mathematical model and the optimization of the system can be obtained through the results listed in

Combination | Solar collector (m^{2}) |
Heat pump (kW) | Tank (L) |
---|---|---|---|

1 | 6 | 3.5 | 300 |

2 | 6 | 5.2 | 200 |

3 | 4 | 5.2 | 300 |

4 | 4 | 3.5 | 200 |

5 | 2 | 3.5 | 200 |

6 | 2 | 5.2 | 300 |

According to ^{2}. The schemes in

Two optimum schemes are considered in this study, which are the minimum annual cost of the system and the annual comprehensive energy efficiency ratio.

Optimum scheme 1: Taking the minimum annual cost as the objective function and the optimized results are combination 1. The solar collector area is 6 m^{2}, the power rating of air-source heat pump is 3.5 kW, the power rating of gas-fired boiler is 18 kW, the thermal storage tank is 300 L, the annual cost is 292 dollar and the annual comprehensive energy efficiency ratio is 1.62.

Optimum scheme 2: Taking the annual comprehensive energy efficiency ratio as the objective function and the optimized results are combination 2. The solar collector area is 6 m^{2}, the power rating of air-source heat pump is 5.2 kW, the power rating of gas-fired boiler is 18 kW, the thermal storage tank is 300 L, the annual cost is 302 dollar and the annual comprehensive energy efficiency ratio is 1.73.

The traditional decentralized heating system includes simple gas system, the heat pump-gas combined heating system and the solar-gas combined heating system. To compare the performance of the hybrid energy system and the traditional decentralized heating system, the optimal scheme of traditional decentralized heating system is conducted considering the annual cost and taking the energy efficiency as the objective function. The optimization schemes of the traditional decentralized heating systems are shown in

System | Solar collector (m^{2}) |
Heat pump (kW) | Tank (L) | Gas-fired boiler (kW) |
---|---|---|---|---|

Gas | 0 | 0 | 0 | 18 |

Heat pump-gas combined | 0 | 3.5 | 300 | 18 |

Solar-gas combined | 4 | 0 | 300 | 18 |

In order to compare the economic performance of the systems, optimum scheme 1 is taken as the benchmark of the solar-heat pump-gas combined system. Compared with the hybrid energy system (C: 292 dollar, EER: 1.62), the annual cost of gas-fired system (334 dollar) increases by 14.22%, heat pump-gas combined system (308 dollar) rises by 5.37%, while solar-gas combined system (286 dollar) reduces by 2.35%. The annual comprehensive energy efficiency ratio of the simple gas system reduces by 0.77, heat pump-gas combined system reduces by 0.18, and the solar-gas combined system decreases by 0.34. The results show that the proposed hybrid system is more economical and efficient than the gas system and heat pump-gas combined system, while it is less economical but more efficient than solar-gas combined system.

In order to compare the system efficient, optimum scheme 2 is taken as the benchmark of solar-heat pump-gas combined system. Compared with the hybrid energy system (C: 302 dollar, EER: 1.73), the annual cost of gas system (334 dollar) rises by 10.49%, heat pump-gas combined system (308 dollar) increases by 1.94%, while the solar-gas combined system (286 dollar) reduces by 5.48%. The annual comprehensive energy efficiency ratio of gas system reduces by 0.88, heat pump-gas combined system decreases by 0.29, and the solar-gas combined system reduces by 0.45. The results show that the proposed hybrid system is more economical and efficient than the gas system and heat pump-gas combined system, while it is less economical but more efficient than the solar-gas combined system.

A hybrid energy system for decentralized heating is designed to satisfy the heating and domestic hot water demands. It makes the rational use of renewable energy and proposes an effective way to deal with the problems of environment and resource.

The numerical study indicates that different heating unit load capacity had strong effects on the system performance. The increase of flat plate solar heat collector area from 1 to 6 m^{2} results in an maximum increase in the annual comprehensive energy efficiency ratio of 0.57 and maximum decrease in the annual cost of 41 dollar.

Considering the economy of the system, the optimum result is that the solar collector area is 6 m^{2}, the power rating of air-source heat pump is 3.5 kW, the power rating of gas-fired boiler is 18 kW, and the thermal storage tank is 300 L. Considering the efficiency of the system, the optimum result is that the solar collector area is 6 m^{2}, the power rating of air-source heat pump is 5.2 kW, the power rating of gas-fired boiler is 18 kW, and the thermal storage tank is 300 L.

The research results indicate that the hybrid energy system contributes more to energy and economic saving than the traditional decentralized heating system. The simulation results show that compared with the gas system and heat pump-gas combined system, the proposed hybrid system is more economical and efficient. Compared with the solar-gas combined system，it is less economical but more efficient. The mathematical model is much helpful in the designing and optimizing of the hybrid system.

Science and technology project of state grid corporation-Research on demand response interaction strategy and simulation technology for integrated energy service business.