A COMPARATIVE ASSESSMENT of Future Heat and Power Sources for the UK Domestic Sector

Jeremy Cockroft, Nick Kelly#

Energy Systems Research Unit, University of Strathclyde, Glasgow, G1 1XJ

#Corresponding Author e-mail: tel: +44(0)141 548 2854 fax: +44(0)141 5525105

ABSTRACT

In 2003 the UK government announced its aspiration for a 60% reduction in CO2 emissions by 2050 relative to 1990 levels. To achieve this radical target action is required across all sectors of the economy to significantly reduce energy demand and to increase the supply of energy from zero or low carbon sources. Focusing on the domestic sector, where energy consumption is currently rising, technologies such as fuel cells, Stirling and internal combustion engine micro-CHP and heat pumps are often cited as the means to reduce carbon emissions. However there is much uncertainty as to the potential environmental benefits (if any) of the aforementioned technologies when set against a picture of changing energy supply and demand. The paper describes an analysis in which the performance of these four different technologies mentioned above wascompared against a common datum of energy supply from condensing gas boilers and grid electricity for a number of scenarios. The aim of the analysis was to determine if significant CO2 savings could be made and the minimum thermodynamic performance criteria that these technologies must attain if they are to yield any environmental benefits. The main finding of the work is that air source heat pumps yield significantly more CO2savings than any of the other technologies examined.

KEYWORDS

heat pumps, fuel cells, cogeneration, modelling, housing, emissions

INTRODUCTION

2003 saw the publication of the United Kingdom’s Energy White Paper [1], the core of which was the aspiration for a 60% reduction in greenhouse gas emissions by 2050 compared to 1990 values. To achieve this goal the UK will need to implement radical changes in the means by which energy is supplied and used. Focusing on the domestic sector the large scale deployment of embedded renewable technologies such as photovoltaics (PV) and micro wind power, micro combined heat and power (μ-CHP) coupled with greatly improved energy efficiency is seen as a means of combating increasing energy demand and reducing carbon emissions. The transition towards low carbon housingis of particular interest to researchers from many disciplines in that the effective integration of renewables and micro power technologies and improving energy efficiency will require significant changes from a technical, economic and social perspective given that the provision of heat and (particularly) power from localsources will involve a radical shift away from the energy supply systems in common use today. Significant resources are being targeted in this area with several large scale research projects underway examining issues ranging redesigning dwellings for low carbon emissions [2] to the integration of micro-power within the electricity supply infrastructure [3]. Surprisingly, despite these research efforts there is currently little published literature on the likely effect of deploying new heat and power sources on UKdomestic CO2 emissions.

With regards to energy efficiency, the energy profile of the domestic sector is changing, driven by conflicting trends. Positive drivers for change include the development of more energy efficient and responsive environmental conditioning equipment. Additionally, new legislation is also being designed to drive down energy demands, for example, the European Energy Performance in Buildings Directive (EBPD) [4] introduces mechanisms such as energy labelling and performance benchmarking for buildings. Counteracting these positive trends are increased expectations for occupant comfort (e.g. increased internal temperatures and demand for hot water); increased demand for electronic equipment; a growing market for domestic air-conditioning; changing demographics (e.g. an ageing population, reducing family sizes, increasing numbers of single-person households); behavioural patterns and socio-economic developments (e.g. increased working-from-home): all are acting to increase domestic energy consumption [5].

Given this complex and changing picture of domestic energy supply and demand driven by social change, legislation and many other factors it is difficult to determine exactly what impact the introduction of new heat and power technologies into the domestic sector will have.

In this paper a simulation-based approach is adopted to assess the performance of new domestic energy supply systems and quantify any resulting environmental benefits. Key questions to be answered include the following. What levels of performance will be required to produce tangible environmental savings? What impact will energy efficiency have on the viability of these technologies? How do they compare to each other and conventional alternatives? This paper sets out to examine these issues within the context of changing energy supply and demand characteristics (e.g. increased production of electricity from centralised renewable sources such as wind and tidal power and improved domestic energy efficiency).

NEW DOMESTIC HEAT AND POWER TECHNOLOGIES

Many new technologies are emerging as potential low carbon heat and/or power sources for the domestic sector, which could be considered as a direct replacement for the domestic boiler. Some of the main examples include: air-source heat pumps; micro-CHP featuring internal combustion engines (ICE), micro-CHP featuring Stirling Engines, proton exchange membrane fuel cells (PEM) and solid oxide fuel cells (SOFC).

Air Source Heat Pumps: while ground source heat pumps have traditionally had a higher coefficient of performance (COP) [6], the capital cost associated with their installation and the lack of availability of land for the evaporator coils in urban areas coupled with ready availability of other fuel sources such as coal, oil and latterly natural gas has meant that the number of ground source heat pumps (GSHPs) installed in the UK is very small [7]. The technology behind air source heat pumps (ASHPs) has been significantly improved in recent years and COPs of more than 3 have been reported in tests and field trials (e.g. [8]). ASHPs potentially have a much greater market potential than GSHPs in the UK due to the fact that they are more easily retro-fitted and could be seen as a direct replacement for the gas boilers that are the main heat source for around 80% of the UK’s homes 9]. There is currently one major field trial of ASHP’s underway in the UK[1] and several others planned.

Stirling Engine Micro-CHP: in the UK there is significant interest in the use of this technology to replace domestic boilers. There are currently several large scale field trials underway, however given the near-to-market status of the technology, performance data from these trials is limited. Useful information on in-situ-performance is available from studies conducted by NRCan [10], where overall device efficiencies of over 80% have been reported. Heat to power ratios are of the order of 12:1 and consequently the electrical power output from these units is low (<1kW).

ICE Micro-CHP: Several internal combustion engine (ICE) combined heat and power (CHP) devices, specifically targeted at the domestic sector have recently appeared on the market. These devices are usually scaled down versions of larger units and are designed to run on natural gas and have sizes ranging in size 1-4kW of electrical output. Some limited performance data is available from the manufacturers, with heat to power ratios of around 3:1 and overall efficiencies of 85% being reported [11].

Fuel Cells:two fuel cell technologies are of interest with regards to domestic heat and power applications: the proton exchange membrane (PEM) and the solid oxide fuel cell (SOFC). SOFCs offer a higher-grade source of heat for the production of steam, high-pressure hot water or low-pressure hot water. Moreover their high operating temperature (800-1000oC) leads to high electrical efficiencies, with 45% electrical efficiency and an overall efficiency of around 85% being reported (Sulzer Hexis, 2005). The high operating temperature also means that reforming of hydrocarbon fuels (e.g. natural gas) to produce the hydrogen needed to power these units can be done internally[13].

While the previous three technologies could be described as market ready, the prospect of heat and power from domestic fuel cells is still several years away. The US DOE published cost targets for stationary fuel cells, with a target of $750/kW by 2010. At these levels the technology would be competitive with alternative technologies, however current costs are significantly greater than this at over $2000/kW [14].

ANALYSIS

In this paper the environmental performance of the low carbon technologies mentioned above is analysed using computer simulation; the approach adopted is to calculate their CO2 emissions when supplying heat and power to three characteristic UK dwellings for a number of different scenarios and comparing these emissions to the conventional alternative, e.g. a boiler and grid electricity. The three characteristic UK dwellings considered in the simulations were:

an apartment with a total floor area of 68m2;

a terraced dwelling with a floor area of 90m2; and

a ‘semi-detached’ dwelling with a floor area of 87m2 .

In the simulations two different demand and supply scenarios are considered; these correspond, very roughly, to the current situation in 2005 and a more energy efficient scenario in 2020. For the 2005 demand scenario the buildings are assumed to be poorly insulated and have poor air tightness (1.5 air changes per hour): typifying the performance of many current UK dwellings. For the second more energy efficient demand scenario the buildings are insulated to current UK building regulations[15, 16], while air tightness has been significantly improved such that infiltration is restricted to 0.5 air changes per hour. In both cases the buildings are subject to intermittent occupancy, which represents occupation by a working family. In all cases the living space in each dwelling was heated to 21oC, while the other spaces were heated to 18oC, temperatures which are typical of UK dwellings [9]

The complementary supply scenarios are as shown in table 1. The first reflects the make up of the UK electricity supply at present and the other represents the envisaged supply in 2020 [17].

Table 1 Electricity Supply Make up (%)

Source / 2005 / 2020
Coal / 33.0 / 12.0
Oil / 0.6 / 0.4
Gas / 33.9 / 48.2
Nuclear / 24.6 / 5.8
Hydroa / 0.9 / 0.6
Other RE / 4.4 / 12.4
Importsb / 2.6 / 20.3
CO2 ‘coefficient’ of grid electricity kg CO2/kWh / 0.42 / 0.30
apumped storage bFrench electricity

Notice that the envisaged change in the supply make up reduces the CO2 coefficient for grid electricity by approximately 28% by 2020.

A total of 24 scenarios are analysed (2 demand/supply combinationsx 3 dwellings x 4 micro generation sources); each of the low carbon technologies is in turn subjected to up to 6 variations in electrical and thermal efficiency leading to a total of 124 different analyses. Specifically, the variations made are: (1) the thermal efficiency is increased while the electrical efficiency is held constant: effectively increasing the overall efficiency and the heat to power ratio of the device; (2) the electrical efficiency of the device is increased while the thermal efficiency is held at the rated value: decreasing the heat to power ratio while increasing the overall efficiency. For the air source heat pump only the COP is varied.

The calculation of each building’sannual CO2 emissions[2] for each combination required the development of a customised calculation tool. The tool useshourly time-varying electrical, space heating and hot water demand profiles, details of the prevailing electricity generation mix (shown in table 2) coupled with models of the micro-generation and conventional heat and power technologies to calculate the hourly CO2 emissions associated with heating and powering the dwellings over the course of a year.

The ESP-r building simulation tool[18] is used to calculate the time-varying space heating demands for each dwelling type. In ESP-r the geometry of the building, fabric, systems and occupant activity are described in a mathematical model. Solution of this model with real climate data and user defined control criteria yields the building’s time-varying mass, energy flows and state variables (e.g. temperature, pressure, etc.) over a simulated period.

UK standard electricity profiles[3] describing the daily variations of household electricity demand were modified for each dwelling: each profile was scaled according to the floor area and number of occupants. For the 2020 scenario electricity consumption patterns are assumed to follow the current trend: increasing at 2% per year [5], hence demand in 2020 is 30% greater than the 2005 case. Note that DTI projections assume that overall electricity demand will increase by 36% over the whole economy by 2020.

Hot water demand profiles are calculated based on an assumption of 36 litres of hot water usage per person per day [21]; this figure is based on data from 1997. Typical profiles of hot water consumption have two distinct demand peaks; in the morning and again in the evening [22]. A combination of the global consumption data and profile information is used to generate the demand profiles for these simulations. The profiles are scaled according to the number of occupants in the dwelling.

Figures 1 and 2 show the simulated heat, electrical and hot water demands fromthe 2005 and 2020 scenarios from the apartment dwelling for a winter day in January. The graphs show total demand (kW) and itsconstituents at each hour. Note the dramatic reduction in the space heating energy requirement (and hence total demand)caused by the improvements in insulation and air tightness.

Given the high-level nature of this study and the distinct lack of suitable models of micro power devices[4], some simple parameterised models of the generation components were used in this analysis. The parameters used for each model are those that dictate the basic performance in relation to supplying the calculated demands. Sensitivity analysis on these key parameters (table 2) gives an approximate indication of the minimum performance criteria needed for each low carbon technology to yield a CO2 saving compared to the provision of heat from a condensing gas boiler (efficiency 91%[5]) and electricity from the grid. In each case the micro-generation device was sized to be capable of meeting the maximum thermal demand (space heating and power) from the building being analysed. Control of the each device was assumed to be on/off. Hence if the hourly load is less than the device capacity, the device will cycle between zero and full output, operating on full load for a fraction of the hour f.

Table 2 Technology Performance Variables

Technology / Variables
Air Source Heat Pump / COP
Fuel Cell, Stirling and ICE Micro-CHP / electrical efficiency, thermal efficiency, overall efficiency, H:P ratio

In the analysis an assumption made was thatany surplus electrical output from the fuel cell, Stirling engine or ICE engine is exported back to the grid. Note however that exported power is not included in the CO2 saving calculation: exporting power from a large number of micro-generators could eventually lead to a larger generator ceasing operation, operating at part load or could displace renewable electricity, hence it is difficult to determine if a CO2 saving (or penalty) results from power export.

For cogeneration devices, the CO2 saving, S (kg CO2/annum)at each hour of the year is calculated by subtracting the emissions associated with provision of heat and power the cogeneration device and any impoted electricity from the emissioms associated with suppying a heat from a condensing boiler and all electricity from grid. For the Stirling, ICE and SOFC units, the saving is calculated using the following equation:

/ (1)

Equation 1 can be re-arranged to give

/ (2)

Where

/ (3)

Equation 1 can therefore be expressed as:

/ (4)

In the case of the air source heat pump, the CO2 savings for each hour of the year are calculated by subtracting the emissions associated with the electricity used by the heat pump from the emissions associated with supplying heat from a boiler:

/ (5)

Balance of plant and distribution system losses are assumed to contribute to the heating of the dwelling and so the efficiencies of these subsystems are not included in the calculations.

RESULTS

Figure 3 shows the predicted CO2 savings from all of the simulations conducted for the 2005 demand scenario. Figure 4 shows the results for the simulations conducted for the 2020 scenario. Importantly the figures show that in certain cases micro-generation and heat pump devices show a negative carbon saving.

For both the 2005 and 2020 scenarios the air source heat pump (ASHP) shows the greatest potential CO2 savings. The savings are greatest with the 2005 electricity supply mix and demand level; the reason for this is that the high heat demands for the 2005 scenario afford the air source heat pump (ASHP) the greatest potential to offset CO2 emissions: the greater the amount of heat produced by the ASHP the greater the CO2 saving compared to a conventional boiler. For the ASHP each kWh of heat produced saves s (kg/CO2):

/ (6)

Inspection of equation 5 indicates that for the air source heat pump (ASHP), the potential CO2 savings per kWh of heat supplied actually increase as the carbon dioxide emissions factor of electricity (E) reduces. Examining the curves displayed in figures 7 and 8 it can be deduced that with the 2005 fuel mix a minimum COP of around 2.0 is required to achieve CO2 savings, while with the 2020 fuel mix this is reduced to 1.4; these values can also be determined from equation 5 by setting s to 0 and rearranging for the COP.