Technology Selection

Retrofit of CHP with Ammonia Fuel

Ammonia (NH3) is a composition of nitrogen and hydrogen. Ammonia is commonly used in fertilizer, cleaner and antimicrobial agents for food products, however use of ammonia as a fuel is an emerging technology. When considering the use of ammonia as a fuel for a low carbon CHP, the hydrogen produced to be used in the Haber-Bosch process (equation below) must be green and nitrogen collected from air.

After the Haber-Bosh process, the ammonia can be mixed with water to be used as a fuel for CHP plant and boilers to produce Heat and electricity, in replacement of Natural Gas.

The advantages of ammonia as a fuel are it can be used in the existing CHP plant by retrofitting some components such as upper engine, check valve and injection system. Ammonia is also desirable as the only byproduct of combustion is water, and where green hydrogen is used to source ammonia, the overall process becomes zero carbon. Ammonia also has the capability to produce the heat demands required by the DH network.

However, it is a concern that during the combustion process there is a release of bad odour. The system to store ammonia requires large amounts of space, which is a concern for Strathclyde Campus. There are also numerous safety risks associated with ammonia fuel as it is a toxic gas. When used it requires safe storage and conditions, otherwise it could lead to an explosion of ammonia, posing large health hazards to humans such as skin irritation. Such safety concerns are amplified by the fact that Strathclyde Campus is a City Centre Location.

Retrofit of CHP with Biomass Fuel

Biomass boilers operate very similarly to traditional gas boilers, providing heat and hot water, however instead of using Natural Gas, the energy source is Sustainably Sourced wood pellets.

Burning wood provides far less climate concerns than the combustion of Natural Gas. There is also potential for removal of waste; each year, roughly 8.5 million tonnes of wood goes into landfill in the UK; this waste wood could be used in either biomass boilers (if converted into the pellets) or burned in wood burning stoves. This would not only provide heat and hot water, but in doing so, it would also ease the pressure on landfill capacity (The Green Age, 2020).

How does a biomass boiler work?

A biomass boiler works in a very analogous way to natural gas boilers, combusting fuel to produce heat which is used to heat water. The biomass boiler has an automatic feed hopper which will require extra room. This hopper stores a huge volume of the wood pellets that are then automatically fed into the boiler. (The Green Age, 2020)

The boiler needs to be refuelled very infrequently, that requires an extra space to store the wood. This is a large obstacle, as Strathclyde University face large Spatial Constraints due to the city centre location.

Retrofit of CHP with Hydrogen Fuel

A Hydrogen fuelled CHP system would pose the potential re use of many parts of the current system, with retrofit of incompatible parts of the CHP engine to suitability for hydrogen. The existing DH network may still be used and there is potential that in the future the national gas grid may be used for hydrogen delivery. The key concern of a hydrogen fuelled CHP is that of safety risk, due to the underdevelopment of the technology and lack of implementation of hydrogen currently. However, projects such as H21 are proving that the current natural gas network is compatible and safe with the use of Hydrogen (Ofgem, 2021).

As the combustion of hydrogen itself produces zero carbon emissions (Department for Business, Energy & Industrial Strategy, 2019), a hydrogen system offers the potential for large carbon savings compared to the current Natural Gas system. The environmental footprint of a hydrogen system is highly dependent on the source of the hydrogen and delivery of hydrogen. Where possible, green hydrogen sourced from renewables should be used. Hydrogen may be delivered by the grid, via trucks or produced onsite via Electrolysis. Truck deliveries to supply hydrogen pose challenges due to spatial constraints of HGV's within the city centre and transport emissions associated with trucking goods. Where produced by electrolysis, the electricity supply used should be from Renewable Generation to minimise environmental impacts. High levels of water consumption also raise concerns, thus the implications on the grid and water supply must be taken into consideration when suggesting the use of electrolysis.

As Strathclyde campus has spatial restrictions due to the city centre location, a hydrogen system would be desirable as compressed gas storage may offer a scenario where not much space is required for the technology. With the ongoing projects such as H21 and H100, health and safety concerns and limitations to the use of Hydrogen are predicted to ease, with governments including Hydrogen in all pathways to carbon-neutral.

Electrification via Ground Source Heat Pump (GSHP)

GSHPs utilise the relatively constant temperature of the soil and are considered one of the most energy efficient renewable energy technologies.

GSHPs can be categorised into two systems:

Open Loop systems where groundwater is used as the heat transfer fluid.

Closed Loop systems where heat exchangers are placed in the ground and do not use water as the heat transfer fluid.

Whilst open loop systems can be more efficient and cost effective when compared to a closed loop system, they often require much more maintenance in the long term. The main types of closed loop heat exchanger are vertical boreholes or horizontal loops, and the coefficient of performance (COP) typically ranges from 3-6 (Cocchi et al., 2013).

Within the setting of the University of Strathclyde, space is extremely limited due to the city centre location, therefore it was decided that vertical borehole heat exchangers would be most feasible. Using the step-by-step calculation process on SynergyBorehole’s website along with the heat demand data acquired from the Estate’s team at the university, an approximate calculation of the number of boreholes required was undertaken. Taking into consideration the spacing required between each borehole to prevent thermal linkage, on top of the already limited amount of space available around the university, it was decided to discount GSHPs based on spatial constraints.

Electrification via Water Source Heat Pump (WSHP)

WSHP is a renewable energy conversion technology which uses low grade heat from water as a heat source. The overall efficiency of the WSHP varies with the type of heat pump that is installed. There are two type of WSHP:

Open loop systems where water is used as the transfer fluid.
Closed loop system where a closed circuit is placed in the water and a fluid is passed through.

In a study looking at the Seoul river, open-sourced heat pumps saved 6.9% of energy compared to the original gas CHP (Jung et al., 2021). The thermal efficiency of WSHP varies with the type of water and the waters environment. The coefficient of performance (COP) also differs through the change in the seasons, the COP of WSHP is higher in the winter than in the summer. On average river WSHP have a COP of 5.2 in heating and 2.6 in cooling (Zhang et al., 2020)

The largest emissions of the WSHP will be indirect emissions. Indirect emissions can be attributed to the energy consumption of the system, which will be taken from the grid. Also attributed to the indirect emissions is the manufacturing of the building materials and refrigerant used and finally the end-of-life emissions (Jung et al., 2021). The 𝐶𝑂2C emissions associated with heat pumps are primarily from the electricity consumed.

Technical Criteria Considered

Safety Risk

Risk of System Failure

Spatial Constraints

Environmental Criteria Considered

Emissions

Waste Production

Failure to recycle at end of life

Noise Pollution

Visual Pollution

Air Pollution (Odour)

Inability of Use of repurposed material

Selection Tool

Table 1 - Probability and Impact and resultant weightings key

Figure 1- Probability and Impact weightings to be linked to table 1, and the resultant outputs, with key showing green are desirable outputs, yellow acceptable and red unacceptable.

For each of the Technical and Environmental criteria noted above, the technologies were assessed using the tool created as shown. Within each category the considered technology is assigned a number for the Probability of occurrence (rare to almost certain) and magnitude of the impact (very low to very high) - table 1. The weighted numbers for probability and impact are multiplied together to produce an output value, which falls either within the range of desirable, acceptable or unacceptable output - figure 1.

Outcome

Table 2- The output values and colour code of the values for each of the 5 considered technology under all technical and environmental criteria.

The outputs of the selection tool are shown in table 2, the selected technologies were chosen upon these outputs. The technologies with the most unacceptable outputs, that could not be mitigated were discounted; ammonia, ground source heat pump and Biomass. The common concern was spatial constraints due to Strathclyde University city centre location, this is unavoidable. A summary detailing the reasons why the technologies were discounted or selected shown in figure 2 below.

Figure 2- Schematic Diagram Illustrating the discounted and proceeding technologies, alongside the key determining factors for the judgement within the arrows.