Modeling and economic analysis of gas engine heat pumps for residential and commercial buildings in various climate regions of Iran

There is an increasing trend in using heat pumps in air conditioning (heating/cooling) systems of residential and commercial buildings. The required power to drive the compressor of vapor compression heat pump cycles may be provided by either an electrical motor or an internal combustion engine. In this paper thermal modeling and economic analysis of gas engine heat pumps (GEHPs) are presented based on energy and mass balance equations as well as the gas engine operating parameters (such as thermal efficiency, fuel consumption and fuel mass flow rate) and heat pump operating parameters (such as evaporator and condenser capacity and compressor input power). Based on the modeling results and with estimating GEHP fuel consumption, the economic analysis of using gas engine heat pumps (in comparison with the electrical heat pumps) at various climate regions of Iran, for both residential and commercial (office) buildings, and for both cooling and heating modes, was performed. Appropriate cost functions for predicting GEHP capital investment were proposed. Three approaches including equivalent uniform annual cost (EUAC), the annual cost of energy consumption, and payback period were applied in the economic analysis.     

地源热泵--建筑节能新技术

在建筑供热空调中采用热泵技术可以有效地提高一次能源利用率，减少温室效应气体CO2和其他大气污染物的排放。本文阐述了利用热泵供热比锅炉直接燃烧供热节能的原理，对主要的热泵供热形式，特别是地源热泵的技术特征、适用范围和经济性作了较详细的介绍。     

Development of system concepts for improving the performance of a waste heat district heating network with exergy analysis

The building sector is responsible for a great share of the final energy demand and national CO₂ emissions in countries like Germany. Nowadays, low quality thermal energy demands in buildings are mainly satisfied with high-quality sources (e.g. natural gas fired in condensing boilers). Exergy analysis, pursuing a matching in the quality level of energy supplied and demanded, pinpoints the great necessity of substituting high-quality fossil fuels by other low quality energy flows, such as waste heat. In this paper a small district heating system in Kassel (Germany) is taken as a case study. Results from preliminary steady-state and dynamic energy and exergy analysis of the system are presented and strategies for improving the performance of waste-heat based district heating systems are derived. Results show that lowering supply temperatures from 95 to 57.7 °C increases the final exergy efficiency of the systems from 32% to 39.3%. Similarly, reducing return temperatures to the district heating network from 40.8 to 37.7 °C increases the exergy performance in 3.7%. In turn, the energy performance of all systems studied is nearly the same. This paper shows clearly the added value of exergy analysis for characterising and improving the performance of district heating systems.     

Domestic demand-side response with heat pumps: controls and tariffs

ABSTRACT

Electric heat pumps feature prominently in projected energy transitions in the UK and elsewhere. Owing to their high electricity consumption, heat pumps are viewed as important targets for demand-side response (DSR). Findings are presented from a field trial of a new control system that aims to optimize heat pump performance, including under time-varying tariff conditions. The trial involved monitoring 76 properties with heat pumps, but without dedicated heat storage; 31 of these received the control system. Interviews were conducted with a subsample of 12 participants. The controller successfully evened out electricity demand over the day (reducing the evening peak), but this was associated with increased late night and daytime temperatures. Interview participants reported some disturbance owing to overnight heating and noise, as well as usability issues with the controller interface and hardware. These issues present risks to the future acceptability of such systems. While the system delivered short-term demand reductions successfully, longer-term demand shifting risked causing unacceptable disturbance to occupants. Future control systems could overcome some of the issues identified in this pioneering trial through more effective zoning, using temperature caps or installing dedicated heat storage, but these may either limit the available flexibility or be challenging to achieve.     

Development and experimental validation of a high-temperature heat pump for heat recovery and building heating

The performance of a high-temperature heat pump unit using geothermal water for heat recovery in buildings is experimentally evaluated. The unit consists of a twin-screw refrigeration compressor, a condenser, an evaporator and an oil cooling system. The effect of the cooled oil temperature on the performance of the heat pump unit is experimentally investigated. Results show that the unit stably produces outlet hot water at a constant temperature of 85 °C and performs well in a wide range of high-temperature conditions with a high energy efficiency ratio. The results also indicate that the key to improving the performance of a high-temperature heat pump unit often depend on the selection of proper cooled oil temperature. The optimum cooled oil temperature is 50-65 °C when the condensing temperature is above 70 °C. At these temperatures, the oil cooling system can increase the energy efficiency ratio of the heat pump by 6.3%.     

Experimental investigation of adsorption chiller for Micro-scale BCHP system application

Building cooling, heating and power (BCHP), is an attracting cogeneration system for its economic and environmental friendly qualities. Therefore, it is meaningful to develop Micro-scale BCHP (MBCHP) system based on adsorption chiller and internal combustion gas engine for the use of residential and light commercial buildings. To guide the optimum matching and operation of adsorption chiller in MBCHP system correctly, this paper deals with the performance of adsorption chiller under varying heating conditions. Experimental results show that the value of COP is high in the operation mode of varying hot water inlet temperature with mass recovery in no heating pattern (VTNH). With the hot water inlet temperature of 65 °C, the value of COP is as high as 0.40 in VTNH mode. Under electricity output conditions from 6.0 to 8.3 kW in MBCHP system, VTNH mode is especially preferred when cooling demand is with priority.     

Economy of exploiting heat from low-temperature geothermal sources using a heat pump

The article describes the economy of exploiting heat from low-temperature geothermal sources for high-temperature heating of buildings using a heat pump. For the exploitation of low-temperature geothermal sources, a two stage heat pump with a heat exchanger was planned. The pump consists of two single stage heat pumps which use different refrigerants at each stage. At stage 1, the calculation of the heat pump is conducted with refrigerant R407c; at stage 2 of the heat pump, the refrigerant R600a is used. The main operational characteristics of a two stage heat pump are presented in the form of diagrams. For the exploitation of heat from geothermal water with a temperature of 45 °C, a profitability evaluation of the investment in the heat pump was carried out, using the method of the net present value. In the research, also the coefficient of profitability and the period of time in which the investment is going to return itself were established.     

The performance of air-source heat pumps in current and future offices

A typical UK office is used as an exemplar for office energy use in the UK. The effect of replacing existing boilers and air-conditioning systems with air-source heat pumps (ASHPs) is investigated for a “current” version of the office, with typical equipment/lighting usage, fabric and internal gains, and also for a “2030” office, where fabric is improved, equipment/lighting made more efficient and, as a result, internal gains reduced. The ASHPs, as a potential carbon-saving technology, performs slightly differently for the two office scenarios. Furthermore, after removing the boiler, it is found to be important whether electric hot water or gas hot water point-of-use heaters are adopted with the ASHPs (assuming that the existing boiler would not be used if the ASHPs is satisfying all space heating requirements). This can be the difference between ASHPs reducing and increasing the carbon emissions of the office. Finally, the carbon intensity of the grid has a large effect on the success of ASHPs technology. This is quantified through a sensitivity analysis, indicating the external conditions for which ASHPs might reduce CO₂ emissions for office buildings. The results suggest that an ASHPs has the potential to reduce CO₂ emissions for certain conditions, but should not be seen as a guaranteed low-carbon technology for all scenarios. As well as assessing the ASHPs as a carbon-saving technology, potential economic benefits are also estimated based on running costs and predicted reduction in energy bills.     

A theoretical study of an innovative ejector enhanced vapor compression heat pump cycle for water heating application

This study presents a novel vapor compression heat pump cycle in which an ejector associated with a subcooler is applied to enhance the heating performance for air-source heat pump water heater application. The heating coefficient of performance (COP_h) and heating capacity of the novel cycle using the non-azeotropic mixture refrigerant R417A are theoretically investigated, for the ranges of evaporating temperature (−15 to 10 °C) and condensing temperature (55-60 °C). The theoretical results show that the COP_h and volumetric heating capacity of the novel cycle are better than that of the conventional heat pump cycle. It is found that for the operating conditions considered, the maximum COP_h and volumetric heating capacity can be improved by up to 1.62-6.92% and 15.20-37.32% over the conventional heat pump cycle, respectively. The performance characteristics of the novel cycle show its promise in air-source heat pump water heater applications.     

Exergoeconomic analysis of the Gonen geothermal district heating system for buildings

This paper presents an application of an exergoeconomic model, through exergy and cost accounting analyses, to the Gonen geothermal district heating system (GDHS) in Balikesir, Turkey for the entire system and its components. This exergoeconomic model is used to reveal the cost formation process and the productive interaction between components. The exergy destructions in the overall Gonen GDHS are quantified and illustrated for a reference temperature of 4 °C. The results indicate that the exergy destructions in the system occur primarily as a result of losses in the cooled geothermal water injected back into the reservoir, pumps, heat exchangers, and pipelines. Total exergy destruction and reinjection exergy of the cooled geothermal water result in 1010 kW (accounting for 32.49%), 320.3 kW (accounting for 10%) of the total exergy input to the Gonen GDHS, respectively. Both energy and exergy efficiencies of the overall Gonen GDHS are also investigated to analyze the system performance, as these efficiencies are determined to be 42% and 50%, respectively. It is found that an increase of the load condition leads to a decrease in the overall thermal costs, which will result in more cost-effective energy systems for buildings.     

Primary energy implications of ventilation heat recovery in residential buildings

In this study, we analyze the impact of ventilation heat recovery (VHR) on the operation primary energy use in residential buildings. We calculate the operation primary energy use of a case-study apartment building built to conventional and passive house standard, both with and without VHR, and using different end-use heating systems including electric resistance heating, bedrock heat pump and district heating based on combined heat and power (CHP) production. VHR increases the electrical energy used for ventilation and reduces the heat energy used for space heating. Significantly greater primary energy savings is achieved when VHR is used in resistance heated buildings than in district heated buildings. For district heated buildings the primary energy savings are small. VHR systems can give substantial final energy reduction, but the primary energy benefit depends strongly on the type of heat supply system, and also on the amount of electricity used for VHR and the airtightness of buildings. This study shows the importance of considering the interactions between heat supply systems and VHR systems to reduce primary energy use in buildings.     

Life cycle primary energy analysis of residential buildings

The space heating demand of residential buildings can be decreased by improved insulation, reduced air leakage and by heat recovery from ventilation air. However, these measures result in an increased use of materials. As the energy for building operation decreases, the relative importance of the energy used in the production phase increases and influences optimization aimed at minimizing the life cycle energy use. The life cycle primary energy use of buildings also depends on the energy supply systems. In this work we analyse primary energy use and CO₂ emission for the production and operation of conventional and low-energy residential buildings. Different types of energy supply systems are included in the analysis. We show that for a conventional and a low-energy building the primary energy use for production can be up to 45% and 60%, respectively, of the total, depending on the energy supply system, and with larger variations for conventional buildings. The primary energy used and the CO₂ emission resulting from production are lower for wood-framed constructions than for concrete-framed constructions. The primary energy use and the CO₂ emission depend strongly on the energy supply, for both conventional and low-energy buildings. For example, a single-family house from the 1970s heated with biomass-based district heating with cogeneration has 70% lower operational primary energy use than if heated with fuel-based electricity. The specific primary energy use with district heating was 40% lower than that of an electrically heated passive row house.     

Heat supply to low-energy buildings in district heating areas: Analyses of CO2 emissions and electricity supply security

In several housing development projects in Norway the requirements related to the mandatory connection to district heating plants have shown to be a barrier for building low-energy residential buildings. The developers have considered the costs related to both low-energy measures and a space heating system that can utilize district heat to be too high to give the project acceptable profitability. In these projects the developers wanted to use a cheaper electric space heating system. Based on models representative for the range of the Norwegian district heating plants, calculations show that the CO₂ emissions related to heating in residential buildings with an energy standard in accordance with the new building regulations and that are connected to the district heating grid, are lower than for similar buildings with a low-energy standard and with heating based on electricity. However, in a long term perspective the differences are marginal when considering the national annual CO₂ emissions. Similarly, increased peak power demand due to electricity-based heating may also be regarded as marginal when compared to the present maximum peak power capacity in Norway.     

Forecasting future cooling demand in London

Cooling of buildings in the UK is responsible for around 15 TWh per year of energy demand, largely powered by electricity with highly related CO₂ emissions. The Greater London Authority wished to understand the potential impact of London's growing need for cooling on UK CO₂ emissions in the period up to 2030. This paper describes a model developed to analyse the cooling requirements for London's key building stock and assess how these would be affected by change in system mix, improvements in system efficiencies, and by varying degrees of climate change.The analysis showed that, if left unchecked, the growth in active cooling systems in London could lead to a doubling of CO₂ emissions from this source by 2030. This growth will be due to increase in building stock, increase in market share of cooling systems, and climate change. The last of these is difficult to predict, but by itself could add 260,000-360,000 tonnes of CO₂ emissions by 2030. This increase can be strongly mitigated, or even offset, by improvements in system efficiency. The difference between no efficiency improvements, and an assumed 1-3% annual efficiency improvement is around 340,000 tonnes by 2030.     

Modelling the carbon-saving performance of domestic ground-source heat pumps

A ground-source heat pump (GSHP) model is formulated and used to estimate the potential of a domestic GSHP as a carbon-saving technology (compared to a conventional gas boiler) with actual thermal data for a dwelling in the UK. The model shows good agreement with current sizing guidelines but shows possible barriers to the carbon-savings (and running cost savings) of GSHPs. This includes an analysis of output temperatures, system sizes and grid CO₂ intensity. The model suggests that GSHPs should be aimed towards a new-build market (rather than retrofit) due to the likely reliance on high surface area/low temperature distribution systems. Also, grid CO₂ intensity needs to be better understood when estimating the electrical consumption CO₂ emissions of GSHPs, both for current and future scenarios.     

The development of a finned phase change material (PCM) storage system to take advantage of off-peak electricity tariff for improvement in cost of heat pump operation

An experimental system consisting a longitudinally finned RT58 phase change material (PCM) in a horizontal cylinder has been conducted to evaluate the heat transfer characteristics of RT58. The investigation forms part of a wider study to investigate a suitable PCM to take advantage of off-peak electricity tariff. The system consisted of a 1.2 m long copper cylinder filled with 93 kg of RT58 with an embedded finned tube at the centre to serve as a heat transfer tube. The experimental data has been reported using hourly temperature profiles, isotherm plots, overall heat transfer coefficients and energy stored. The results show a quadratic relationship between heat transfer coefficient and the inlet HTF temperature within temperature range (62-77 °C) investigated. Increasing charge inlet heat transfer fluid temperature by 21.9% increased heat transfer coefficient by 45.3% during charging and 16.6% during discharge. The potential implication of integrating PCM storage system to an air source heat pump to meet 100% residential heating energy load for common buildings in UK has demonstrated that with an improvement in heat transfer, store size can be reduced by up to 30%.     

A new energy analysis tool for ground source heat pump systems

A new tool, suitable for energy analysis of vertical ground source heat pump systems, is presented. The tool is based on analytical equations describing the heat exchanged with the ground, developed in Matlab^® environment. The time step of the simulation can be freely chosen by the user (e.g. 1, 2 h etc.) and the calculation time required is very short. The heating and cooling loads of the building, at the afore mentioned time step, are needed as input, along with the thermophysical properties of the soil and of the ground heat exchanger, the operation characteristic curves of the system's heat pumps and the basic ground source heat exchanger dimensions. The results include the electricity consumption of the system and the heat absorbed from or rejected to the ground. The efficiency of the tool is verified through comparison with actual electricity consumption data collected from an existing large scale ground coupled heat pump installation over a three-year period.     

A thermodynamic analysis of a transcritical cycle with refrigerant mixture R32/R290 for a small heat pump water heater

In this study, a thermodynamic analysis on the performance of a transcritical cycle using azeotropic refrigerant mixtures of R32/R290 with mass fraction of 70/30 has been performed. The main purpose of this study is to theoretically verify the possibility of applying the chosen refrigerant mixture in small heat pumps for high temperature water heating applications. Performance evaluation has been carried out for a simple azeotropic mixture R32/R290 transcritical cycle by varying evaporator temperature, outlet temperature of gas cooler and compressor discharge pressure. Furthermore, the effects of an internal heat exchanger on the transcritical R32/R290 cycle have been presented at different operating conditions. The results show that high heating coefficient of performance (COP_h) and volumetric heating capacity can be achieved by using this transcritical cycle. It is desirable to apply the chosen refrigerant mixture R32/R290 in small heat pump water heater for high temperature water heating applications, which may produce hot water with temperature up to 90 °C.     

The potential of wastewater heat and exergy: Decentralized high-temperature recovery with a heat pump

There is a large potential in the heat losses from the wastewater leaving a building. We present a novel concept for recovering this heat. Instead of recovering it in a mixed state, the recovery immediately after use is evaluated. This allows the exploitation of the higher temperatures found at the points of warm water usage. By integrating a heat pump to utilize this heat, we can produce a higher temperature heat supply while maintaining a low temperature-lift requirement. This leads to the possibility of directly regenerating the hot water supply through wastewater heat recovery. The concept is a result of research into low exergy building systems, and is part of the IEA ECBCS Annex 49. We have modeled the annual performance of two different system scenarios, which result in a potential average annual coefficient of performance (COP) of over 6. The first scenario supplies up to 4400 kWh of heat for all hot water events with only 790 kWh of electricity, while the second scenario regenerated directly the hot water supply just for bathroom fixtures at 2400 kWh with just 410 kWh of energy. This is a significant reduction in the demand for hot water supply of a building compared to most modern installations.