Carbon emissions reduction and net energy generation analysis in the New Zealand electricity sector through to 2050

Carbon Emissions Pinch Analysis (CEPA) and Energy Return On Energy Investment (ERoEI) analysis are combined to investigate the feasibility of New Zealand reaching and maintaining a renewables electricity target of above 80% by 2025 and 2050, while also increasing electricity generation at an annual rate of 1.5%, and with an increase of electricity generation in the distant future to accommodate a 50% switch to electric vehicle transportation. To meet New Zealand’s growing electricity demand up to 2025 the largest growth in renewable generation is expected to come from geothermal generation (four-fold increase) followed by wind and hydro. To meet expected demand up to 2050 and beyond, including electric vehicle transportation, geothermal generation will expand to 17% of total generation, wind to 16%, and other renewables, such as marine and biomass, will make up about 4%. Including hydro, the total renewable generation in 2050 is expected to reach 82%

Area targeting and storage temperature selection for heat recovery loops

Inter-plant heat integration across a large site can be achieved using a Heat Recovery Loop (HRL). In this paper the relationship between HRL storage temperatures, heating and cooling utility savings (heat recovery) and total HRL exchanger area is investigated. A methodology for designing a HRL based on a ΔTmin approach is compared to three global optimisation approaches where heat exchangers are constrained to have either the same Number of Heat Transfer Units (NTU), Log-Mean Temperature Difference (LMTD) or no constraints (actual global optimum). Analysis is performed using time averaged flow rate and temperature data. Attention is given to understanding the actual temperature driving force of the HRL heat exchangers compared to the apparent driving force as indicated by the composite curves. The cold storage temperature is also varied to minimise the total heat exchanger area. Results for the same heat recovery level show that the ΔTmin approach is effective at minimising total area to within 5 % of the unconstrained global optimisation approach. The study also demonstrates the efficiency of the ΔT min approach to HRL design compared to the other methods which require considerable computational resources

California’s Renewables Portfolio Standard (RPS) requires 33% renewable electricity generation by 2020 - Dream or Reality?

Progress on California’s Renewable Portfolio Standard (RPS), which requires 33% of all retail electricity sales to be served by renewable energy sources by 2020, excluding large hydro, is reported in this paper. The emerging renewable electricity mix in California (CA) and surrounding states which form the Western Electricity Coordination Council (WECC) is analysed using the Carbon Emission Pinch Analysis (CEPA) and Energy Return on Energy Invested (EROI) methodologies. The reduction in emissions with increased renewables is illustrated and the challenge of maintaining high EROI levels for renewable generation is examined for low and high electricity demand growth. The role of the California government in facilitating progress towards a more sustainable renewable electricity future is also highlighted. The investigation shows that wind and solar PV collectively form an integral part of California reaching the 33% renewables target (excluding large hydro) by 2020. Government intervention of tax rebates and subsidies, net electricity metering and a four tiered electricity price has accelerated the uptake of renewable wind and solar PV. Residential uptake of solar PV is also reducing overall California electricity grid demand. Emphasis on new renewable generation is stimulating development of affordable wind and solar technology in California which has the added benefit of enhancing social sustainability through improved employment opportunities at a variety of technical levels

Design and operation methods for better performing heat recovery loops

Inter-plant integration via a heat recovery loop (HRL) is an economic method for increasing total site process energy efficiency of semi-continuous processes. Results show that both the constant storage temperature approach and variable storage temperature approach have merit. Depending on the mix of source and sink streams attached, it may be advantageous to change the operation of an existing HRL from a constant temperature storage to a variable temperature storage. To realise the full benefits of this change in operation, a redistribution of the existing heat exchanger area may be needed

Optimal waste stream discharge temperature selection for dryer operations using thermo-economic assessment

A typical drying process that has liquid and gas discharge streams has been analysed and the impact of selecting various combinations of soft temperatures on heat recovery, utility targets, area targets, capital cost and total cost is reported. The method is based on the plus-minus principle and traditional pinch analysis methods for utility, area and capital cost targeting with the modification of using a ΔT contribution. Results show that there is significant benefit from optimising discharge temperatures for total cost. To achieve minimum energy consumption and total cost, heat recovery from the dryer exhaust air is necessary. Heat recovery from liquid heat sources is shown to be preferable over gas streams due to a higher film coefficient resulting in less heat exchanger area and capital cost. There is also value in making process modifications, such as combining streams or removing small streams to be solely heated by utility, to reduce the number of network heat exchangers. For the best case, the discharge temperatures of the leaving streams are 18.0 °C for water condensate (liquid stream) and 52.4 °C for the exhaust air (gas stream)

Minimising energy use in milk powder production using process integration techniques

Spray drying of milk powder is an energy intensive process and there remains a significant opportunity to reduce energy consumption by applying process integration principles. The ability to optimally integrate the drying process with the other processing steps has the potential to improve the overall efficiency of the entire process, especially when exhaust heat recovery is considered. However, achieving the minimum energy targets established using pinch analysis results in heat exchanger networks that, while theoretically feasible, are impracticable, unrealistic, contain large number of units, and ultimately uneconomic. Integration schemes that are acceptable from an operational point of view are examined in this paper. The use of evaporated water is an important factor to achieve both energy and water reductions. The economics of additional heat recovery seem favourable and exhaust heat recovery is economically justifiable on its own merits, although milk powder deposition should be minimised by selecting an appropriate target temperature for the exhaust air. This will restrict the amount of heat recovery but minimise operational risk from heat exchanger fouling. The thermodynamic constraints caused by the operating temperatures of the dryer and the poor economics exclude the use of heat pumps for exhaust heat recovery in the short to medium term

Optimal stream discharge temperatures for a dryer operation using a thermo-economic assessment

The application of traditional pinch analysis to processes involving waste streams require the discharge temperatures of the waste streams to be estimated prior to performing the pinch analysis

An investigation of milk powder deposition on parallel fins

One method to reduce the energy consumption of industrial milk spray dryers is to recover waste heat from the exhaust dryer air. A significant challenge associated with this opportunity is the air contains a small amount of powder that may deposit on the face and surfaces of a recuperator. This paper introduces a novel lab based test that simulates powder deposition on a bank of parallel plate fins at exhaust dryer air conditions. The fin bank acts like the face of a typical finned tube row in a recuperator. The aim of this study is to look at how deposition on the front of fins is affected by the air conditions. Results show similar characteristics to other milk powder deposition studies that exhibit a dramatic increase in deposition once critical stickiness levels are reached. As powder deposits on the face of the fins, the pressure drop across the bank increases until eventually an asymptote occurs, at which point the rates of deposition and removal are similar. For very sticky conditions, deposition on the face of the fins can cause a rise in the pressure drop by as much as 65%. The pressure drop has also been successfully related to the percentage of open frontal area of the fins with and without deposition. Deposition inside and at the rear of the fin bank was found to be minimal

Tube shape selection for heat recovery from particle-laden exhaust gas streams

Heat recovery from exhaust gas streams is applicable to a wide variety of industries. Two problems encountered in exhaust gas heat recovery are: the high heat transfer resistance of gases and the presence of entrained particulate matter, which can limit the use of extended surface area. Standard heat exchangers use round tube. This study uses Computational Fluid Dynamics (CFD) to investigate whether round or another shape is the best tube selection for exhaust heat recovery. Tube shape rankings are based on taking into account heat transfer, gas flow resistance and foulability. Foulability is inferred from the average wall shear stress around the front or back of each shape. An estimated asymptotic fouling resistance is used to calculate an equivalent fouled j factor, jf. CFD results suggest the best tube for exhaust heat recovery is an elliptical tube. The ellipse shape produced j/f and jf/f ratios (where f is the tube bank friction factor) over 1.5 times larger than that of standard round tube. A flattened round tube is also promising and may be the practical and economic optimum

Options for solar thermal and heat recovery loop hybrid system design

Integration of solar thermal energy into low temperature pinch processes, like dairy and food and beverage processes is more economic when combined with a Heat Recovery Loop (HRL) to form a hybrid inter-plant heat recovery system. The hybrid system shares common infrastructure and improves solar heat utilisation through direct solar boosting of the HRL intermediate fluid’s temperature and enthalpy either through parallel or series application. The challenge of dealing with variable solar energy supply is less of a problem in the hybrid system because the HRL with its associated storage acts as an enthalpy buffer which absorbs temperature and flow rate fluctuations on both the heat supply (including solar) and heat demand side simultaneously. Three options for integrating solar thermal directly into HRLs are applied to a large multi-plant dairy case study to demonstrate the hot utility savings potential of the Solar-HRL hybrid system. HRL performance with Variable Temperature Storage (VTS) and solar is dynamically modelled with historical plant data. The series configuration is shown to be consistently better than parallel configuration for the same thermal storage volumes and similar heat exchanger areas