BSEE


he last few years have seen significant improvements in the energy performance of commercial heating technology, driven by a combination of end user demand and legislation. However, we are not resting on our laurels because we recognise we have already made most of the obvious and easier to achieve changes. Engineers will now have to go further and deeper to keep up with fast moving energy targets and to help end users meet other obligations like local planning laws and corporate social responsibility (CSR).


T Commied


The gasfired hot water industry has made huge strides in energy eciency, but the best is yet to come, according to Steve Addis, product manager of boiler, water heater and heat pump company, Lochinvar


‘ In general, it is


a good principle that we take a flexible approach to the design of both heang and hot water systems


Legislation will continue to drive the market and, although we are leaving the European Union, that does not mean the UK will abandon the challenging targets it has already set. Our industry is committed to the EU’s 2030 targets of a 27 per cent increase in energy efficiency and 40 per cent reduction of greenhouse emissions. Eventually, the commercial hot water sector will be exclusively using condensing technology, but the legislation is not entirely prescriptive and the ErP has left the door open for non- condensing solutions for a while longer. Condensing water heaters provide the highest levels of efficiency, and they also meet the NOx emission targets, but users will still be able to use atmospheric water heater replacements for some time to meet ErP requirements. The use of condensing technology means that many heating and hot water products have already reached their maximum level of efficiency. Increasingly, we will need to adopt a system approach with designers making greater use of integrated and hybrid systems in which conventional and renewable systems are combined to hit higher energy targets. Heat pumps have seen a surge in popularity in homes in recent years, but most, if not all, are installed with back up from immersion heaters to ensure they are able to deliver hot water in the coldest conditions and when demand is high. This approach is not always practical in commercial buildings where hot water demand is much higher and because of the need to store water at 65˚C to avoid the build-up of legionella bacteria.


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In order to stay within constrained budgets, specifiers often revert to tried and tested methods of providing heating and hot water by using traditional gas-fired methods. However, they can also use heat pumps or solar thermal in the same system with the gas-fired systems as


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Integrated approach will help industry reach efficiency heights


back up for times of highest demand. This reduces running costs, but also has the added benefit of extending the operating life of the equipment.


This type of integrated or hybrid solution is a good way to keep initial capital outlay relatively low, when compared with renewable-only solutions, and means some end clients, who might previously have considered renewables to be beyond their budget, can afford a renewable element. This may also help them satisfy planning requirements and meet their CSR obligations.


Mulvalent


There are a range of solutions open to specifiers, including combined heat and power (CHP), but there is increasing interest in the use of multi-valent systems where energy is gathered from a number of separate sources including solar thermal; heat pumps; and condensing gas-fired boilers and/or water heaters.


The performance of heat pumps continues to improve and air source heat models (ASHPs) were still eligible for government subsidy under the Renewable Heat Incentive (RHI) scheme when this publication went to press. It was one of the few survivors from the government’s recent review of subsidy schemes and is a significant, but so far poorly exploited, benefit that can significantly reduce the payback period on some projects. Until recently, it was hard to make a commercial case for a heat pump in a straight comparison with condensing gas-fired boilers in UK weather conditions. On a cold, winter’s day the carbon impact and the energy cost were always likely to favour the boiler when compared with an electrically powered ASHP. However, one particular technical advance is tipping the argument in favour of heat pumps: ‘economised vapour injection’ (EVI).


Systems that use EVI have a much more efficient refrigeration cycle because the compression process is separated into two stages within a single compressor. As a result, most of the refrigerant is cooled during compression, which reduces the electrical energy consumed. This is particularly beneficial at lower evaporating temperatures, i.e. when the outdoor air temperature is around 0°C or below. It also increases the evaporating effect, the useful amount of heat that the refrigerant can extract from the outside air.


In applications where the building relies on electricity from the Grid, there is an even greater advantage in operating a modern ASHP at even lower temperatures and it is significantly more carbon efficient than using other fossil fuels, such as oil and coal.


Gas Absorption Heat Pumps (GAHP) also have potential and are capable of delivering impressive coefficients of performance (CoPs). Their efficiency is also less affected by changes in outdoor temperature.


A gas absorption system will only lose around 10 per cent of its operating capacity when outdoor temperature falls from 5°C to -5°C, but an equivalent electrically powered heat pump will lose more than 30 per cent. So, by correctly sizing a GAHP a system designer can provide a reliable source of hot water at a CoP of around 1.4 (seasonally adjusted across a whole year). That is more than 40 per cent better than a typical gas- fired condensing boiler.


Compensaon


GAHPs work particularly well if they are integrated with a condensing boiler to provide low temperature hot water for underfloor heating or low temperature radiators and the use of weather compensation control in the heating mode will


16 BUILDING SERVICES & ENVIRONMENTAL ENGINEER JANUARY 2019 Read the latest at: www.bsee.co.uk


also significantly enhance system efficiency. GAHP’s can also be used to supply pre-heated hot water to a gas-fired condensing water heater They can be used as part of multi-valent systems and, in such a system, the use of a suitably flexible and properly sized thermal store becomes the critical element. If the system is sized and controlled correctly this will ensure the non- renewable plant only runs when required, so minimising fossil fuel use.


The use of a thermal store capable of accepting multiple energy sources, in tandem with the latest heat pump technology, allows the design engineer to provide an extremely flexible, but high output solution. The thermal store acts as a large, low resistance header that can accept heat from up to three sources. This smooths out the system capacity to maximise efficiency. It also prevents legionella risk because it does not store domestic hot water for long periods.


Ideally, the store should be sized to provide 20 litres per kW output from the heat pump plus an additional 10 litres per kW output from the system boiler. This approach is not particularly challenging in a technical sense, but it does require proper pre- planning and assessment of the usage patterns within the building and, therefore, the likely best configuration for the system.


Integrating technologies in this way requires good control strategies and a proper commissioning period to ensure the various parts of the system work in correct sequence. The system should be set up to ensure the renewable technologies are the first to respond to any call for heating and hot water; with gas-fired boilers acting as back-up and firing only for periods of particularly high demand.


However, the control strategy, and therefore commissioning process, is vital to the long-term success of this type of system and should only be approached following consultation with the building manager/end user to establish likely patterns of occupant behaviour and the resulting hot water and heating demand.


It also requires an integrated, joined up supply chain to complement the technical integration built into the system design. That is a challenge, but one that our industry will increasingly have to meet in order to keep improving energy efficiency and reducing emissions.


www.lochinvar.ltd.uk


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