The problem of heat losses
Heat networks have long been promoted by policy makers as an important approach to enable low carbon heat distribution for buildings to help meet the UK’s zero carbon target. However, traditional heat networks are subject to significant heat losses as the heat transfer fluid within the network is often at a much higher temperature than its surroundings. This is a major limitation and a large source of inefficiency.
In the latest UK Building Regulations Part L software (SAP 10.2), heat networks that follow the CIBSE “Heat Network: Code of Practice for the UK” should use a Distribution Loss Factor (DLF) of 1.5 if there is no data for network in the SAP software for the specific network. This equates to a 33% loss of all generated heat. A DLF of 2 must be used if the code of practice has not been followed which means half the heat generated is wasted.
The evolution of heat networks
The first generation of district heating networks, which began over a hundred years ago, often used pressurised steam at temperatures over 100°C. The second generation used pressurised hot water with temperatures around 100°C. Third generation networks, which began from the 1970s, supplied temperatures around 80°C while, more recently, fourth generation networks have aimed to drive temperatures lower and are based on water distribution around 60°C. These types of networks are all based on central plant systems.
The ambient loop
The so-called “fifth generation” heat network or “ambient loop” networks can overcome the problem of heat loss by circulating water that is a similar temperature to ambient conditions. The heat within the low temperature heat transfer fluid is then upgraded by individual water-to-water heat pumps within each property to provide space heating and hot water.
As ambient loop grids are based on decentralised plant, they are not constrained in the same way as other heat networks which have to supply heat at the temperature required by the most demanding end user. In a fifth-generation heat network, users are able to generate and supply low temperature, e.g. underfloor heating at 35°C, and benefit from higher efficiencies, while those that require higher temperatures are free to meet their own heating needs.
Project example - achieving net zero carbon
We have proposed an ambient loop heat network, in a new residential scheme in south Bedfordshire which is currently in the planning process. The scheme comprises 70 homes including bungalows for the elderly and 30% affordable homes. The scheme commits to zero operational carbon and zero-carbon construction which will be a first for Bedfordshire.
As a trial, the community borehole infrastructure will be shared by all affordable homes and bungalows for the elderly with the aim of providing these units with highest efficiency heating and cheapest energy bills. This will be compared with other homes in the development which will be heated using local air source heat pumps.
Beside the use of heat pumps, zero-carbon operational energy will be achieved through passive design and a large rooftop solar array as illustrated below.
Zero carbon construction will be achieved using low embodied carbon timber construction. The low embodied carbon design will be informed by life cycle analysis and any remaining embodied carbon will be offset using a recognised framework, following UKGBC guidelines.
The scheme commits to 5-year post occupancy monitoring for energy/carbon, overheating and air quality. We aim to quantify the benefits of the ambient loop network which have access to stable temperatures deep underground in winter as a source of heat. The temperature over 10m below ground is typically around 11°C which increases the efficiency of the system compared to other means of heat generation such as air source heat pumps which need to extract energy from the air in winter which can go below freezing.
The buildings are designed so that the local rooftop solar PV array powers the local modulating heat pump within the property during the day. This charges the buffer vessel and thermal mass within the building with enough heat to keep the home warm throughout the evening and night. Following a fabric first approach, the building envelope will have sufficient insulation to ensure heat losses are less than the amount of heat stored.
Benefits of ambient loop networks
Beside carbon and energy bill reductions, other benefits include: zero fossil fuels and combustion emissions; low noise emissions compared to air source heat pumps; no visible infrastructure; reduced expense of pipework insulation; reduced installation cost through shared infrastructure compared to individual ground source installations; and the ability to have flexible modular expansion.
Another benefit of smaller ambient loops is that pumping is distributed, rather than via a central pump-set, which means there is no need for central plant and costly central management and billing. Homeowners are free to switch energy (electricity) providers as they choose.
The Bear, in Deptford, is an example of another project we have design using an ambient loop network. As a block of 33 apartments the scheme will see another benefit related to reduced overheating risks compared to an apartment block with centralised high temperature hot water networks. In this scheme water-to-water heat pumps within each unit are relatively small, around 500 by 500 by 360 HxWxD (mm).
Heating and cooling networks
Small-scale and district-scale fifth generation heat networks could become an important part of future zero-carbon built environments. The larger scale networks come into their own when they serve both heating and cooling demands or if waste heat sources are available from local industrial processes. In this case scenario, heat rejection from cooling helps to charge the network for heating users and vice versa.
As a simple example, reversible water-to-water heat pumps can be used to extract heat from a shared ground array in winter to heat homes, and when providing cooling the heat rejection to the network can charge the ground array.
Whole life carbon perspective
It is important to take a whole life carbon perspective when determining the energy strategy in order to accurately consider long-term operational carbon savings from more efficiency systems against upfront embodied carbon cost from equipment. As such we recommend whole life carbon analysis during the concept design stage.
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