Powering the UK with Christmas Trees: How much electricity could all the UK’s Christmas trees produce?

2nd January 2020, Richard Howard, @RDHoward

Every year in early January a large number of UK households simultaneously produce a sizeable amount of biomass in the form of spent Christmas trees. 7 million [1] trees to be precise, which are no longer fit for purpose and are disposed of in a variety of different ways.

This is a sizeable amount of biomass. What if we were to collect all these trees and burn them in a commercial biomass energy unit to produce electricity? How much energy would be produced? Could it power the UK for a period of time? Could Christmas trees or a similar form of biomass power the UK in the long term?

The rough calculations below give us an interesting insight into the biomass industry but unfortunately show the infeasibility of the biomass industry as a major supplier of renewable energy. A lot of energy is stored in one Christmas tree but looking at the land requirements to grow those trees reveals it is not feasible as a reliable source of electricity. We have also done some similar analysis on solar power for some more positive analysis.

The headline figures are:

• One average Christmas tree could generate approximately one day’s worth of electricity for the average UK home – around 10kWh.
• All the UK’s seven million Christmas trees could power the UK electricity grid for 2 hours.
• If we wanted to power the UK’s electricity grid solely from Christmas trees it would require approximately 70 million hectares or three times the UK land area.
• In contrast, high-end solar panels would require 7% of the UK’s land area.

These figures are estimates and include a large number of assumptions. For example, Christmas trees are not bred or cultivated as an energy crop, but we will approximate their energy content to commercially grown pine. We will only consider using biomass for electricity generation, and we will ignore the energy required to process the trees (manufacture of fertilisers, energy to power agricultural machinery and transportation etc). Those assumptions aside, however, the correct values are unlikely to be radically different from the numbers calculated below.

Power generated by Christmas Trees

• 7 million trees consumed per year in the UK [1]
• Average Christmas tree height 6ft to 7ft = 1.8m to 2m [2]
• Average weight = 15kg [3]
• Energy density of wet woodchips (commonly pine) = 11.85MJ/kg = 3055Wh/kg [4]
• Efficiency of converting biomass to electricity = 0.2-0.25 (use nominal value of 0.225) [5]
• Electricity per tree = 3055*0.225*15 = 10.3kWh – the average UK household’s daily electricity (averaged over the entire year)
• Total UK electricity from Christmas trees = 7,000,000*10.3 = 72.1GWh

Therefore all the UK’s Christmas trees could power the UK grid for about 1.5-3 hours depending on the time of day and time of year [6].

How Much Land Is Required to Supply the UK with Christmas Trees?

• It takes 10-12 years for a Christmas tree to fully grow [1]
  • Note: The farmer has to weed, fertilise, prune, and pest control throughout those 10-12 years.
• 1,000-1,500 trees per acre = 2,500-3,750 trees per hectare – take nominal as 3,125 per hectare [7,8]
• 7,000,000/3,125 = 2,240 hectares are needed to grow one year’s supply of the UK’s trees
  • Note: Not all trees are grown in the UK. A large number of UK trees are imported – £3 million worth of trees per year [9]
• But it requires approximately 15,500 hectares to maintain a constant supply of trees.
  • Not as simple as 1,900*11 years since the trees are grown in nurseries for the first 3-4 years of their lives [1]. Therefore, we will presume that the trees need 7 years outside and we will ignore the nursery space.

How much land is required to power the UK from just Christmas trees?

• UK electricity production in 2018 was 334TWh [10]
• 334TWh/10.3kWh = 32.4 billion Christmas trees to produce enough electricity to power the UK for a year
• That would require 10.4 million hectares (32,400,000,000/3,125) or 41% of the UK land area to grow one year’s crop
• Therefore, in order to have a sustainable supply we would need approximately x7 this value (for seven year’s supply), 10.4*7 = 72.8 million hectares or nearly x3 the UK land area.

What about photovoltaic solar power?

Biomass is essentially a form of solar power – sun shines on the plant’s leaves, the plant photosynthesises converting carbon dioxide and water into biomass and oxygen. But as we can see above, this process is highly inefficient – in Europe 2% of sunlight can be converted into carbohydrates at best [14].

As a demonstration of how efficient, if we covered the UK with high-end solar panels we would have 5TW of solar capacity. Obviously these panels are not outputting 100% electricity all the time and in 2018 the UK solar capacity factor was 11.5%, giving a theoretical maximum electrical energy output from solar of 4,900TWh from the entire UK land area. This means that only 7% of the UK’s land area needs to be covered with solar panels to generate all the UK’s 334TWh of electricity need in 2018. Our calculations are similar to others which vary from 1% to 12% of the UK land area [11,12]

Calculations:

• UK area = 25 million hectares
• High-end Solar PV panel power per meter = 200W/m2 [13]
• Total theoretical UK solar power capacity = 25,000,000*200W = 5TW
• UK solar power output in 2018 = 12.9TWh [10]
• UK solar capacity factor = 12,900/(13*24*365) = 0.115 = 11.5%
• Total theoretical UK solar power output = 5*24*365*0.115 = 4,912TWh
• UK electricity production in 2018 was 334TWh [10]
• Land area required to meet UK electricity demand = (334/4,900)*100 = 6.7%

References

1. https://www.bctga.co.uk/gallery/press-releases/86-facts-figures-advice-for-christmas-trees.html
2. Estimate based most common internet quieres about tree heights
3. Estimate based on advice on internet forums
4. https://web.archive.org/web/20100110042311/http://www.woodgas.com/fuel_densities.htm
5. https://www.biomasscenter.org/policy-statements/FSE-Policy.pdf
6. UK grid requirements are in the 25-45GW range www.electricinsights.com
7. https://www.profitableplantsdigest.com/how-to-start-a-christmas-tree-farm/
8. http://theconversation.com/growing-revenue-how-does-the-christmas-tree-business-stack-up-52544
9. https://www.growninbritain.org/homegrown-christmas-trees/
10. https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/791297/Press_Notice_March_2019.pdf
11. https://www.cclcomponents.com/panasonic-hit-n330-330w-hybrid-solar-module-black-frame-white-backsheet?gclid=CjwKCAiAo7HwBRBKEiwAvC_Q8cZq0wKsuOs7JrFDGy_miAfZW4syayend5SzkRb4_lNowOXWA7tOVBoCURIQAvD_BwE
12. https://www.energylivenews.com/2018/05/18/uk-would-need-to-cover-12-of-land-with-solar-panels-to-supply-itself/
13. https://www.solarpowerportal.co.uk/news/if_solar_covered_one_percent_of_the_uk_it_would_meet_the_countrys_2356
14. MacKay, Sustainable Energy – Without the Hot Air, 2019, Green Books