Viewpoint: Choosing between AC or DC coupled Solar Energy Storage
With numerous Energy Storage solutions coming to the UK market what are the benefits and differences for installing AC or DC coupled Energy Storage in the UK. Moixa (www.moixatechnology.com) has installed over 400 energy storage systems in homes and SME’s in the UK. We have delivered several major pilots e.g. MASLOW -­‐ Energy Storage Demonstrator for DECC (Department of Energy and Climate Change), local energy pilots with Utilities and communities (Innovate UK) and with DNO’s, projects and customer sales. This has generated considerable technical, field and installation experience as well as measured benefits of storage systems (1-­‐ case-­‐studies). We also recently published a ‘Right Sizing Solar and Storage’ viewpoint (2) for a low subsidy regime, which shows smaller systems often deliver maximum payback case. In January the BRE (3) published a consumer guide to storage and key questions to ask installers or sales representatives as there is a risk that Energy Storage; which is complex, could be miss-­‐sold or wrong types of systems installed reducing customer benefits. For domestic storage installation, there are two main approaches as outlined below: Energy Storage System installation (UK choices) AC Coupled System DC Coupled System Energy Storage system is connected Energy Storage is connected to DC to the AC / mains side, typically by side of system, typically between the consumer unit Solar PV panels and the inverter Benefits – extra solar stored for self-­‐consumption • Easy add-­‐on to new or existing • New install or PV expert/MCS solar independent of legacy system install check if suitable on existing • Solar and Mains chargeable – • Typically cheaper as just batteries access cheap night and smart or DC switch apparatus tariffs or grid services benefits • Potential off-­‐grid or back-­‐up use (but at loss of FIT payments) • Significantly better payback • Electrician skill only to install • DNO G83/59 process unlikely to be needed on retrofit install unless • UPS/Backup option (AC , DC loops) inverter change • Install Location choices Disadvantages • Slightly higher cost as includes • Reduced metered Feed in Tariff extra inverter / power controls cash as battery/conversion losses • Process – filing of G83 post install • Unknown impact to existing MCS or G83/G59 pre if multiple or large status or FIT risk • Typically needs current clamp or • May invalidate inverter warranty connection to meter, so located or need bespoke inverter (Tesla) near consumer unit • No benefit from night or smart tariffs as no mains charge • No Grid services revenue in winter • Not suitable for micro-­‐inverters • Location restrictions – e.g. next to inverter, clamps or meters needed Example System Providers Typically AC Coupled Typically DC Coupled Moixa -­‐ MASLOW – all in one system TESLA battery (+ new inverter) SMA Sunny Island Growatt battery + DC Switch Powervault – undercounter unit GoodWe DC switch + battery pack Sonnen Batterie (Germany) Power Nedap Enphase – AC battery (US) Wattstor In Moixa’s view there are clear advantages to AC coupled systems such as MASLOW, which are easily installed and do not risk or impact existing MCS or FIT payments. Such systems can deliver a much greater payback for customer and future proofing; by accessing mains charging, night tariffs, emerging smart tariffs and grid services income. Use of batteries that can be both solar and mains charged in the winter are expected to become a significant opportunity for improved payback to customers and solutions to the grid capacity challenges. The IMECHe (4) recently (published analysis on the brewing 2025 power supply crises in the UK which could see a supply gap as nuclear plants close rising to 55% over the next 10 years. In Nov 2015 National Grid issued a NISM (notice of inadequate supply margin notice) and spent millions of pounds on balancing costs as capacity margin limits were breaked to prevent blackouts, and spends nearly a £1bn annually on balancing services. Such costs are expected to rise significantly and ultimately passed onto consumers in the form of higher bills. This will further improve the benefits of AC Coupled mains chargeable battery systems and enable discount consumer tariffs. MCS and RECC are also working on much needed advice, expected this spring, since some solutions may risk impact to Feed-­‐in-­‐Tariff or existing MCS approved installs. Similarly DECC is soon to open a consultation on energy storage and explore potential derogations to licensed utilities and network operators or a new license category for energy storage. There are also hybrid approaches that can deliver a range of services, and different technologies and sizing may be better in some countries. E.g. Net-­‐metering has a different impact on location and sizing of batteries, tariff or tax relief, e.g. in the US or the Netherlands. In Japan storage might be deployed to night charge rather than improve self-­‐
consumption as solar is net export rather than generation metered. For more information about Energy Storage projects and MASLOW systems, please visit www.meetmaslow.com , www.moixatechnolgy.com or contact info@moixatechnology.com References (1) Moixa UK case studies -­‐ http://www.moixatechnology.com/case-­‐studies.php (2) Moixa Right Sizing Solar and Storage -­‐ www.bpva.org.uk/members/bpva/news/moixa-­‐techology-­‐right-­‐sizing-­‐solar-­‐and-­‐new-­‐
storage-­‐opportunities-­‐in-­‐a-­‐post-­‐fit-­‐regime/ (3) BRE – Solar Consumer Guide -­‐
http://www.bre.co.uk/filelibrary/nsc/Documents%20Library/NSC%20Publications/881
66-­‐BRE_Solar-­‐Consumer-­‐Guide-­‐A4-­‐12pp-­‐JAN16.pdf (4) IMECHe -­‐ http://www.energylivenews.com/2016/01/26/uk-­‐could-­‐face-­‐power-­‐
supply-­‐crisis-­‐by-­‐2025/