Morbilidad

The exclusive electricity supply of virtually all UK domestic buildings is the National Grid, with an electrical efficiency and carbon intensity (calculated as primary fuel input to electricity delivered to dwelling) that varies throughout the day and year. This variation is in response to the mixture of generation on-line at any point, including fossil fuel generators, nuclear power stations, renewable power sources, and storage (such as pumped hydro). In 2009, thermal efficiency (i.e. electricity generated over average energy content of fuel input) was 36.4% for coal-fired generation and 46.7% for combined cycle gas turbines (CCGT) [1]. This low electrical efficiency is inherent to centralised electricity generation, where heat from the energy conversion process is ejected to the atmosphere, and electrical losses occur over the transmission and distribution network. In the current climate of energy awareness, where concerns over carbon emissions, energy costs and security of energy supply have prompted drives to reduce both carbon emissions and primary energy consumption, there is an impetus to maximise electrical efficiency.

In the last decade, energy studies in the UK have typically considered the long term marginal carbon intensity (CI) of grid electricity as 0.43kgCO2/kWh [2], as introduced by DEFRA. The approximate carbon intensity of electricity generated by coal-fired stations and CCGT plant has been quoted as 0.96kgCO2/kWh and 0.44kgCO2/kWh respectively [3]. Investigations of electrical micro-generation technologies, that is power sources with electrical capacities typically below 50kWe (or in some definitions below 5kWe), typically use the marginal carbon intensity when calculating the relative carbon savings between electricity generated by the micro-generator and the National Electricity Grid (NEG).

Climate change research is a complex topic, out with the domain of this project; however, atmospheric carbon dioxide (CO2) concentration has been identified as a major contributor to global warming. International and UK studies [4][5][6][7] have called for large changes in the rate of CO2 emissions associated with energy use, e.g.

reductions of 60% between 2000 and 2050, in order to stabilise atmospheric carbon dioxide concentrations within levels predicted to limit the effects of global warming on the global climate.

The UK government has set ambitious greenhouse gas emission reduction targets of 80% (of 1990 emissions) by 2050, with an interim target of 34% by 2020 [8]. The European Union (EU) set wide ranging targets to integrate emissions reductions, supply of energy from renewable sources, and energy efficiency improvements, with “20-20-20” calling for 20% improvements (versus 1990 levels) in each of these areas [9][10]. Within the context of these targets, the UK government has made specific commitments to reduce the carbon footprint of dwellings [11][12].

The interest in renewable technologies, which in some definitions includes low-carbon technologies such as micro-Combined Heat and Power (micro-CHP or µCHP), is driven by the issues of climate change, security of supply, and affordable energy. Renewable or low-carbon micro-generation technologies include µCHP, solar photovoltaic (PV), solar thermal, heat pumps, biogas- and biomass-fired boilers, and wind turbines. To secure investment and drive adoption of renewable technologies, the European Union agreed to the Renewable Energy Directive 2009 (2009/28/EC), which requires 20% of energy (from electricity, heat, and transport) consumed by member states to come from renewable sources by 2020. Within this framework, the UK government has agreed to a 15% target for energy (for the purposes of transport, heat and electricity) to be supplied by renewable sources by 2020 [13][14]. The devolved administrations in the UK (Scotland, Northern Ireland and Wales) have produced targets for renewable energy production in support of central government [15][16][17]. The Scottish government has targeted 100% of electricity to be generated from renewable sources by 2020 [15].

Energy policies and technology uptake are not only driven by climate change concerns, economic factors or security of supply concerns (due to resource scarcity and geopolitical circumstances). Recent market research [67] on µCHP systems discusses the impact of Japanese and German energy policy changes on µCHP uptake since the

Fukushima-Daiichi nuclear power station disaster. With Japanese energy policy priorities shifting to incorporate distributed generation as a means of meeting peak electrical demands on the national grid, annual sales of 1kWe µCHP systems have more than doubled to 18,000 units. In Germany, the disaster prompted a rethink of the national energy strategy, with a planned end to nuclear electrical generation by 2022. This so called “Energie U-turn” presents an opportunity for distributed generation, supported by the re-introduction of the German µCHP purchase subsidy [67].

In this thesis, an alternative approach to domestic energy supply is considered at a discrete dwelling level, where a building-integrated micro-generation system provides electricity in combination with the NEG. Micro-Combined Heat and Power (µCHP) systems are one form of building-integrated micro-generation, and will be the primary focus of this thesis. In contrast to centralised electrical generation systems, the investigated µCHP systems utilize a much higher proportion of primary input energy, through the recovery of heat from an electrical generation process, and the avoidance of transmission and distribution losses [20]. These µCHP systems can also be referred to as co-generation systems, and in a domestic context they predominately have a rated electrical output (Pe) of 5kWe or less. This fits within the UK government’s definition of micro-generation, as defined in recent strategy documents [21]. In support of the policies discussed previously, the UK government has supported [18] combined heat and power research, development and deployment (for µCHP and larger systems), including domestic field trials of µCHP systems [19].

There is a distinction between micro-generation technologies that generate electricity only, and µCHP systems. To understand the economic and carbon reduction potential of an electricity-only generator, the demand profile and potential for generation need to be understood for a single fuel. Where storage is employed to maximise on-site utilisation of generated electricity, the behaviour of the storage device needs to be understood. µCHP adds several layers of additional complexity; the need to understand the demand profiles and supply potential for two types of end use energy demands (under varying load conditions), the co-incidence of both demands, the storage of heat and/or electricity, and the responsiveness of demand to supply. This

final point does not apply to electrical systems (with the exception of changing efficiencies of storage technologies over the range of state-of-charge levels), but it is a significant matter for thermal systems. The thermal mass within thermal stores and space heating distribution systems introduces the dynamic response of thermal demand profiles to the profile of thermal generation. The distinction between static and dynamic demand profiles is drawn in Section 1.6.3, and it is argued that this distinction is often overlooked in the investigation of µCHP performance, where many studies are confined to the response of supply to demand.

The issues co-incidence of thermal and electrical demand and dynamic response of demand to supply, especially within the operating constraints of different µCHP technologies, is the core of the research issue investigated in this project. In order to conceive, model and analyse any energy supply approach, the transient nature of demand must be characterised and understood, in both daily and annual contexts. To this end, this research project was undertaken to explore and build an understanding of the relationships between transient demand and supply. Part of this research exercise is to understand the technological constraints placed on µCHP systems, and their resultant ability to follow variable thermal and electrical demand. With this understanding, operating and control practices can be defined and investigated to maximise µCHP system performance.