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%

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

Reducing Undesirable Powder Deposition

This paper describe how to reduce powder deposition

Heat Integrated Milk Powder Production

Dairy processing is critical to New Zealand’s (NZ) economy producing NZ$13 billion in exports for 2012 while consuming 32 PJ of fossil fuels for process heat. Three quarters of NZ dairy exports are milk powders. This thesis presents methods to reduce process heat use in Milk Powder Plants (MPP) through improved heat integration and addresses key technical challenges preventing industrial implementation. My original contributions to literature include: (1) a novel design method called the Cost Derivate Method (CDM) that cost optimally allocates area in direct heat exchange networks, (2) a new design methodology for integration of semi-continuous process clusters using a Heat Recovery Loop (HRL) with a Variable Temperature Storage (VTS) system for improved heat recovery, (3) an experimentally validated deposition model for predicting critical air conditions that cause milk powder fouling, and (4) a thermo-economic assessment tool for the optimisation of industrial spray dryer exhaust heat recovery projects via a Liquid Coupled Loop Heat Exchanger (LCHE) system. By applying Pinch Analysis to an industrial MMP, this work confirms that heat must be recovered from the milk spray dryer exhaust air (~75 °C) to achieve maximum heat integration in MPPs. For stand-alone MPPs exhaust heat is best used to indirectly preheat the inlet dryer air reducing steam use by 12.7 A key barrier preventing exhaust heat recovery implementation in NZ MPPs is the possibility of milk powder fouling. Dryer exhaust air contains a low concentration of powder that when exposed to low temperatures at high humidity becomes sticky. For a heat exchanger face air velocity of 4 m/s, experimental data from milk powder fouling tests of flat plates, tubes and fins indicates particulate fouling becomes severe when the exhaust air temperature reaches 55 °C. Higher face velocities are shown to lower this critical exhaust temperature for avoiding severe fouling, which gives potential for increased heat recovery but for increased pressure drop. Lower face velocities show the opposite effect. Designing exhaust heat recovery systems entail an acute trade-off between heat transfer, pressure drop and fouling. Two important design parameters are the number of tube rows in the exhaust heat exchanger and the face velocity. The outputs of a thermo-economic spreadsheet tool suggest LCHE systems for a dryer producing 23.5 t/h is economic. With a face velocity of 4 m/s and 14 rows of finned round tube, the project had an estimated payback of 1.6 years, a net present value of NZ$3 million and internal rate of return of 71 %. This tool will empower industry with greater confidence to uptake exhaust heat recovery technology as a vital method for improving the heat integration of MPPs in NZ

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

Integration of solar heating into heat recovery loops using constant and variable temperature storage

Solar is a renewable energy that can be used to provide process heat to industrial sites. Solar is extremely variable and to use it reliably thermal storage is necessary. Heat recovery loops (HRL) are an indirect method for transferring heat from one process to another using an intermediate fluid (e.g. water, oil). With HRL’s thermal storage is also necessary to effectively meet the stop/start time dependent nature of the multiple source and sink streams. Combining solar heating with HRL’s makes sense as a means of reducing costs by sharing common storage infrastructure and pipe transport systems and by lowering nonrenewable hot utility demand. To maximise the value of solar in a HRL, the means of controlling the HRL needs to be considered. In this paper, the HRL example and design method of Walmsley et al. (2013) is employed to demonstrate the potential benefits of applying solar heating using the HRL variable temperature storage (VTS) approach and the conventional HRL constant temperature storage (CTS) approach. Results show the VTS approach is superior to the CTS approach for both the non-solar and solar integration cases. When the pinch is around the hot storage temperature the CST approach is constrained and the addition of solar heating to the HRL decreases hot utility at the expenses of increased cold utility. For the VTS approach the hot storage pinch shifts to a cold storage pinch and increased heat recovery is possible for the same exchanger area without solar. With solar the VTS approach can maintain the same heat recovery while also reducing hot utility still further due to the presence of solar, but only with additional area. When the pinch is located around the cold storage temperature, solar heating can be treated as an additional heat source and the benefits of CTS and VTS are comparable

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

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

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

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