Decarbonisation of large industrial areas with low temperature hydronic heating
Decarbonisation of gas heated large areas by switching to low temperature hydronic heating
Hydronic Heating with Low Temperature Hot Water (LTHW) , when produced with Geothermal, Solar or Air to Water Heat Pumps can cut down Green House Gas (GHG) emissions to zero and reduce the amount of energy purchased from utlity companies
Introduction
Old Industrial and Commercial Buildings are heated by either natural gas unit heaters or roof top units for cooling with gas burner for space heating. Switching to low temperature hot water (LTHW) for heating can be a substitute to natural gas. LTHW can be produced with either renewable enrgy sources (such as Solar Thermal, Solar Hybrid, etc...) or semi-renewable sources (such as Air to Water Heat Pumps or Geothermal Heat Pumps).
Recent advancement in DC inverter technology and the introduction of new refrigerant such as R32, allow Air to Water Heat Pumps to work at ambient temperatures as low as -30°C (-22°F) and can supply hot water at temperature as high as 60°C (140°F).
This Blog explains how to retrofit an existing space heating system, using Natural Gas as a primary heat source, with a LTHW system as a primary heat source. Existing gas heating system can be kept in place as an emergency or seconday heat source.
Example of an existing gas heating system
Direct or Indirect Fired Gas Unit Heaters can be found in many existing buildings. This type of heaters is used for either space heating inside warehouses or for heating infiltrated air near loading docks and external doors. They are simply controlled by a thermostat that turns ON gas burner as well as circulation fan. Fan draws air from the open space and blow it through the gas heated coil. Flue gas is either evacuated to the outdoor via a chimney (indirect fired Unit Heaters) or simply mixed with blown air (Direct Fired Unit Heaters).
This is an extremely simple and reliable way of providing space heating, but it's extremely inefficient and harmful to the environment. Combustion of Natural Gas generates Carbon Monoxide, Carbon Dioxide and other toxic GHG.
Gas Heated Roof Top Units (RTU) are used for Cooling Large Office Spaces (in the summer) and Heating these spaces in winter. RTU is a more sophisticated technology than a unit heater and can be ducted to serve multiple zones (Offices, Meeting Rooms, common areas, etc...). RTUs are favored by many designer and installers because they are located outdoor and are inexpensive to buy and install.
Old RTU use single or two stage compressor for cooling and have either multiple satges gas burner or modulating gas burner. This makes them slightly more efficient than Unit Heaters, however they both have the disadvantage of burning combustible fuel (gas or propane). Since RTU are located outdoors, they have a shorter life span than unit heaters (located indoor) and require more seasonal and annual maintenance.
Retrofit Equipment and other components
Selecting and installing proper retrofit equipment running on LTHW is the backbone of energy saving and decarbonisation. The reason behind it, is that LTHW can be produced with Air to Water Heat Pumps, Geothermal Heat Pumps, Solar Thermal, Solar Hybrid or simply of combination of multiple technologies. Equipment can be split in two categories: Energy Producing Equipment and Energy Consumming / Distributing Equipment. The two categories will be detailed in the coming sub-sections:
Energy Producing Equipment
Air Source Heat Pump, has the ability to produce chilled water for 7⁰C) cooling and Hot Water for Space Heating and Domestic Hot Water Heating (35-60⁰C) - Outdoor Operating Temperatures -30/-25⁰C To +45⁰C
Liquid to Water Ground Source Heat Pump, has the ability to produce chilled water for 7⁰C) cooling and Hot Water for Space Heating and Domestic Hot Water Heating (35-55⁰C). Can Be used for heat Recovery as well in places with Simultaneous cooling and heating .
Evacuated Tubes: Solar Thermal Panels for producing Hot Heat Transfer Fluid (HTF) in extreme Nordic Climate. This type of panels can be used for all hydronic heating applications: Space heating, DHW, Process Heating, Swimming Pools, Snow Melting, etc...
Hybrid Solar Panel for producing electrcity and Hot Heat Transfer Fluid (HTF). Electricty can used locally, stored in batteries or sold to the grid. Thermal Energy can be used in any hydronic heating application (same as in Evacuated Tubes)
Energy Consuming/Distributing Equipment
Distributing Cooling and Heating in indoor spaces can be done via hydronic Air Handling Unit, Fan Coil Units or Unit Heaters. Configuration of Equipment (ducted or ductless) depends on the application and the type of served indoor space. Large Spaces such as warehouses or storage areas where cooling isn't required and temperature gradient isn't a big issue. Hydronic Unit Heaters running on LTHW are the best fit:
Ducted and Ductless Fan Coil Units, with multiple configurations for both cooling and heating for occupied indoor spaces such as offices, appartments, meeting rooms, etc...
How does a Retrofit Low temperature Hot Water System Looks Like?
Unlike Gas Unit Heaters, LTHW Hydronic System is a centralised system. That means Hydronic Heating/Cooling Water is produced, Centrally stored in Buffer Tanks. Buffer Tanks act as a separation between Thermal Energy Production and Thermal Energy distribution. Buffer tanks, wehther for chilled or hot water, are maintained at their setpoints by the controller of the Energy Producing Equipment. Energy Consuming equipment get their chilled/hot water pumped from the buffer tanks. A schematics of a 30 000 ft² Industrial Building, that was recently retrofitted is shown below:
Building includes 6 000 ft² of office / commercial space and the rest is used as either a warehouse or workshop. There is a simultaneous heating and cooling demand in the office area, which explains the presence of Liquid to Water Heat Pump (for Heat recovery) and the four-pipe system design. Hydronic Air Handling Units serving office spaces have 2 separate hydronic coils: One for Hot Water and another one for Chilled Water.
Case Analysis - Existing Gas Heating System
Building's Nominal Heating Load is 300 576 Btu/hr and its Nominal Cooling Load is 149 279 Btu/hr (Loads include envelope and fresh air). Existing System has Gas Unit Heaters in the warehouse and Roof Top Units with gas burners for the office / commercial spaces. In this analysis we will only be calculating savings for space heating and DHW heating. Building is lcoated in a place where hte lowest winter temperature is -15°C (5°F).
Total Fresh Air and Envelope Annual/Monthly Heating Demand
Global Annual Heating Demand:
The Combined Space Heating and DHW Heating annual demand is 217 936 KWh. Since existing Eqpt gas burners' efficiency is less than 100%, Gas consumption is 244 259 KWh/year or 23 153 m³/year.
Primary Energy Factor for existing system is 1.23 (we need to buy 1.23 m³ of gas for every 1m³ of gas heating demand).
Total Domestic Hot Water Annual/Monthly Heating Demand
Case Analysis - New Retrofit Hydronic Heating System
Our HSS080V2LS 7 Tons Air to Water Heat Pump has an output capacity of 50-60MBH at -15°C (5°F). Tp provide 300 MBH, we have installed 6 Pc x HSS080V2LS Air to Water Heat Pumps with a Gas Boiler as an emergency/backup heat. Schematics is shown in the above section of this blog.
Combined annual electricty and gas consumption for LTHW system is now 124 557 KWh/year instead of 224 259 KWh/year.
Gas Boiler Annual Consumption is now 74 533 KWh/Year which is almost one third the consumption before the retrofit.
Heat Pumps alone consume 50 024 KWh/year and produce (224 259-74 533 ) = 149 726 KWh/year.
Heat Pumps annual COP (Coefficient of Perfromance) is 149 726 / 50 024 = 2.99.
The annual Coefficient of Performance of the whole system is simply the building's energy demand divided by the combined electricty and natural gas consumption;
COP (System) = 224 259 / 124 557 = 1.499.
This means for every KWh of energy we buy from either utility companies, we generate 1.499 KWh of the thermal energy.
Annual Savings resulting from the retrofit are as follows:
224 259-124 557 = 99 702 KWh /year. This represents 44.5% of annual savings.
The abobe annual savings, will contribute to the reduction of GHG emissions. More Heat Pumps can be added to the system for more reduction of natural gas consumption or using an electric boiler instead of gas boiler as a backup heater. Best scenario depends on the availability and cost of energy sources where project is being executed. Presence of Dual Energy Tariff, which aim at reducing the peak demand on the electric grid in peak winter time, influence the selection of retrofit equipment for the project.
Keep up the good work to move forward inch by inch to reduce GHG