Environomic multi-objective optimisation of a district heating network considering centralized and decentralized heat pumps

Concern for the environment has been steadily growing in recent years, and it is becoming more common to include environmental impact and pollution costs in the design problem along with construction, investment and operating costs.     

Numerical simulation of steady state heat transfer in a ceramic-coated gas turbine blade

As gas turbine entry temperature (TET) increases, thermal loading on first stage blades increases and, therefore, a variety of cooling techniques and thermal barrier coatings (TBCs) are used. In the present work, steady state blade heat transfer mechanisms were studied via numerical simulations. Convection and radiation to the blade external surface were modeled for a super alloy blade with and without a TBC. The effects of surface emissivity changes, partial TBC coatings and uncertainties in external heat transfer coefficient were also simulated. The results show that at 1500 K TET, radiation heat transfer rate from gas to an uncoated blade is 8.4% of total heat transfer rate which decreases to 3.4% in the presence of a TBC. The TBC blocks radiation, suppresses metal temperatures and reduces heat loss to the coolant. These effects are more pronounced at higher TETs. With selective coating, substantial local temperature suppression occurs. In the presence of radiation and/or TBC, the uncertainties in convection heat transfer coefficient do not have a significant effect on metal temperatures.     

The role of actor-networks in the early stage mobilisation of low carbon heat networks

Low carbon heat networks (LCHNs) offer great potential for carbon and heating cost reduction. Despite these benefits, LCHNs provide for just two per cent of heat demand in the UK, when estimates suggest they have the potential to provide for around 43 per cent. These low levels of LCHN provision are in stark contrast to the Nordic nations which exemplify some of the highest quality and most extensive heat networks in the world. It is within this context that the Pioneer Cities project (the project) was launched by the UK government to help local authorities overcome barriers to the deployment of LCHNs. This paper reports the findings of an evaluation of this project, drawing on 86 interviews across five local authorities, analysed using elements of Actor Network Theory (ANT). The evaluation found that the project’s success has been limited. Participating local authorities have encountered challenges regarding marketisation, public sector retrenchment and inexperience in mobilising LCHNs. These factors militate against the formation of the robust actor-networks required to deploy LCHNs. Analysis using ANT reveals insights into why LCHNs remain elusive in the UK and suggests that policy makers need to strengthen local authorities’ ability to lead and deliver complex infrastructure projects.     

The performance of an open-loop lake water heat pump system in south China

A district heating and cooling (DHC) system that utilizes lake water as heat source–sink of heat pumps has been constructed in Xiangtan, a city in Hunan province in south China. An initial analytical study had been carried out before the construction. In this paper, a simplified two-dimensional model is developed to simulate the steady lake water temperature (LWT) distribution during continuous operation. The simulation results indicate that the impacts of the discharge on entering water temperatures (EWT) and the ecological environment of lake are acceptable. Field test results showed that the COP values of the system were, respectively, 0.7–0.85 higher in cooling season and about 0.46 higher in heating season than those of the air-source heat pump (ASHP) units at the same sink and source temperatures. An acceptable payback period of 5.6 years was found through an economic analysis based on the comparison between the initial and operating costs of the system and those of the distributed ASHP units that would have been installed according to initial scheme.     

Heat accumulators in CHP networks

In a Combined Heat and Power (CHP) network, it is sometimes optimal to install a device for storing heat from one period of time to another. Several possibilities exist. If the electricity demand is high, while at the same time the district heating load is too small to take care of the heat from the CHP plant, it could be optimal to store heat from peak periods and discharge the storage under off-peak. It might also be optimal to store heat during off-peak and use it under the district heating peak load. The storage is then used for decreasing either the district heating demand or for decreasing the electricity load used for space heating. The paper shows how a mixed integer program is developed for use in the optimization process. As a case study, the CHP system of Malmö, Sweden, is used. Further, a sensitivity analysis is elaborated in order to show how the optimal solution will vary due to changes in certain input data.     

Combined heat and power in industry and buildings

Combined heat and power (CHP) has huge potential to deliver energy savings and emissions reductions, and in many cases cost reductions too. But the market and regulatory framework is the key to delivering large-scale installations, and government has a poor record in delivering an appropriate framework.     

Inverse problem and variation method to optimize cascade heat exchange network in central heating system

Urban heating in northern China accounts for 40％ of total building energy usage.In central heating systems,heat is often transfened from heat source to users by the heat network where several heat exchangers arc installed at heat source,substations and terminals respectively.For given overall heating capacity and heat source temperarure,increasing the terminal fluid temperature is an effective way to improve the thermal performance of such cascade heat exchange network for energy saving.In this paper,the mathematical optimization model of the cascade heat exchange network with three-stage heat exchangers in series is established.Aim at maximizing the cold fluid temperature for given hot fluid temperature and overall heating capacity,the optimal heat exchange area distribution and the medium fluids' flow rates are determined through inverse problem and variation method.The preliminary results show that the heat exchange areas should be distributed equally for each heat exchanger.It also indicates that in order to improve the thernmal performance of the whole system,more heat exchange areas should be allocated to the heat exchanger where flow rate difference between two fluids is relatively small.This work is important for guiding the optimization design of practical cascade heating systems.     

On the robustness, effectiveness and reliability of chemical and mechanical heat pumps for low-temperature heat source district heating: A comparative simulation-based analysis and evaluation

Our physical environment is endowed with unlimited amount of natural and artificial sources of energy at various low exergetic levels which leaves them almost impossible to be thermally utilized at such source states. The thermal upgrading of these low exergetic energy sources could render them amenable to various practical thermal usages. This paper provides a comparative study through simulations, of the effectiveness, robustness and reliability of the often two most promising heat upgrading technologies (the chemical and mechanical heat pumps) systems for the sustainable heat upgrading of low-temperature heat sources for district heating. The simulation results reveal that for a low to medium energy demand, low-temperature heat source upgrading using the chemical heat pumps seems more promising than the mechanical heat pumps, while the mechanical heat pump is best suited for high energy demand space heating. In the simulation, use was made of an artificial low-temperature heat source (a nearby pharmaceutical industry waste heat) with source temperature state of between 25 and 35 °C as the feed to the upgrading units. The high energy demand (assumed to be able to serve the space heating energy requirements of a given locality) was estimated to be 923 TJ/annum at a consumer-side temperature level of about 95 °C.

The selected most robust system (the mechanical heat pump-based process) for upgrading the low-temperature heat source at such given high energy demand has been conceptually designed.     

Condensing boiler applications in the process industry

Major challenging issues such as climate change, energy prices and fuel security have focussed the attention of process industries on their energy efficiency and opportunities for improvement. The main objective of this research study was to investigate technologies needed to exploit the large amount of low grade heat available from a flue gas condensing system through industrial condensing boilers. The technology and application of industrial condensing boilers in various heating systems were extensively reviewed. As the condensers require site-specific engineering design, a case study was carried out to investigate the feasibility (technically and economically) of applying condensing boilers in a large scale district heating system (40 MW). The study showed that by recovering the latent heat of water vapour in the flue gas through condensing boilers, the whole heating system could achieve significantly higher efficiency levels than conventional boilers. In addition to waste heat recovery, condensing boilers can also be optimised for emission abatement, especially for particle removal. Two technical barriers for the condensing boiler application are corrosion and return water temperatures. Highly corrosion-resistant material is required for condensing boiler manufacture. The thermal design of a “case study” single pass shell-and-tube condensing heat exchanger/condenser showed that a considerable amount of thermal resistance was on the shell-side. Based on the case study calculations, approximately 4900 m² of total heat transfer area was required, if stainless steel was used as a construction material. If the heat transfer area was made of carbon steel, then polypropylene could be used as the corrosion-resistant coating material outside the tubes. The addition of polypropylene coating increased the tube wall thermal resistance, hence the required heat transfer area was approximately 5800 m². Net Present Value (NPV) calculations showed that the choice of a carbon steel condenser ensured cash return in a relatively shorter period of time (i.e. 2 years) when compared to a stainless steel condenser (i.e. 5-7 years). Moreover, the NPV for the stainless steel was more sensitive to the change of the interest rate.     

Co-generation of biofuels for transportation and heat for district heating systems—an assessment of the national possibilities in the EU

Biomass gasification with subsequent synthesis to liquid or gaseous biofuels generates heat possible to use in district heating (DH) systems. The purpose here is to estimate the heat sink capacity of DH systems in the individual EU nations and assess the possibilities for biomass-gasification-based co-generation of synthetic biofuels for transportation and heat (CBH) for DH systems in the EU countries. The possibilities are assessed (i) assuming different levels of competiveness relative to other heat supply options of CBH corresponding to the EU target for renewable energy for transportation for 2020 and (ii) assuming that the potential expansion of the DH systems by 2020 is met with CBH. In general, the size of the DH heat sinks represented by the existing national aggregated DH systems can accommodate CBH at a scale that is significant compared to the 2020 renewable transportation target. The possibilities for CBH also depend on its cost-competitiveness compared to, e.g., fossil-fuel-based CHP. The possible expansion of the DH systems by 2020 represents an important opportunity for CBH and is also influenced by the potential increase in the use of other heat supply options, such as, industrial waste heat, waste incineration, and CHP.     

Seasonal performance rating of heat pump water heaters

Seasonal performance evaluation methods for water heaters are reviewed and an experimental method for rating air-source heat pump water heaters is presented. The rating method is based on measured heat pump performance during heat-up operation of particular products rather than a generic simulation model of heat pump performance. The measured performance is used in a correlation model of the heat pump unit in an annual load-cycle system performance simulation based on the TRNSYS simulation package.     

Uncertainties in physical property effects on viscous flow and heat transfer over a nonlinearly stretching sheet with nanofluids

The purpose of this paper is to investigate a numerical analysis for the flow and heat transfer in a viscous fluid over a nonlinear stretching sheet utilizing nanofluid. The governing partial differential equations are converted into highly nonlinear ordinary differential equations by a similarity transformation. Different water-based nanofluids containing Cu, Ag, CuO, Al₂O₃, and TiO₂ are considered in our problem. Furthermore, four different models of nanofluid based on different formulas for thermal conductivity and dynamic viscosity on the flow and heat transfer characteristics are discussed. The variations of dimensionless surface temperature, dimensionless surface temperature gradient as well as the flow and heat transfer characteristics with the governing parameters are graphed and tabulated. Comparison with published results for pure fluid flow is presented and it is found to be in excellent agreement.     

Heat exchanger optimization for geothermal district heating systems: A fuel saving approach

One of the most commonly used heating devices in geothermal systems is the heat exchanger. The output conditions of heat exchangers are based on several parameters. The heat transfer area is one of the most important parameters for heat exchangers in terms of economics. Although there are a lot of methods to optimize heat exchangers, the method described here is a fairly easy approach. In this paper, a counter flow heat exchanger of geothermal district heating system is considered and optimum design values, which provide maximum annual net profit, for the considered heating system are found according to fuel savings. Performance of the heat exchanger is also calculated. In the analysis, since some values are affected by local conditions, Turkey's conditions are considered.     

Assessment of a domestic heat pump 1: The house/heat-pump interaction

The operating environment for a domestic air-air heat pump installation is examined using data from a field monitoring study. The unit, which is used for heating only, has a seasonal COP of 2.7 ± 0.2. The system experiences a wide range of return air temperatures, which are also low compared with those of normal designs. We show that this is partly caused by the heating patterns adopted by the occupants, by which the primary living area is well heated but the secondary area receives background heating only.     

Effect of heat exchanger material and fouling on thermoelectric exhaust heat recovery

Thermoelectric devices are being investigated as a means of improving fuel economy for diesel and gasoline vehicles through the conversion of wasted fuel energy, in the form of heat, to useable electricity. By capturing a small portion of the energy that is available with thermoelectric devices can reduce engine loads thus decreasing pollutant emissions, fuel consumption, and CO₂ to further reduce green house gas emissions. This study is conducted in an effort to better understand and improve the performance of thermoelectric heat recovery systems for automotive use. For this purpose an experimental investigation of thermoelectrics in contact with clean and fouled heat exchangers of different materials is performed. The thermoelectric devices are tested on a bench-scale thermoelectric heat recovery apparatus that simulates automotive exhaust. It is observed that for higher exhaust gas flowrates, thermoelectric power output increases from 2 to 3.8 W while overall system efficiency decreases from 0.95% to 0.6%. Degradation of the effectiveness of the EGR-type heat exchangers over a period of driving is also simulated by exposing the heat exchangers to diesel engine exhaust under thermophoretic conditions to form a deposit layer. For the fouled EGR-type heat exchangers, power output and system efficiency is observed to be 5-10% lower for all conditions tested.     

Practical modelling of micro combined heat and power applications for local authority district heating schemes

The primary purposes of this paper are to describe a practical model which can be used to assist economic evaluation of district heating proposals, with particular reference to potential micro combined heat and power (CHP) applications, and to discuss the results of applying this model to a proposal for a particular scheme. The problems of realistic simulation of demand patterns, and the sensitivity of micro-CHP project values to scale, fuel prices, differential purchase/selling tariffs for electricity, duty cycle and plant utilization factors are discussed. Of several options, two were shown to be economically viable, relative to existing methods of supply (gas-fired boilers); however, the margin of benefit is small relative to wisely chosen modern boiler installations. A brief discussion of alternative methods of finance is provided; ceteris paribus, the proposed scheme would not be likely to attract venture capital from would-be lease-hire agencies. There is scope however for pilot schemes, whose results could be used to define more closely the limits of uncertainty of, for example, annual availability and duty cycle influences on the cost economics of operations.     

The relative competitive positions of the alternative means for domestic heating

This study strives to clarify to what extent the improved insulation of new buildings affects the economically rational choice of heating supply. In the Danish context, natural gas furnaces show to be economically rational for the society only in few cases. District heating should be advocated only for areas with a high concentration of heat demand, primarily areas with apartment buildings. An increased use of heat pumps (HP) should be advocated as the ambient air HP show to be the cheapest of the individual heating systems—and in most cases cheaper than the collective heating systems. The taxation of energy products in relation to space heating should be revised as the economics from the consumer's point of view do not comply with those from the society's point of view.