title: New high temperature transcritical CO2 heat pump concept for spray drying

authors: Kong, Lana

subjects: high temperature heat pump

abstract: The need to decarbonise the industrial sector requires new innovations in high temperature heating. The development of an industrial high temperature heat pump presents not only great emissions reduction potential but also cost saving potential for industrial process heat users. Compared to boilers with efficiencies ranging from 70 to 99%, heat pumps have the ability to leverage renewable electricity to supply heating as a standalone technology (air-source heat pumps), with one unit if electricity being able to supply multiple units of heat (Coefficient of Performance > 1) or integrate with existing boilers to reduce steam demand (air-source or waste heat recovery).
This thesis investigates a novel high temperature transcritical CO2 heat pump concept using a case study in milk powder spray drying. A series of cycle architectures were modelled that incorporated internal heat exchangers and cascade arrangements to assess their effect on thermodynamic performance for not only large temperature lifts (>150 °C) but also large temperature glides (>150 °C). A transcritical-transcritical cascade configuration was found to achieve a sink temperature of 200 °C with a COP of 2.22 at moderate discharge pressures (150 bar) compared to what is currently available in the literature (>200 bar). Expansion work recovery, using ejectors and expanders, was further explored to reduce exergy losses, achieving incremental COP gains up to 2.35. However, it was determined that, at this stage, the marginal increases in COP did not overcome the challenges in complexity of integrating an expander or ejector within the system.

A comparison of integration options was undertaken that compared the high temperature heat pump integration with fuel switching to electrode or biomass boilers. Hybrid configurations of heat pump and boilers were also assessed at varying utility prices to determine whether an optimum sink temperature existed to minimise the operational cost. With this, a multi-temperature heat pump configuration was found to improve the COP of the heat pump to 2.55 for air heating to 200 °C, which emphasises the importance of temperature profile matching in heat pump design. Operating costs depended on the electricity-to-biomass price ratio and grid emissions intensity. In all cases, electrode boiler conversions had the highest operational cost. Price ratios below 2.75 were favourable for full electrification using a heat pump, while price ratios above 3.67 favoured a full biomass conversion. Between these two price ratios, the optimal hybrid configuration was to supply the air heating up to the temperature level for the fluidised bed (102 °C) using a heat pump and supply the remainder of the heating using a biomass boiler. In comparison to coal, the proposed solutions achieved emissions savings between 55 and 94 %.

The wider potential for the high temperature heat pump concept was then estimated, revealing a broader potential for CO2 high temperature heat pumps to supply hundreds of petajoules per year of low-carbon heat across a diverse range of industrial sectors. Due to being able to produce both large temperature glides and large temperature lifts, the CO2 high temperature heat pump was particularly suited for a range of drying processes (further than milk powder spray drying). Potential source and sink streams for the heat pump were identified for each application to estimate possible COPs, which ranged between 2.0 and 3.05 for sink outlet temperatures ranging from 110 to 280 °C. Emissions reduction was shown to be viable for nearly all regions when displacing coal, but requires cleaner electricity grids or higher COPs when displacing natural gas.

This thesis contributes an in depth thermodynamic analysis of both a novel high temperature heat pump cycle architecture and a novel application of high temperature heat pumps into the milk powder spray drying process. Additionally, this thesis contributes a cross-sectoral analysis for implementation, collating potential source and sink streams across a variety of applications.

text: New high temperature transcritical CO2 heat pump concept for spray drying The need to decarbonise the industrial sector requires new innovations in high temperature heating. The development of an industrial high temperature heat pump presents not only great emissions reduction potential but also cost saving potential for industrial process heat users. Compared to boilers with efficiencies ranging from 70 to 99%, heat pumps have the ability to leverage renewable electricity to supply heating as a standalone technology (air-source heat pumps), with one unit if electricity being able to supply multiple units of heat (Coefficient of Performance > 1) or integrate with existing boilers to reduce steam demand (air-source or waste heat recovery).
This thesis investigates a novel high temperature transcritical CO2 heat pump concept using a case study in milk powder spray drying. A series of cycle architectures were modelled that incorporated internal heat exchangers and cascade arrangements to assess their effect on thermodynamic performance for not only large temperature lifts (>150 °C) but also large temperature glides (>150 °C). A transcritical-transcritical cascade configuration was found to achieve a sink temperature of 200 °C with a COP of 2.22 at moderate discharge pressures (150 bar) compared to what is currently available in the literature (>200 bar). Expansion work recovery, using ejectors and expanders, was further explored to reduce exergy losses, achieving incremental COP gains up to 2.35. However, it was determined that, at this stage, the marginal increases in COP did not overcome the challenges in complexity of integrating an expander or ejector within the system.

A comparison of integration options was undertaken that compared the high temperature heat pump integration with fuel switching to electrode or biomass boilers. Hybrid configurations of heat pump and boilers were also assessed at varying utility prices to determine whether an optimum sink temperature existed to minimise the operational cost. With this, a multi-temperature heat pump configuration was found to improve the COP of the heat pump to 2.55 for air heating to 200 °C, which emphasises the importance of temperature profile matching in heat pump design. Operating costs depended on the electricity-to-biomass price ratio and grid emissions intensity. In all cases, electrode boiler conversions had the highest operational cost. Price ratios below 2.75 were favourable for full electrification using a heat pump, while price ratios above 3.67 favoured a full biomass conversion. Between these two price ratios, the optimal hybrid configuration was to supply the air heating up to the temperature level for the fluidised bed (102 °C) using a heat pump and supply the remainder of the heating using a biomass boiler. In comparison to coal, the proposed solutions achieved emissions savings between 55 and 94 %.

The wider potential for the high temperature heat pump concept was then estimated, revealing a broader potential for CO2 high temperature heat pumps to supply hundreds of petajoules per year of low-carbon heat across a diverse range of industrial sectors. Due to being able to produce both large temperature glides and large temperature lifts, the CO2 high temperature heat pump was particularly suited for a range of drying processes (further than milk powder spray drying). Potential source and sink streams for the heat pump were identified for each application to estimate possible COPs, which ranged between 2.0 and 3.05 for sink outlet temperatures ranging from 110 to 280 °C. Emissions reduction was shown to be viable for nearly all regions when displacing coal, but requires cleaner electricity grids or higher COPs when displacing natural gas.

This thesis contributes an in depth thermodynamic analysis of both a novel high temperature heat pump cycle architecture and a novel application of high temperature heat pumps into the milk powder spray drying process. Additionally, this thesis contributes a cross-sectoral analysis for implementation, collating potential source and sink streams across a variety of applications.

year: 2026

title: How pro-poor is the selection of seasonal migrant workers from Tonga under New Zealand’s Recognised Seasonal Employer (RSE) Program?

authors: Gibson, John

subjects: development

abstract: Temporary migration programs for unskilled workers are increasingly being proposed as a way to both relieve labour shortages in developed countries and aid development in sending countries without entailing many of the costs associated with permanent migration. New Zealand’s new Recognised Seasonal Employer (RSE) program is designed with both these goals in mind, enabling unskilled workers from the Pacific Islands to work in horticulture and viticulture in New Zealand for a period of up to seven months. However, the development impact on a sending country will depend not only on how many workers participate, but also on who participates. This paper uses new survey data from Tonga to examine the process of selecting Tongans to work in the RSE, and to analyze how pro-poor the recruitment process has been to date. We find that the workers recruited come from largely agricultural backgrounds and have lower average incomes and schooling levels than Tongans not participating in the program. We also compare the characteristics of RSE workers to those of Tongans applying to permanently migrate to New Zealand through the Pacific Access Category, and find the RSE workers to be more rural and less educated. The RSE therefore does seem to have succeeded in creating new opportunities for relatively poor and unskilled Tongans to work in New Zealand.

text: How pro-poor is the selection of seasonal migrant workers from Tonga under New Zealand’s Recognised Seasonal Employer (RSE) Program? Temporary migration programs for unskilled workers are increasingly being proposed as a way to both relieve labour shortages in developed countries and aid development in sending countries without entailing many of the costs associated with permanent migration. New Zealand’s new Recognised Seasonal Employer (RSE) program is designed with both these goals in mind, enabling unskilled workers from the Pacific Islands to work in horticulture and viticulture in New Zealand for a period of up to seven months. However, the development impact on a sending country will depend not only on how many workers participate, but also on who participates. This paper uses new survey data from Tonga to examine the process of selecting Tongans to work in the RSE, and to analyze how pro-poor the recruitment process has been to date. We find that the workers recruited come from largely agricultural backgrounds and have lower average incomes and schooling levels than Tongans not participating in the program. We also compare the characteristics of RSE workers to those of Tongans applying to permanently migrate to New Zealand through the Pacific Access Category, and find the RSE workers to be more rural and less educated. The RSE therefore does seem to have succeeded in creating new opportunities for relatively poor and unskilled Tongans to work in New Zealand.

year: 2008-06

title: A study of iwi communication between Te Tau Ihu iwi and intergenerational diasporic whānau

authors: Wright, Karamea Moana

subjects: diaspora

abstract: This study looks at the online communication methods used between Te Tau Ihu iwi and intergenerational uri who live in the diaspora. Preferences and perceptions of methods of communication utilized by both iwi, and tāngata who live in the overseas diaspora were explored. Most important are the implications for uri living away from Aotearoa, and their desire to remain connected to iwi in New Zealand. This is a mixed methods study, comprising of a survey of fifty-eight emigrant respondents living abroad, and interviews with one member of the first, second, and third generations of emigrants to the United States, with affiliation to at least one of the eight tribes who claim tāngata whenua status of Te Tau Ihu.

The implications of the findings from the study provide both iwi and tāngata the opportunity to create a synergistic relationship of communicating. Assisting iwi to develop and implement a strategy for relevant communication, and iwi members engaging in a dialogue with iwi about relevance of communication is critical in strengthening ties between iwi and future generations regardless of location. “He aha te mea nui, he tangata, he tangata.”

text: A study of iwi communication between Te Tau Ihu iwi and intergenerational diasporic whānau This study looks at the online communication methods used between Te Tau Ihu iwi and intergenerational uri who live in the diaspora. Preferences and perceptions of methods of communication utilized by both iwi, and tāngata who live in the overseas diaspora were explored. Most important are the implications for uri living away from Aotearoa, and their desire to remain connected to iwi in New Zealand. This is a mixed methods study, comprising of a survey of fifty-eight emigrant respondents living abroad, and interviews with one member of the first, second, and third generations of emigrants to the United States, with affiliation to at least one of the eight tribes who claim tāngata whenua status of Te Tau Ihu.

The implications of the findings from the study provide both iwi and tāngata the opportunity to create a synergistic relationship of communicating. Assisting iwi to develop and implement a strategy for relevant communication, and iwi members engaging in a dialogue with iwi about relevance of communication is critical in strengthening ties between iwi and future generations regardless of location. “He aha te mea nui, he tangata, he tangata.”

year: 2019