Storage of solar power using a battery Energy Storage System – updated

A case study of a domestic installation updated after five months’ use.

This report describes my experience of a renewable energy storage system developed by a Cornish start-up company operating from the Falmouth University campus – Wattstor Ltd.  The WATTSTOR™ was commissioned at my Penryn home on 14 March 2015. The data here for this period of two and a half months shows my imported energy from the utility company has stayed stable at 62% compared with the average for the same periods in 2013 and 2104. Even more to the point, the comparison shows my local usage of my PV-generated power over the period has approximately doubled.

A Solar PV system generates power when the sun shines. As for most sources of renewable energy, the power generated may not be immediately usable in the property, or even by the National Grid (to which the surplus power is exported). Storage of this surplus power in a battery bank is a practical way around this because surplus power is then exported only when the batteries are fully charged. When the sun goes down the battery system powers the house. The next day the process starts again. This simple action silences the criticisms of ‘intermittency’ and ‘uncontrollability’ of renewables. It also reduces grid import dramatically and relieves some constraints on the National Grid infrastructure. The other added benefit is that my Feed-in Tariff (FiT) payments from the government are not affected as these are based on the amount of renewable power generated, not on the amount of power exported.

My WattstorTM can store up to 6kWh of power and can, within that capacity, meet demands of up to 5kW of electrical load. The stored capacity is enough to get my home through the night without importing any significant utility company power. The tables below give evidence from my meter readings of Grid Import and Solar Generation over the period 14 March – 31 May in each of the three years 2013 to 2015.

Table 1 – Demand reduction

Meter Readings (kWh)

Solar PV only
no storage

Solar PV
with WattstorTM

Date

Meter

2013

2014

2015

14 March

Import (WPD)

21,266.8

23,689.1

26,007.9

24 July

Import (WPD)

21,901.0

24,342.6

26,249.9

 

Imported kWh

634.2

653.5

242.0

 

 

Average (2013-2014) = 643.85

Reduction in import = 62.4%

 

Table 2 – Local use of locally- generated power

Meter Readings (kWh)

Solar PV, no storage

Solar PV with WattstorTM

Date

Meter

2013

2014

2015

14 March

PV (TGM)

6,671.7

9,751.1

13,073.4

24 July

PV (TGM)

8,352.0

11,528.4

14,768.7

 

Generated kWh

1,680.3

1777.3

1,695.3

Imported cf. locally-generated power:

37.7%

36.8%

14.3%

The findings here are hugely significant for any solar PV owner (and for national energy policy):

  • Demand reduction: The actual, measured, reduction in demand (imported electricity) is 62.4% over the period.  The power I am now drawing from the National Grid is 37.6% of my average demand over the two previous years: I am drawing less than 2/5 of the power from the grid than before installing the WattstorTM.
  • Local use of local generation: Comparing my imported energy with PV-generated energy shows a reduction from an average of just under 40% to 14.3%. With the WattstorTM, my locally-generated PV provides a far higher proportion of my demand.  (With no Export meter, my exact usage cannot be measured). Note the comparative reduction in this period including mid-summer is less dramatic than in the earlier period. However, the percentage with the Wattstor drops also – from 16.7% to 14.3%.

Interestingly, if this performance were to be repeated on all registered renewable energy storage systems across the UK – 650,000 and growing – we could probably close down power stations instead of building new ones – especially the nuclear one at Hinckley C where the government is intending to give £24bn to the French and Chinese to build for us!

Richard Hopper CEng MIET
24 July 2015

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