The development and calibration of a generic dynamic absorption chiller model

Although absorption cooling has been available for many years, the technology has typically been viewed as a poorly performing alternative to vapour compression refrigeration. Rising energy prices and the requirement to improve energy efficiency is however driving renewed interest in the technology, particularly within the context of combined cooling, heat and power systems (CCHP) for buildings. In order to understand the performance of absorption cooling, numerous models are available in the literature. However, the complexities involved in the thermodynamics of absorption chillers have so far restricted researchers to creating steady state or dynamic models reliant on data measurements of the internal chiller state, which require difficult-to-obtain, intrusive measurements. The pragmatic, yet fully-dynamic model described in this paper is designed to be easily calibrated using data obtained from the measurements of inflows and outflows to a chiller, without resorting to intrusive measurements. The model comprises a series of linked control volumes featuring both performance maps and lumped mass volumes, which reflect the underlying physical structure of the device. The model was developed for the ESP-r building simulation tool. This paper describes the modelling approach, theory and limitations, along with its calibration and the application of the model to a specific example

The influence of thermal storage on microgeneration flexibility

In a future power system, the ability to manipulate generation and load will be a critical factor in providing a secure and stable supply of electrical energy to consumers. Using a simulation-based approach, this study assesses the ability of thermal storage to help deliver flexibility in the operation of domestic micro-generation technologies without sacrificing householder comfort and convenience. A typical UK detached dwelling is modelled along with its heating system, which features a retro-fitted air source heat pump (ASHP). The model is used to determine the maximum possible temporal shift for different capacities and configurations of thermal storage, taking into account the influence of climate, building fabric, control settings and occupancy. The limits of time shifting are dictated by the living space temperature and the hot water temperature delivered to the occupants. The storage mechanisms examined are: the basic thermal inertia of the building fabric; increasing the space heating set point temperatures to increase fabric storage and inserting a dedicated thermal buffer between the ASHP and the heat distribution system. The simulation results indicate that back-shifting of the ASHP start/stop times of between one and two hours are possible without causing serious discomfort or inconvenience to the occupants

The influence of thermal storage on microgeneration flexibility

In a future power system, the ability to manipulate generation and load will be a critical factor in providing a secure and stable supply of electrical energy to consumers. Using a simulation-based approach, this study assesses the ability of thermal storage to help deliver flexibility in the operation of domestic micro-generation technologies without sacrificing householder comfort and convenience. A typical UK detached dwelling is modelled along with its heating system, which features a retro-fitted air source heat pump (ASHP). The model is used to determine the maximum possible temporal shift for different capacities and configurations of thermal storage, taking into account the influence of climate, building fabric, control settings and occupancy. The limits of time shifting are dictated by the living space temperature and the hot water temperature delivered to the occupants. The storage mechanisms examined are: the basic thermal inertia of the building fabric; increasing the space heating set point temperatures to increase fabric storage and inserting a dedicated thermal buffer between the ASHP and the heat distribution system. The simulation results indicate that back-shifting of the ASHP start/stop times of between one and two hours are possible without causing serious discomfort or inconvenience to the occupants

Impact of residential energy system sizing and control over heat pump’s system cost and reliability

Using a simulation-based approach, this work analyses the impact that different energy unit sizes and control methodologies will have over the capital and running costs of an air source heat pump (ASHP) system to be installed in a refurbished dwelling. A total of 9 different heating configuration options were investigated and the cumulative cash flow over a period of 10 years (including initial investment) was utilized to compare the systems from a customer perspective. Additionally, in selected cases, the cycling of the heat pump was calculated in order to estimate the life-span of the device. The building and heat pump systems were simulated using TRNSYS energy system models. The results revealed the sensitivity of the system’s costs and life-span to its operating characteristics. For example, operating the system as a direct gas boiler replacement resulted in capital costs above £10,000 and the unit's life span reduced by half in comparison to more favourable operational strategies. The results highlight the fact that the successful technical and financial performance of heat pumps within the UK’s residential market will depend of designers, installers and end-users’ awareness regarding optimal operational strategies for this technology

Performance assessment of tariff-based air source heat pump load shifting in a UK detached dwelling featuring phase change-enhanced buffering

Using a detailed building simulation model, the amount of thermal buffering, with and without phase change material (PCM), needed to time-shift an air source heat pump's operation to off-peak periods, as defined by the UK 'Economy 10' tariff, was investigated for a typical UK detached dwelling. The performance of the buffered system was compared to the case with no load shifting and with no thermal buffering. Additionally, the load shifting of a population of buffered heat pumps to off-peak periods was simulated and the resulting change in the peak demand on the electricity network was assessed. The results from this study indicate that 1000 L of hot water buffering or 500 L of PCM-enhanced hot water buffering was required to move the operation of the heat pump fully to off-peak periods, without adversely affecting the provision of space heating and hot water for the end user. The work also highlights that buffering and load shifting increased the heat pump's electrical demand by over 60% leading to increased cost to the end user and increased CO2 emissions (depending on the electricity tariff applied and time varying CO2 intensity of the electricity generation mix, respectively). The study also highlights that the load-shifting of populations of buffered heat pumps wholly to off-peak periods using crude instruments such as tariffs increased the peak loading on the electrical network by over 50% rather than reducing it and that careful consideration is needed as to how the load shifting of a group of heat pumps is orchestrated

The implementation of discrete demand management algorithms within energy systems modelling

Traditionally, demand side management (DSM) programs have been driven by utilities. With the prospect of growth in the utilization of building-integrated micro-generation, DSM offers opportunities for additional energy savings and CO2 emission reductions through better utilisation of local renewable energy resources. This paper examines the feasibility of using discreet demand management (DDM) to improve the supply/demand match. For many combinations of micro-generation and DDM controls, it is necessary to know the environmental conditions (i.e. temperatures and lighting levels) within the buildings being modelled. One method would be to embed all the renewable energy technologies and DDM algorithms within a detailed simulation program. An alternative method, investigated in this study, involves coupling two existing tools: a dynamic building simulation program (ESP-r) and a demand/supply matching program (MERIT) that incorporates DDM algorithms and renewable energy system technologies. These two programs interact at the time-step level and exchange calculated parameters (relating to loads, supply potentials and prevailing environmental conditions) to enable an evaluation of DDM techniques in terms of energy saving and occupant impact. This paper describes the technique and presents simulation results relating to a number of building cases

Neutrino-nucleus coherent scattering as a probe of neutron density distributions

Neutrino-nucleus coherent elastic scattering provides a theoretically appealing way to measure the neutron part of nuclear form factors. Using an expansion of form factors into moments, we show that neutrinos from stopped pions can probe not only the second moment of the form factor (the neutron radius) but also the fourth moment. Using simple Monte Carlo techniques for argon, germanium, and xenon detectors of 3.5 tonnes, 1.5 tonnes, and 300 kg, respectively, we show that the neutron radii can be found with an uncertainty of a few percent when near a neutrino flux of $3\times10^{7}$ neutrinos/cm$^{2}$/s. If the normalization of the neutrino flux is known independently, one can determine the moments accurately enough to discriminate among the predictions of various nuclear energy functionals.Comment: 10 pages, 5 figure

A daily representation of Great Britain's energy vectors : Natural gas, electricity and transport fuels

In much of Europe there is a strong push to decarbonise energy demands, including the largest single end-use demand – heat. Moving heat demands over to the electrical network poses significant challenges and the use of hybrid energy vector and storage systems (heat and electrical storage) will be a critical component in managing this transition. As an example of these challenges (facing many developed countries), the scale of recently available daily energy flows through the UK’s electrical, gas and transport systems are presented. When this data is expressed graphically it illustrates important differences in the demand characteristics of these different vectors; these include the quantity of energy delivered through the networks on a daily basis, and the scale of variability in the gas demand over multiple timescales (seasonal, weekly and daily). As the UK proceeds to migrate heating demands to the electrical network in its drive to cut carbon emissions, electrical demand will significantly increase. Additionally, the greater variability and uncertainty shown in the gas demand will also migrate to the electrical demand posing significant difficulties for the maintenance of a secure and reliable electrical system in the coming decades. The paper concludes an analysis of the different means of accommodating increasingly volatile electricity demands in future energy networks

Household-differentiated demand modelling for communities

Micro-generation schemes are increasingly being proposed for and incorporated in new-build housing developments. Community composition dependant demand prediction for such schemes is poorly understood and modelled. Using a previously developed higher-order Markovchain occupancy model, differentiated for different household types, an occupancy-driven electricity demand model has been developed from high resolution appliance-level data to realistically distribute demand cycles for individual households. The model incorporates a novel event-based method for linking the time-of-day probability of appliance cycles relative to occupancy, which allows accurate replication of expected demand patterns and improves computational efficiency compared to existing models. Additional socio-economic and behavioural factors are also included to better capture demand diversity