Electrical Storage & Grid Interaction

The integration of electrical storage within a µCHP system has not been investigated to the same extent as thermal storage because the prevalent CO2 accounting method for grid-connected micro-generation systems assumes that all of the electricity which is generated, but not used instantaneously on-site, will be exported and used elsewhere, displacing the equivalent quantity of electricity from central generation [118]. In calculating the reduction in CO2 emissions footprint for a dwelling with µCHP, versus a base-case scenario without on-site generation, the change in net electrical import is typically considered. In the base-case scenario, net electrical import is equal to the total consumption of electricity within the dwelling. With a µCHP system, net import is the difference between total electrical demand and the total µCHP generated electricity. Net import can also be calculated as the arithmetic difference between actual electrical imports and electrical exports; both methods will produce the same value. Difference in net import can be used in a carbon footprint reduction calculation if exported electricity is assigned the same carbon intensity of grid electricity assigned to import electricity. This approach has been used by the majority of the research in the field, with the exception of those studying time-varying carbon intensities of grid electricity due to the daily and seasonal mixtures between fossil fuel, renewable and nuclear generation. Pout and Hitchin [118] point out that the UK building regulations and Standard Assessment Procedure (SAP), used to assess the energy efficiency of new

domestic properties in the UK for compliance purposes, both use this CO2 accounting convention. However, economic evaluations of µCHP commonly apply a reduced price to export compared with import, which is reflective of market conditions (unless influenced by regulation or subsidy).

In an earlier paper, Hitchin & Pout [124] argue that the carbon intensity used for both displaced electrical import (between scenarios with and without µCHP) and electrical export should use the incremental carbon intensity. They argue that incremental CI should include both direct and indirect effects, i.e. CI of marginal plant whose generation would be displaced by generation from CHP, and avoided new generation that would not be installed and operated due to the deployment of a fleet of µCHP. Hawkes & Leach [59] considered the ability for mass penetration of µCHP to displace central generation by modelling the availability of prime movers using a heat-led operating regime. Their investigation included SE, ICE and generic FC-based systems, where the reported capacity credit increased from 48% for SE to 75% for ICE and 92% for FC, due to decreasing heat-to-power ratio.

Others have investigated the impact of µCHP penetration on the NEG by simultaneously modelling multiple buildings, usually with a range of demand scenarios, applying centralised control signals to manage aggregated demand and export profiles. Investigations by Peacock & Newborough [57] and Boait et al [147] found that, for particular electrical demand profiles, up to 40-50% of electrical generation can be exported from a dwelling, depending on prime mover technology, capacity and operating regime.

Peacock & Newborough [45] aggregated the µCHP modelling results of 50 dwellings, using a demand profile dataset, to investigate resultant electrical peak load, load factor and energy flows due to mass penetration of µCHP at a local level. They investigated SE prime movers, with 15% ƞe, incorporating transient output and efficiency characteristics during start-up. Comparing thermal load following with an aggregated control operating regime (to smooth the electrical load profile for the group of dwellings), they concluded that aggregated control methodologies can

significantly increase capacity factors versus heat-led operation. This addressed the issue of low capacity factor raised by their earlier research [53], where they compared capacity factors for penetration levels across several prime mover technologies and capacities.

The majority of µCHP modelling has focused mainly on dwelling-centric operating regimes, such as thermal load following, electrical load following and hybridised operating practices to minimise operating cost, usually referred to as “least-cost”. Newborough [61] broadly characterises µCHP systems as either network-connected, where power can flow to and from the national grid, or autonomous systems that have little or no interaction with the grid. He defines several operating regimes where the prime mover operates at constant output either continuously, or for distinct pre- configured time periods defined by household occupancy. He discussed the potential for utilising such operating regimes for a system incorporating electrical storage. Agar & Newborough [62] discuss the challenge in identifying a prime mover technology that could operate under such regimes without some drawback.

Peacock & Newborough [57] investigated both thermal and electrical load following, and explored the concept of restricted and unrestricted thermal dumping (or thermal surplus). They assessed the environmental and economic impact of thermal dumping for 1kWe 15% ƞe SE-based and 1kWe and 3kWe 50% ƞe FC-based µCHP systems. They found that switching from restricted to unrestricted thermal dumping reduced thermal cycling (from 1,898 to 1,182 for SE) whilst penalising CO2 savings and cost; relative CO2 savings of 10% were reduced to a carbon penalty of 3%.

In their investigation of SE-, ICE- and FC-based µCHP, De Paepe et al [77] implemented a thermal load following operating regime with a seasonal operating restriction, in that the space heating was switched off during the 4 summer months. The value of seasonal restriction was recognised by Peacock & Newborough [57], who reported significantly reduced prime mover run-times during summer months, due to limited thermal demand (for heat-led), and the need to restrict thermal dumping (electric-led) to maximise CO2 savings. Seasonal restriction has been applied in economic modelling,

where Hawkes & Leach [58] switch operating regimes and control strategies between seasons to limit running costs, based on seasonal electricity export prices.

Many studies have investigated multiple discrete building variants, or the same building variant with differing characteristics, in an attempt to understand the impact of demand scenarios on µCHP performance. Future scenarios with reduced building heat loss were investigated by Hawkes & Leach [125], who concluded that SOFC-based µCHP systems maintain carbon savings as thermal demand reduces, whilst ICE- and SE- based systems do not. Hawkes & Leach [60] compared the economic and carbon reduction cases for application of FC-, ICE- and SE-based µCHP in existing, refurbished and newly-built homes. They conclude that government policy supporting both energy efficiency measures (to reduce thermal demand) and µCHP (to satisfy thermal demand) can be justified. However, they warn that high heat-to-power ratio technologies in dwellings with low or inconsistent heat demand, which could correspond with new-build or smaller refurbished homes, should not be granted policy support. It is prudent to note that whilst heat-to-power ratio is a function of the prime mover technology, it is also a function of design of particular systems; hence advances in SE technology could dramatically decrease its heat-to-power ratio.

To understand the relationship between thermal demand and relative carbon savings (RCS), Peacock & Newborough [3] used full factorial design to create, by multiple linear regression, a relationship between carbon emissions from a μCHP system and the thermal demand of the dwelling, rated thermal and electrical outputs of the CHP system (for a particular set of operating constraints and control methodology). Again using a matrix of prime mover design variants, they concluded that RCS (as a percentage of carbon footprint of the conventional boiler system) increased with increasing thermal demand of the building.

Operating regime has a significant impact on the relationship between relative CO2 savings and thermal demand, as supported by Shaneb et al [80]. In their optimisation sizing exercise, they conclude that µCHP systems with higher electrical capacities save more CO2 when heat-led, and less when electric-led.