Although the main message still stands that hot water for domestic use is best generated in a decentralized solution, internal heat distribution in networks in residential collective buildings as well as district heating, are still a popular solutions. The low efficiency of domestic hot water systems is well known by field practitioners. For many new residential multifamily buildings hot water delivery times and heat losses in distribution systems have been getting steadily worse.
To serve in a central system Multi Family Buildings with DHW high distribution temperatures are preferred. Especially in luxury apartments in inner city the vdelivery time of hot water at the required temperature ‘must’ be short.
The most common refrigerants used in heat pumps today are R410A, R134a and R407C. Until recently the maximum output temperature for domestic heat pumps using these refrigerants was about 55°C with R134a achieving slightly higher temperatures, however now temperatures above 60°C can be achieved. By using slightly different temperature and pressure characteristics the refrigerant can be flashed off at a higher temperature, increasing the output temperature, although this can reduce the overall thermal output.
High Temperature Heat Pumps
High temperature heat pumps are considered to be products capable of producing an output temperature of at least 65°C and higher. There is a number of heat pump designs capable of achieving high temperature outputs, including:
- Cascade systems with two separate refrigeration cycles;
- Enhanced Vapour Injection (EVI);
- Products with optimised design for specific refrigerants such as CO2 an other natural refrigerants;
- Gas driven heat pumps.
Whilst these products have been specifically designed for high temperature operation, the designs of “conventional” heat pumps are increasingly being improved to reach 60-65°C at reasonable efficiency.
Cascade Heat Pumps
A cascade heat pump consists of two single-stage cycles (a low temperature and a high temperature cycle using different refrigerants) which are thermally connected by a heat exchanger or a storage tank. The low temperature cycle mainly uses the refrigerant R410a which is able to evaporate at a very low air temperature and condenses at a relatively low pressure and temperature of about 45°C. This process transfers heat to the evaporator of the high temperature cycle with a second refrigerant like R134a. Cascade systems are capable of reaching temperatures of up to 80°C.
The applications are various, like: the Suurstoffi project, the Swiss Holiday Village Reka in Blatten, Eco Quarter Capazur Nice, the Leyhoeve care home in Tilburg, Collective HT HP at the Jacques Urlusplantsoen in Leiden a Renovation of privately owned flats.
Enhanced Vapour Injection (EVI)
The EVI technology requires an additional loop to be added to the standard heat pump cycle. This loop enables a small proportion of the condensed refrigerant to be extracted and expanded through an expansion valve and into a counter flow heat exchanger which acts as a subcooler. The additional subcooling increases the evaporator capacity. The resulting superheated vapour is then injected into the compressor part way through the compression process.
Do’s and don’ts are described by Emerson and a number of projects can found in China.
CO2 Heat Pumps
This technology uses R744 or CO2 as the refrigerant in a transcritical Heat pump cycle. This results in a cooling phase of the CO2 fluid in the gascooler above the critical pressure of 71 bar, where with conventional refrigerant it would be the evaporator. This transcritical cycle is excellent for the production of hot water at temperatures varying from 60 to 95oC. Especially in Japan and China the monobloc heat pump for single family houses is very popular. For larger systems the technology using CO2 as refrigerant is also available from a number of manufacturers and applied in large projects in hotels and multi family buildings.
Solar supported systems
Solar Thermal and Solar PV can be installed in combination with heat pumps. Much used in single family buildings it can also be applied in collective systems in multifamily buildings, either as:
- Direct combination to generate DHW in a large storage tank or fresh water system;
- Direct combination as heat source coupled to the evaporator of the heat pump;
- Regeneration of the groundsource for the heat pump.
Fresh Water Systems
A fresh water station (FWS) produces domestic hot water, transferring heat energy from a buffer tank directly to incoming domestic water from the mains water supply. The FWS only heats up in the continuous flow when the tap is turned on, thus being in fact instantaneous water heater using the heat from the storage tank. Systems can have a capacity to supply larger multifamily buildings, but are also available in smaller sizes with capacities down to 24kW’s for single family buildings. The storage tank of the FWS can be connected to any heat generator. Basically this type of systems has first been applied in solar thermal systems to avoid legionella. How it works is easily explained in a small video by Daikin.
Fresh water systems in multifamily buildings do have an interesting and growing market in Switzerland, Austria and Germany with a growing number of suppliers. A number of examples is displayed by this suppliers on their websites. A selection of these is in the list of example projects .
Especially with hot water as an energy carrier in a collective distribution system for a building there are lessons to be learned from studies on low temperature distribution for the 4th Generation of District Heating, often with great attention for comfort and Legionella:
- Michael Markussen – Heat Pumps for Domestic Hot Water Preparation in Connection with Low Temperature District Heating
- Dietrich Schmidt – Low Temperature District Heating for Future Energy Systems
- Linita Karlsson – Overcoming issues with Legionella in DHW in LTDH systems
A great number of articles have been published on this challenge, especially in retrofit. In the final report under the Annex a first attempt has been made to analyse the experiences.
Under the Technical Collaboration Program on Heat Pumping Technologies the HPT-Annex 47 focused on databasing a number of district heating projects with heat pumps. The new HPT-Annex 57 focuses on the ‘Flexibility in Multi-Vector Energy Systems and Thermal Networks by the Implementation of Heat Pumps’. Important work has been done under the SHC Task 55 ‘Integrating Large SHC Systems into DHC Networks‘ as well as work done under TCP on District Heating and Cooling.
Connecting to district heating is always displayed as a chance because of the high density of energy demand in inner-city areas with large ‘one-point’ demand in the substation of a multifamily building. However this is especially for domestic hot water not a good solution as the internal distribution system in the building has high energy losses up to 50%. The trend in district heating developments is towards lower supply temperatures as the system losses are high, especially with a decreasing demand for space heating the losses for ‘only’ supplying heat for generating domestic hot water are considerable.
Thermal grids (TG) is defined as grids where energy is transported and exchanged between different consumers/prosumers. This grid type is known under different names like Anergy networks or the brand name Ectogrid. The concept is that some buildings is extracting heat from the system and other extracts cooling from the grid, the temperature in the grid is usually between 28°C and 8°C depending on the load and usually close to the temperature of the surrounding soil. The main challenge is to balance the loads in the grids.
The Suurstoffi project in Risch-Rotkreuz (Switzerland) is a district energy system with a low temperature network fed from a borehole field ground storage.
In the last 10 years numerous research and commercial initiatives have been undertaken in Europe to develop abandoned coal mining fields into low-temperature resources. One of the most successful is the Minewater project of the municipality of Heerlen, the Netherlands, where a low-temperature district heating system was launched in operation in October 2008, under the European Interreg IIIB NWE programme and the 6th Framework Program project EC-REMINING-lowex.