Thursday, 7 July 2022

Is using your home for energy storage really practical?

Electricity demand varies through the day, and the cost of supply depends largely on the demand peaks. Therefore reducing peak demand can bring cost savings. Most of us are going to be using electric heating in future, mostly with heat pumps, and this will be a major part of peak demand in cold weather. (Peak time currently is typically 4-7pm, including cooking time for many households.)

You can turn off your home heating for an hour or so, or at least turn it down for a bit. This could be useful to reduce demand during peaks or at other times when there is a short lull in renewable energy supply. The grid could be supplied from energy storage instead, but this is generally expensive whereas homes with masonry construction have ‘built in’ thermal storage. Turning off the heating you are using the fabric of your house as an energy storage system for free. (Well not quite free perhaps, you might need a controller like the Homely thermostat). However there are difficulties with this approach. On the whole, I do not believe it is sensible. The main reasons being:

When you turn the heating off in your house (or down) it gets colder. This means it has to be warmer than you need to start with, which means you are likely to have bigger bills overall.
Most homes cool quite quickly over the first hour or two. Preheating for an hour or two in advance does not help very much.
Some people are impervious to cold - many of us are not. Even quite small changes in temperature bring discomfort. 
Energy demand flexibility can be achieved more easily with other kinds of demand such as EV charging and especially with bi-directional vehicle to grid charging. It makes no difference to your wellbeing when your car is charged, as long as it is ready when you need it. 

In this post I explore (a) the rate at which homes cool, (b) how sensitive we are to cooling and (3) potential for demand response in buildings compared to EVs.

Buildings cool quickly to start with, then more slowly. 
This chart shows temperature cooling rates from a real house versus a simulation from one of the models I developed, reported in CODE [1]. Modelled data has fewer lumps and bumps but by and large the behaviour is similar. You can see that the temperatures drop quite quickly to start with and then tail off. This is because air heats up quickly while surfaces (walls, windows, furniture etc). respond more slowly. When heating is on the air temperature is normally warmer than the surfaces. When the heating goes off, the air cools quickly until it gets below the surface temperature and then more slowly because it gets heat from the warmer surfaces. The temperature we feel (the operative temperature) is an average (more or less) of the air temperature and the surface temperature.
Diagram comparing model temperature and field temperature data from a similar building, from CODE [1]

Insulation reduces the rate of cooling but it is still rapid at first
Homes that are not well insulated lose heat faster than the better insulated homes but the initial drop in temperature is always rapid. This chart shows a model for three homes that are identical except for the type of walls: one has solid walls, one has filled cavity walls, and one has solid walls with external insulation . A heat pump is in use and the overnight thermostat setting is 18°C, heating goes off at 4pm and the external temperature is about 6.0°C at that time. The uninsulated solid wall house cools fastest and the house with external wall insulation des the best - but the temperature still drops by about a degree within the space of an hour.
Temperature profiles from a model of similar homes with different types of wall and insulation

Preheating for a few hours increases the final temperature but also the initial drop.
Preheating for only a few hours means the final temperature is not so cold but the overall temperature drop is greater. This is because the surfaces take longer to warm up than the air so when the heating goes off the surface temperature has not caught up. This chart shows the cavity wall model with no preheat, a short preheat and a longer one.
Modelled cavity wall home with different durations of preheat

Using temperature data from a set of 25 homes (all social housing, with fairly good insulation levels), I have estimated the mean temperature drop in each home, when it is 5°C outside – typical winter conditions. After the heating went off, the drop over the first hour would typically be 0.8°C; after two hours, 1.5°C and after 3 hours, 2°C. Admittedly some of the households were using thermostats as an on/off switch rather than for maintaining a steady temperature.

Studies on thermal comfort during temperature ramps – mostly students in offices
Most studies of the potential for heating flexibility use simple rules for what is comfortable in the home such as a minimum of 18°C and/or a maximum drop of 1°C per hour. However, we are surprisingly sensitive to reductions in temperature - women more so than men (in general). I am sure many female readers will agree with this – and men too who have been used as hot water bottles (such as my long-suffering beloved). 

Unfortunately, studies of thermal comfort all too frequently rely on students who are young and healthy, in office-style environments. At home we are often less active and many of us are no longer young and fit. Even for young adults there is evidence that the ASHRAE standards for thermal comfort are not accurate. ASHRAE allow ramps – up or down - of 1.1°C in 15 minutes, or 2.2°C in an hour [2].

Many participants report discomfort after less than 1°C temperature drop.
For example: Favero (2021) [2] used university campus staff, median age 26 years. They had an initial setpoint of 22°C and a minimum temperature ramp of 1.4 degrees/hour. They found many participants reporting discomfort after a drop of much less than 1°C. (see diagram). However, temperature rises were much more acceptable – here the ASHRAE standards were too tight rather than too lax. This study used 29 women and only 9 men, which is probably why they did not find a statistically significant difference with gender.
This figure from Favera et al (2021) shows many participants were uncomfortable with less than 1°C drop from a starting temperature of 22°C even though this is well within acceptable limits according to the ASHRAE standard (the dashed lines). However very few complained about temperature rises of a similar magnitude. 
Figure from [2], indicating when test subjects reported discomfort due to increasing or decreasing temperature, compared to the ASHRAE standard thermal comfort limits.

Skin responds faster to cooling conditions than warming conditions.
Wu et al (2020) [3] used 30 college students, half male and half female. Their paper title refers to ‘moderate’ temperature ramps but this was at least 4°C over 40 minutes and the minimum temperature was 20°C, so hardly cold. As well as asking the participants how they felt, they also monitored skin temperature. Under cold conditions (less than 22°C) they found skin temperature changed twice as fast when cooling than when warming. So maybe that explains why we are more sensitive to cooling. Unfortunately, they did not look at gender effects.

Women are more sensitive than men.
Zhang and Zhu (2022) [4] also used men and women between 25 and 35 years old and tested them with temperature ramps between 24°C and 30°C. They found that women were definitely more sensitive than men to cool environments and everyone was more sensitive to cool than warm environments. 

Occasional discomfort will be acceptable to some, given a good incentive.
Noticing the cold does not necessarily mean it is unacceptable, and I should imagine many people will be prepared to accept some discomfort occasionally if there is a good reason why – and if they get an incentive for doing so. For example the Homely thermostat is designed to work with flexible tariffs such as Octopus Agile, which vary by the half hour. Homely adjusts the heating when electricity is most expensive, so as to maintain the temperature within the comfort limits you have set. The incentive is lower bills - 30% reductions are claimed (dating from before the recent price rises). However, I don’t think this is going to be an easy sell, especially if it happens frequently. It may be more popular with young, all-male households.

A house with a heat pump can typically reduce demand by about 1.5kW, for a short time.
A medium- sized well insulated house might need 3.2kW heating power on a cold day (20°C inside, 2°C outside, 180 W/K). A heat pump could provide this with, say, 1.5 kW. However, assuming you have some lights and appliances on at the same time they will provide some of the heat, so you could save 1.5 kW or so, for a short time. If it goes on much longer you would want to have some heat; if you accepted a chilly 17°C instead of 20°C, then you could save 0.25 kW indefinitely. Personally, I would be very reluctant, and complaining. At the end of the off-period there will be a spike in demand as the heating system works to replace the lost heat.

An EV can reduce demand by 2-4kW - if it is charging at the time. 
Suppose you have an EV and you normally drive 30 miles/day using 10 kWh. You can replace that charge any time between about 8pm and 6am. Your charging power could be up to 4 kW normally, but maybe your smart charger does it more slowly to avoid heating the battery. The demand response potential for your car is probably more than turning off the heating - but only if you happen to be charging your car at the time when it is needed. 

Two-way charging can reduce demand or supply to the grid, most of the time
If on the other hand you have a vehicle-to-grid two-way charger than instead of reducing demand by not charging you can reduce demand by using stored energy from your vehicle for your home. If you do not need it at the time you can feed power into the grid. You could provide 2-4 kW for several hours, as long as you are plugged in and reasonably charged at the time. If your normal range is 150 miles (50 kWh) and you only need 20% of that on a typical day, you have up to 40 kWh to support the grid. 

Bi-directional chargers do have extra bits in them including an inverter so they can convert DC back to AC efficiently. As of a year ago, the extra cost of a two-way charger was £3700 [5]. This is not very much less than a dedicated 4kWh battery for your home – Green Business Watch suggests about £4,300 for this. Hopefully costs will come down.

Flexible households will be outnumbered by flexible EVs.
There are more cars than houses in the UK. Many households have more than one. Not all of them are electric yet, of course, and doubtless some will give up their cars rather than buy an EV – perhaps they will opt for a car share scheme instead. However, the ultimate number of EVs is likely to be in the same ball park as the number of houses. If the two-way chargers are cheap enough so half of these have two-way charging capability, I am pretty sure that will be more than the number of households prepared to turn off the heating when asked by the grid, especially if this happens frequently.

In conclusion
Being able to turn off your electric heating for an hour or two could be very useful for reducing peaks in electricity demand. However, for most homes, turning off heating for even one hour means accepting sizable (0.5-1.5°C) drops in room temperature. Allowing for this means heating your house more than you need most of the time or accepting pretty chilly conditions – some might find this discomfort acceptable but I suspect this would be a (mostly young/male) minority. High levels of insulation reduce the temperature drop a little – but less heat would be needed anyway so the saving for the grid would be less. The reduced demand period will be followed by a demand spike to replace the lost heat.

Electric vehicle charging could deliver somewhat greater levels of flexibility, with no spikes afterwards. However this requires that the vehicle is charging at the time – unless the chargers are two way so that the battery can deliver to the grid as well as vice versa. At the moment chargers for this are quite expensive but hopefully that will change. 

If you have room and cash to invest you can install a battery or a thermal store for your house so that you can reduce power demand without turning the heating off. However, it makes sense to use the car battery if you have one available, if only because it is likely to be bigger.

[2] Favero, Matteo, Sartori, Igor and Carlucci, Salvatore (2021) Human thermal comfort under dynamic conditions: An experimental study, Building and Environment. 

[3] Wu, Qingqing; Liu, Jianhua; Zhang, Liang, Zhang , Jiawenl and Jiang, Linlin (2020) Study on thermal sensation and thermal comfort in environment with moderate temperature ramps, Building and Environment. 

[4] Zhang, Shuai and Zhu, Neng (2022) Gender differences in thermal responses to temperature ramps in moderate environments, Journal of Thermal Biology 

1 comment:

  1. It is true that the cost of electricity supply depends on peak demand, but there is currently a disconnect: what 99% of electricity customers pay has no link to peak demand; it is only suppliers that face higher costs when peak demand is higher.
    There is no financial incentive for customers to reduce peak demand. In fact, most customers would pay more if they pre-heated and then turned down electric heating during the peak period, as described, because overall consumption would increase.
    It is only when customers switch to time-of-use tariffs (where they pay more for electricity at periods of high demand) that using your home for energy storage becomes viable.


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