There is a growing need for cooling, and district cooling is like district heating, a natural part of the urban infrastructure in smart cities. The benefits of district cooling can be explored and implemented in four steps or via four generations:
- 1st interconnecting buildings with a network-saving capacity
- 2nd, installing a chilled water tank to optimize the operation of the network
- 3rd, installing more efficient cooling sources within the range of the network and taking into account the storage capacity
- 4th installing heat pumps for combined heating and cooling benefit from the storage and ambient energy sources
By Anders Dyrelund, Senior Market Manager, Ramboll and Frederik Palshøj Bigum, Energy Planner, Ramboll
It is our experience each generation contributes to better cost-effectiveness. However, the time frame depends on the local conditions, and not least, the focus on green solutions and urban development. In particular, the 4th generation can be very cost-effective as the heat pump generates revenues from energy sales for heating and cooling and, not least, cooling capacity.
Several cases demonstrate that district cooling is a natural twin of hot water district heating and that both networks are a natural part of the energy infrastructure in modern cities. In fact, not only in warm climate zones but also in the city centers generally and for campuses in mild climate zones.
Previously, the thermal demand for active cooling was unknown as it was part of the electricity consumption. Today, a good indoor climate and reliable low carbon cost-effective energy supply are among the overall objectives in many cities by the UN Sustainable Development Goals (SDGs) accepted in most countries. The EU recognises that a good and cost-effective indoor climate is essential in modern society and that the combined district heating and cooling infrastructure is a means by cost-effectively utilising surplus energy and renewable energy in cities.
The demand for active cooling of buildings is increasing, not least due to the global megatrends: urbanization and industrialization and the SDG’s. The UN Climate Panel IPCC has predicted that the demand for active cooling will explode in the next century due to these megatrends and increase by 25% due to climate change.
This is a challenge, which we already notice in some industrialised cities in warm climates where load shedding is normal mainly due to uncontrolled individual chillers and lack of energy planning.
Surprisingly, the benefits of district cooling are still unknown, even in many modern European cities. In this article, we will highlight the benefits of district cooling by presenting a natural development of district cooling in parallel with the four generations of district heating.
The 1st generation – interconnect buildings and use the best chillers.
What is the actual demand for cooling in buildings? Often, even the designers and the facility management do not know. Modern buildings full of big data often lack the essential information of the actual thermal cooling demand. The electricity meter, which measures the monthly consumption of the chiller, does not tell the whole story about the maximal cooling capacity demand in MW and the cooling energy demand in MWh. We have undertaken many surveys and found that the costly installed cooling capacity in buildings often exceeds the actual demand or includes unnecessary large inactive spare capacity.
By clustering buildings in a district cooling network, it is possible not only to measure the actual consumption of the whole building hour by hour but also the total consumption of the network. Our experience shows that the cooling demand depends a lot on the use of the building and its internal installations, and, accordingly, many buildings do not have their maximal consumption at the same time.
A reasonable estimate is that the factor that reflects the over-capacity (actual demand divided by installed capacity minus spare capacity) could be a factor between 0.5 and 0.9 and that the simultaneity factor (maximal capacity to the district cooling network divided by the total of all measured maximal demands at the buildings) could be between 0.8 and 1.0 depending on the number of buildings.
Our surveys also demonstrate various cooling installations in the buildings on a campus or in a city district. There are large and small installations. There are efficient and inefficient installations.
Some of them are vital and must be installed in the building to supply critical demand, e.g., an industrial process, and some of them are worn out and call for relatively expensive investments. The O&M costs of many small cooling machines are also significantly more significant than the O&M costs of large machines relative to the capacity.
Therefore, on a campus or in a city district, a good strategy is:
- to interconnect the buildings in small clusters with a cooling network,
- to use the most efficient and vital chiller plants to supply all cooling and
- to scrap the small and worn-out installations one by one.
Based on one year of operation, analyzing how much is needed and which plants can be scrapped will be possible.
A challenge in the design of the cooling network is to agree on the supply temperature, as the buildings may have different needs for supply temperature in the design case. Also, how many of us have not tried to walk into a supermarket, restaurant, pub, or similar to really be cold on a summer day – the result of unnecessary low temperatures?
So, first of all, upgrading the coils in ventilation systems may reduce the need for the lowest supply temperature. Secondly, reducing the supply temperature to supply more consumers may be more cost-effective.
The impact of a bit lower temperature on the COP factor and, thereby, on the cost of cooling has not as much influence on the bottom line as the saved costs of investments in cooling capacity by interconnecting the building to the network.
If a consumer needs a significantly lower supply temperature, e.g.-20 or, 0 degrees Celsius, it is best practice to boost the temperature with a local compressor. On some campuses, e.g., at the Technical University of Denmark, many refrigeration machines are connected to the campus cooling network.
In the design of the cooling network, it is also essential to look at the further development of the system towards the 4th generation and to improve the resiliency by ring connection or by distributed production and connecting buildings in such a way that they can be supplied from one end or the other in case of break down.
It is also important to consider where to connect a mobile chiller in case of unforeseen significant capacity demand – an impossible option in most buildings. Accordingly, the district cooling network is an efficient tool for shaping and minimizing the total installed capacity.
The 2nd generation – chilled water storage facilities
By monitoring the actual cooling load and operation of the ambient cooling sources and the chillers to the network and the fluctuating electricity price (and heat prices in case of absorption chillers) hour by hour, it is possible to simulate the profitability of a short-term day-to-day chilled water storage tank. The tank has five benefits:
- It can provide valuable peak capacity defined by the difference between the maximal hourly peak demand and the maximal daily average demand. This may be a design criterion for the tank’s volume, e.g., six max load hours of the capacity value.
- It can provide a much larger instant spare capacity in an hour or two to compensate for a sudden breakdown
- It can open for optimal generation of cooling (all days except the warmest days), taking into account ambient cooling sources, the electricity price, and the out-door temperature, e.g., generating cooling in night hours at low electricity prices
- It can improve the performance of the chillers and help the operators
- It can store make-up water and maintain the pressure in the district cooling network
The 3rd generation – more efficient cooling
The large central chillers in the first generation are more efficient than the average building-level chillers. However, they can be improved further. The extension of the cooling network and the economy of scale factor may open for competing cooling sources in combination with the chilled water storage.
- High-temperature surplus heat from waste incinerators, absorption heat pumps, and gas turbine-driven compressors might be more cost-effective than electric chillers – depending on the local conditions. An example is the extensive DC system in the city center of Tokyo, which generates cooling by absorption chillers and gas turbine-driven chillers.
- A DC network can utilize efficient ambient cooling sources, e.g., drain water, seawater, deep lake water, or groundwater. The colder the source in the summer, the more efficient the system. If an ambient cooling source is unavailable in the network, consider establishing a connection to a new chiller plant near an ambient cooling source. A good example is the Toronto deep lake water cooling project.
- Utilizing free ambient cooling improves the COP of the electric chillers or absorption chillers. Moreover, they can be optimized for the cost of the heat source and electricity.
- In the specific case of ground source cooling, the warm well can be cooled by, e.g., cold seawater or cold air in the winter. The annual energy balance of the groundwater will normally be a precondition for approval from the environmental authorities.
4th generation – combined heating and cooling
The efficient third-generation cooling-only system can be even more efficient, especially in case there is a heat demand for heating buildings, for hot tap water, or for industrial processes, e.g., desalination. In that case, it is a shame to waste the heat in ambient cooling sources at a relatively warm temperature, typically 25-30 degrees Celsius.
By adding one more compressor in serial connection with the compressor chiller, it is possible to upgrade the temperature to, e.g., between 60 -75 degrees Celsius, which is sufficient for heating modern buildings and for generating hot tap water. Two or three compressors can even be integrated into one unit, reducing the costs.
It is not free to upgrade the temperature to a proper temperature, but it is cost-effective compared to generating heat from traditional air-water heat pumps. The additional investment in upgrading from one chiller to two chillers (or a heat pump) is only roughly 30% of the cost of a heat pump with air coolers, and the COP factor for the additional electricity consumption is twice as significant as the COP factor of the heat pump.
Combined heating and cooling is a new vital and efficient sector coupling similar to the well-known efficient combined heat and power. Typically, one unit of electricity can generate three units of useful heat and two units of useful cold. Nothing is wasted.
Moreover, if the heat demand and the heat capacity of the heat pump exceeds the heat capacity for the efficient combined heating and cooling, the available heat pump capacity can generate heat and dump the surplus cooling in the ambient sources.
Thereby, the ambient cooling source turns into a heat source. The ambient energy in seawater can, e.g., be used for cooling in summer and heating in winter. The warmer the ambient source is, the more efficient it is for heating. Wastewater is, e.g., better for this purpose than groundwater and air.
The optimal location of the energy plant for combined heating and cooling depends on the local conditions, mainly the location of the district heating and cooling grids, the available space, and the location of the ambient energy source for heating and cooling.
It is important to optimise the total generation and storage of heat and cold in the interconnected district heating and cooling system in the warm season. There are several options:
- The heat pump can be combined with ground source cooling (a so-called ATES system). Thereby, the heat pump will serve as peak load in summer, boosting the temperature of the groundwater. The heat pump can reduce the temperature in the cold well and increase its capacity, and the warm well can store ambient heat to generate more efficient heat in winter
- A larger heat storage, e.g., a heat storage pit, will open for optimal use of cheap heat from other sources, and the heat pump
- A larger cold storage will also open for more optimal operation of the heat pump
- A combination of heat pumps and chillers can reduce the combined production in case of low heat demand or low value of the heat in warm periods
- In case there is no chiller, sufficient ATES, or storage capacity available, an option is to reduce the supply temperature from the heat pump to the lowest possible and eject the heat into the ambient source.
More critical for the cost-effectiveness is, however, that the heat pump can be utilised to generate more heat in the winter season in the competition with other heat sources, having access to all the ambient heat sources like wastewater, surplus heat from industrial processes such as data centers, and even drain water and air.
4th generation DC system is identical to the 4th generation district heating (DH) system. The characteristics of the 4th generation DH and DC system are that it benefits from
- interconnection of all buildings with networks benefitting from economy of scale,
- access to all cost-effective facilities for the generation of heating and cooling
- utilisation of sector couplings between electricity, heating, and cooling via heat pumps
- extensive storage facilities for heating and cooling, and
- access to all cost-effective ambient sources for the centralised heat pumps and optimal location of the heat pumps considering the cost of space for the heat pumps, cost of branch lines from ambient sources, and the costs of the network for heating and cooling
In Taarnby, Denmark, the heat pumps are installed at the waste-water treatment plant thereby minimising the length of the ambient heat pipe.
The benefit of combined heating and cooling is even more evident in systems where the heat supply dominates, as in the 4th generation low-temperature DH system and where low electricity prices open up for heat pumps based on ambient heat sources. In this system, we can see DH’s economy of scale benefits compared to individual small heat pumps.
In these heat pump-based systems, cooling is a waste product, and the location of the heat pumps must be coordinated with the cooling potential to gain income from the sale of cooling capacity and cooling energy.
From zero to 4th generation
Going from individual buildings to DC through all four generations can be considered a step-by-step strategy for investing. It is also a way to identify all the benefits of DC one by one. This is illustrated in the table below for a typical cluster of buildings.
By interconnecting buildings going from individual buildings to 1st generation, it will be possible to measure the actual demand for all buildings as a whole. If this demand is 20% lower than the installed capacity in the base case, it can be even lower.
By installing a chilled water tank going from 1st to 2nd generation, it is possible to gain several benefits. By measuring the total load fluctuations and electricity price fluctuations in the 1st generation, it is possible to estimate the benefit of a chilled water storage tank and design its volume.
Connecting the tank directly between the network pumps and the production pumps will improve the operation. The capacity of 2 MW (or 25% of the total capacity) in the table is typical for storing the daily peak of comfort cooling to night hours. However, the instant capacity that can supply the network in case of disruption is equal to the total capacity of the network pumps. In this case, 8 MW.
The DC network and the chilled water tank are open for integration of more efficient cooling sources going from 2nd to 3rd generation. In the table, this is ground source cooling, but it could also be other sources, e.g., fluctuating sources, which the chilled water tank could integrate. The simplest would be to generate all cooling at night hours at low electricity prices and lower outdoor temperatures.
Finally, by investing in a heat pump instead of a new chiller, going from the 3rd to the 4th generation, it will be possible to harvest the synergy between heating and cooling, in particular in combination with the ground source cooling and ambient sources which can be used both for heating and cooling or heating only.
The generic case described in the table is inspired by the Danish case in Taarnby, in which most of the heat from the heat pump is transferred to the district cooling plant via an ambient heat pipe from the ambient heat source, in this case, treated wastewater. Thus, the 4th generation DC is identical to the 4th generation DH.