It&#;s a lot easier to store heat in water than it is to store heat in air. Why not make a household heating system that can ease strain on the electric grid by taking advantage of that fact?
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Longtime Silicon Valley energy industry executive Jane Melia asked herself that question in when she was looking for a more eco-friendly replacement for her home&#;s aging gas furnace. She ended up buying a more efficient electric heat pump to replace it, but wondered, &#;&#;What about hot water? I need another heat pump for that.&#;
Then, Melia reasoned, why not combine water and space heating in a single unit? That&#;s the genesis of Harvest Thermal, the startup she co-founded in , and the Harvest Pod, a device that&#;s now being tested in homes in California&#;s Bay Area.
The pod connects to a standard heat-pump water heater on one end and to an air handler on the other (that&#;s the part of a forced-air heating system with fans that push air through a home&#;s air ducts). Hot water is piped through coils in the air handler, warming the air that&#;s being blown throughout the house.
This isn&#;t a new idea, Melia emphasized. Similar &#;&#;hydronic&#; air-handler systems are made by a number of major water-heater manufacturers and are becoming a feature of high-efficiency heating systems in large buildings and homes alike.
(Harvest Thermal)Where Harvest Thermal differentiates itself, Melia said, is in its software and controls to optimize that hot-water-to-hot-air cycle to help shift a home&#;s demand for electricity in ways that are valuable for balancing the power grid.
By overheating water when electricity is plentiful on the grid, then using that stored heat to reduce electricity demand when the grid is under stress, &#;&#;we&#;re shifting 65 percent of [a home&#;s] load to the lower-cost time of the day,&#; she said, referring to periods when electricity costs less.
That makes for a combined air and water heating system that can reduce carbon emissions not just by switching from fossil gas to electricity but also by reducing its demand for electricity at times when the grid tends to rely on the most carbon-intensive sources of generation.
Shifting electricity demand to midday periods when California&#;s grid is flooded with solar power also helps soak up more of that sometimes-curtailed zero-carbon electricity, as this graphic indicates.
(Harvest Thermal)&#;That&#;s how we&#;re reducing emissions by 90 percent compared to gas, which is good, but also by about 50 percent compared to heat pumps for air and water heating,&#; she said. And unlike systems that require homeowners to turn down their thermostats to reduce electricity use, &#;&#;the heating is stored up in the [water] tank, so you don&#;t deprive people of heating when you have to conserve electricity.&#;
These are the features that have prompted Peninsula Clean Energy, the community choice aggregator serving the Bay Area&#;s San Mateo County, to put $250,000 into a pilot test of the Harvest Pods in a select set of homes. &#;&#;They&#;want to electrify homes and [are] concerned about the cost of electrifying homes &#; but what about the cost to the grid?&#; Melia explained.
Electric water heaters have been the target of utility demand-response and load-shifting programs for decades, largely in colder climates and in more rural areas. The National Rural Electric Cooperative Association estimates that U.S. rural co-ops save about 500 megawatts per year in such water-heater programs across 35 states.
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But the vast majority of those programs are built around electric resistance water heaters, which use electricity to directly heat metal coils to heat water. That&#;s a very energy-intensive way to heat water compared to heat pumps, which work like air conditioners in reverse, using condensing liquids to move temperatures from outdoors to indoors.
Heat-pump water heaters can also be controlled to reduce electricity demand. But their inherently greater energy efficiency means they can&#;t offer as much load-reduction capacity as resistance water heaters can during times of grid stress. Simply put, because heat pumps use less electricity in a more stable and constant pattern, they offer less of a peak to reduce.
Meanwhile, the push to fight climate change by replacing fossil-gas-fueled heaters with heat pumps is likely to drive up grid demand overall as people switch from gas to electric. At a grand scale, that could mean creating new grid peaks to manage. At a local scale, that could mean adding more electricity demand to a home than its electrical panel or grid connections can handle, forcing expensive and time-consuming upgrades.
The short answer to this question is &#;no&#; although it can make a significant contribution depending on the configuration of your system.
On initial consideration, it might be hard to understand why this should be the case. Most heat pumps are designed to heat water (to run your central heating system) to 45-50°C and domestic hot water is used at 38-42°C. However, unlike a combi boiler, a domestic heat pump cannot produce hot water quick enough to provide an adequate flow of water to baths, taps, and showers. Also, most domestic heat pumps are not designed to be able to provide hot water at the same time as providing space heating, with the demand for hot water taking priority.
One way to get around this problem is to store hot water in a cylinder until it is needed. However, care is needed when storing water between 20 and 45°C due to the risk of legionella, particularly if the water remains in the cylinder for long periods of time. Heating the water in the cylinder to 60°C for at least 30mins or 55°C for at least 5-6 hours is the conventional way to kill legionella bacteria in domestic properties, although for safety and simplicity, the usual advice is to keep the cylinder thermostat set to 60°C.
The simplest way to achieve 60°C is to use an electric immersion heater, fitted into the top of the hot water cylinder. For maximum efficiency (unless you are having a hybrid system installed) it is likely that the existing hot water cylinder will need to be replaced with one that has a much larger coil capacity, to compensate for the fact that a heat pump will deliver water at a lower temperature to the cylinder than a conventional boiler would do. The greater surface area of the larger coils aids the transfer of heat from them to the water in the cylinder.
If you don&#;t have space for a hot water cylinder (with a diameter of 55cm or more) then there are other options available. These include:
A hybrid system
A hybrid system is designed so that the heat pump provides your space heating needs and a boiler (which could be your existing gas or oil boiler) provides your hot water. Another reason for having a hybrid system is to cope with a home with a high heat demand. This is because the largest size of heat pump that can be installed on a standard (single phase) domestic electrical supply is 14kW (although an upgrade to a three-phase supply may be possible, at a cost)
Further information and advice on hybrid heat pumps can be found on the Energy Saving Trust website
A heat battery
A heat battery can store spare heat (or electricity) in a material when it changes phase from a solid to a liquid. These materials are referred to as phase change materials. When hot water is needed the phase change material change back to a solid, releasing heat. As heat batteries are generally smaller and lighter than a cylinder or thermal store they can be a good option if space is limited.
Instantaneous water heaters
An example of an instantaneous water heater is an electric shower &#; it uses electricity to produce hot water only as you needed it. Instantaneous water heaters can also be fitted to supply a single tap and can be economical because the hot water is only generated as it is needed. However, an instantaneous water heater is not as well suited to providing hot water to a bath, due to the volume and flow speed needed for a bath.
And potentially, a high-temperature heat pump,
A high-temperature heat pump can produce heat between 60 and 80°C by using a different refrigerant to a more conventional heat pump. However, this type of heat pump is more expensive to buy and run. High-temperature heat pumps are reported to provide 2.5 units of heat for every unit of electricity consumed; a conventional heat pump is more likely to produce between 3 and 4 units of heat for each unit of electricity consumed.
Summary
This blog is intended as a general guide only to the options available for the delivery of your hot water supply if you have an air source heat pump installed. The sizing and design of a heating system (whether or not this includes an air source heat pump) is a matter for a professional installer who will be able to discuss the options through with you. To find one go to the Microgeneration Certification Scheme website.
If the idea of enhancing your home&#;s energy efficiency through strategic retrofitting and delving into the realm of low-carbon alternatives intrigues you, then our SuperHomes project could be a perfect match. Our community members are at the forefront of redefining sustainable living. For further details, feel free to visit www.superhomes.org.uk
Contact us to discuss your requirements of Air Source Heat Pump Hot Water. Our experienced sales team can help you identify the options that best suit your needs.