Integration of renewable energy into the transport and electricity sectors through V2G

Abstract

Large-scale sustainable energy systems will be necessary for substantial reduction of CO₂. However, large-scale implementation faces two major problems: (1) we must replace oil in the transportation sector, and (2) since today's inexpensive and abundant renewable energy resources have fluctuating output, to increase the fraction of electricity from them, we must learn to maintain a balance between demand and supply. Plug-in electric vehicles (EVs) could reduce or eliminate oil for the light vehicle fleet. Adding “vehicle-to-grid” (V2G) technology to EVs can provide storage, matching the time of generation to time of load. Two national energy systems are modelled, one for Denmark, including combined heat and power (CHP) and the other a similarly sized country without CHP (the latter being more typical of other industrialized countries). The model (EnergyPLAN) integrates energy for electricity, transport and heat, includes hourly fluctuations in human needs and the environment (wind resource and weather-driven need for heat). Four types of vehicle fleets are modelled, under levels of wind penetration varying from 0% to 100%. EVs were assumed to have high power (10 kW) connections, which provide important flexibility in time and duration of charging. We find that adding EVs and V2G to these national energy systems allows integration of much higher levels of wind electricity without excess electric production, and also greatly reduces national CO₂ emissions.

Introduction

To mitigate climate change and reduce dependence on external energy supplies, many countries have adopted policies to increase both energy conservation and the share of renewable energy resources (Da Silva et al., 2005; Duic et al., 2003, Duic et al., 2005; Gross, 2004; Hvelplund and Lund, 1998; Lund et al., 2000, Lund et al., 1999, Lund et al., 2003, Lund et al., 2005; Toke, 2005). In some countries and regions e.g. in the EU such policies moreover include increasing the share of combined heat and power (CHP). With a high share of both wind power and CHP Denmark is one of the frontrunners in the implementation of such policies, and thus serves as valuable national case study of large-scale integration of new energy technologies.

Danish energy supply was traditionally based on the burning of fossil fuels. Denmark has very little hydro power potential and during the 1960s and 1970s the electricity supply came solely from large steam turbines located near the big cities. However, after the first oil crisis in 1973 Denmark has become a leading country in terms of implementing CHP, district heating, energy conservation and renewable energy. With these changes, Denmark has been able to maintain the same total primary fuel consumption (for all uses, including transportation) for a period of more than 35 years. More than 15% of oil consumption has been replaced by renewable energy and even more by coal and natural gas, and consequently the Danish energy system has been changed from the situation in 1972, in which 92% out of a total of 833 PJ was oil, to the situation of 2006 in which only 40% is oil. In the same period both transportation, electricity consumption as well as the area of heated space have increased substantially. Today the share of Danish electricity production from CHP is as high as 50% on an annual basis, and approximately 20% of the electricity demand is supplied from wind power (Lund, 1999, Lund, 2000; Lund and Andersen, 2005; Lund and Ostergaard, 2000; Lund and Hvelplund, 1997; Maeng et al., 1999).

However, if Denmark is to proceed in replacing more fossil fuels by renewable energy, two problems arise. One is the transportation sector, which is almost totally fuelled by oil. Consumption was 140 PJ in 1972 and is expected to be 180 PJ or more in 2020. Thus the transportation sector will account for almost all the expected oil consumption.

The second problem is the integration of electricity production from CHP and wind power. Due to the relatively high wind penetration, and the fact that CHP was not operated for balancing until recently, Denmark has had problems of maintaining a balance between electricity supply and demand. So far Denmark already has faced excess electricity production. As is known to those familiar with regions having high penetration of wind power, the problem of excess wind is more difficult to manage than insufficient wind. Excess wind is not a threat to system reliability, as it can always be “spilled” by feathering turbine blades. Rather, excess wind means that the system design has not achieved the best economic return on wind capacity investment and has not minimized emissions. The other aspect of electricity integration, more specific to Denmark, is the high proportion of CHP. Although efficient, CHP must be operated for heating, so during cold periods with low electric demand, CHP, like wind, may contribute to excess electricity production.

To minimize excess electricity production, Denmark has already implemented several measures, including changes in the regulation of distributed CHP plants (Andersen and Lund, 2007; Lund and Andersen, 2005). Additional technological options analyzed and considered to date include electric boilers and heat pumps (Blarke and Lund, 2007; Lund, 2003; Lund and Münster, 2003a), flexible demand, electricity for transportation (Lund and Munster, 2006; Mathiesen et al., 2008), reorganising energy conversion in relation to waste treatment (Münster, 2007) and various energy storage options (Mathiesen and Lund 2007).

“Vehicle-to-grid” (V2G) power technology is one of the many energy storage technologies, which may be part of making a flexible energy system that can better utilise fluctuating renewable energy sources. V2G is built on top of plug-in electric drive vehicles (EVs), which already displace petroleum by using electricity as the carrier for transportation energy. V2G refers to adding the capability to deliver power from the vehicle to the grid, but “V2G” is also used to imply that power flow, whether to or from the vehicle, is controlled in part by needs of the electric system, via a real-time signal. Consequently, the V2G technology provides potential solutions to both the problems mentioned above. Apart from two brief deterministic calculations (Kempton et al., 2007; Kempton and Tomic, 2005b) and one unpublished report of a US national model (Short and Denholm, 2006), there is no published model of how large-scale V2G would affect an entire national energy system.

This paper focuses on battery electric cars, but V2G may also be drawn from fuel cell vehicles or plug-in-hybrid vehicles. For battery cars, the grid-connected batteries charge during low demand hours and discharge when power is needed. Each vehicle must have three required elements: a connection to the grid for electrical power flow, control or logical connection for communication with the grid operator and auditable meters for power metering on-board the vehicles. The operational control logic must allow the grid operator some control, but override grid operator control in order to, for example, minimize battery wear or charge to prepare for vehicle operation. In other words, reliability for the driver must be maintained for each individual vehicle, whereas reliability for the electric grid is obtained through aggregation of many vehicles. We assume vehicles with high power line connections (10 kW) because this provides flexibility when the vehicle is charged (as well as other operational benefits like fast recharge during lunch stops on trips). EVs with 20 kW charging and V2G are already in limited production. However, contemporary plug-in hybrids are typically much lower power, 1.5 kW, thus requiring overnight charging with little time flexibility. The concept of V2G has been described in detail in references (Kempton and Kubo, 2000; Kempton and Letendre, 1997; Kempton and Tomic, 2005a, Kempton and Tomic, 2005b; Tomic and Kempton, 2007; Williams and Kurani, 2006, Williams and Kurani, 2007), with the main governing equations derived in Kempton and Tomic (2005b).